## Heat Science Notes

Important Points :

• Heat is a form of energy. Particles of matter (atoms, molecules, etc.) possess potential energy and kinetic energy.
• Total energy (potential energy + kinetic energy) of all particles of matter in a given sample is called its thermal energy.
• When two bodies at different temperatures are in thermal contact with each other, there is transfer of thermal energy from a body at higher temperature to a body at lower temperature.
• This energy in transfer is called heat. It is expressed in joule, calorie and erg.
• Temperature is a quantitative measure of degree of hotness or coldness of a body.
• It is expressed in °C, °F or K (kelvin).
• Temperature determines the direction of energy transfer.
• Ways of heat transfer : conduction, convection and radiation.

[Note : heat = heat energy. In the textbook, both the terms are used. Latent heat :

→ Latent heat : When there is a change of state of a substance, [solid ↔ liquid, liquid ↔ gas (vapour), solid ↔ gas (vapour)] heat energy is absorbed by the substance or heat energy is removed from the substance at constant temperature.

→ This heat energy is called latent heat of transition (or transformation or change of state). Latent heat per unit mass of the substance is called specific latent heat.

→ The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion, and that constant temperature is called the melting point of the substance.

→ The amount of heat energy absorbed at constant temperature by unit mass of a liquid to convert into gaseous phase is called the specific latent heat of vaporization and that constant temperature is called the boiling point of the substance.

→ Specific latent heat is expressed in J/kg, erg/g, cal/g, kJ/kg and kcal/kg. Heat absorbed or given out in change of state = mL, where m is the mass of the substance. Regelation:

Regelation: The phenomenon in which ice converts to liquid due to applied pressure and then re-converts to ice once the pressure is removed is called regelation.

Anomalous behaviour of water:

→ Anomalous behaviour of water : If water is heated from 0 °C to 4 °C, it contracts instead of expanding. At 4 °C its volume is minimum. If the water is heated further, it expands, i.e., its volume increases.

→ This abnormal behaviour of water in the temperature range 0 °C to 4 °C is called anomalous behaviour of water. The density of water is maximum at 4 °C. Hope’s apparatus is used to study anomalous behaviour of water.

Dew point and humidity :

→ Dew point temperature : If the temperature of unsaturated air is decreased, a temperature is reached at which the air becomes saturated with water vapour. This temperature is called the dew point temperature.

→ Absolute humidity : The mass of water vapour present in a unit volume of air is called absolute humidity. Generally it is expressed in kg/m.

→ Relative humidity : The ratio of the actual mass of water vapour content in the air for a given volume and temperature to that required to make the same volume of air saturated with water vapour at the same temperature is called the relative humidity. This ratio, multiplied by 100, gives the percentage relative humidity. At the dew point the relative humidity is 100%.

→ Take a bottle of cold water out of a refrigerator and keep it outside for a while. Observe the outer surface of the bottle.

→ Drops of water can be observed on the outer surface of bottle. In the same way, if we observe the leaves of plants/ grass or windowglass of a vehicle in the early morning we see water droplets collected on the surface. → Through these two observations, we sense the presence of water vapour in the atmosphere. When air cools, due to decrease in temperature it becomes saturated with water vapour. As a result, the excess water vapour gets converted into tiny droplets. The dew-point temperature is decided by the amount of vapour in the air.

→ Unit of heat : Heat can be expressed in various units, e.g. joule (J), erg, calorie (cal), kilocalorie (kcal). The amount of heat necessary to raise the temperature of 1 g of water by 1 °C from 14.5 °C to 15.5 °C is called one calorie.

→ The amount of heat necessary to raise the temperature of 1 kg of water by 1 °C from 14.5 °C to 15.5 °C is called one kilocalorie.
1 kcal = 103cal, 1 cal = 4.18 J,
1 kcal = 4.18 × 103 J.

Specific heat capacity :

→ Specific heat capacity (c): The amount of heat energy required to raise the temperature of a unit mass of an object by 1 °C is called the specific heat capacity or simply specific heat of the object. It is expressed in various units such as J/kg . °C, erg/g . °C, cal/g . °C, kcal/kg .°C.

 Substance Specific heat (c) (cal/g.°C) Water 1.0 Paraffin 0.54 Kerosene 0.52 Aluminium 0.215 Iron 0.110 Copper 0.095 Silver 0.056 Mercury 0.033
• Heat absorbed by an object = mcΔt, where m is the mass of the object and ΔT is the increase in the temperature of the object.
• Heat lost (given out) by an object = mcΔT
• Here ΔT is the decrease in the temperature of the object. → Principle of heat exchange : If a system of two objects is isolated from the environment by keeping it inside a heat resistant box, then no energy can leave the box or enter the box. In this situation, heat energy lost by the hot object = heat energy gained by the cold object. In due course, the two objects attain the same temperature.

→ Sublimation: The phenomenon in which a solid directly passes to the gaseous state without passing through the intermediate liquid state is known as sublimation. Iodine, ammonia and camphor possess the property of sublimation.

→ Effect of pressure on the melting point of a substance : At a given pressure, a given solidmelts at a fixed temperature. A change in pressure changes the melting point of the substance. In the case of the solids which expand on melting, an increase in the pressure raises the melting point of the solid, e.g., lead and wax.

→ In the case of the solids which contract on melting, an increase in pressure lowers the melting point of the solid, e.g., ice, antimony and bismuth. → Effect of pressure on the boiling point of a liquid : The boiling point of a liquid depends on the pressure on its surface. An increase in pressure raises the boiling point of a liquid while a decrease in pressure lowers its boiling point.

## Effects of Electric Current Science Notes

Important Points :

• A material which has very low electrical resistance is called a good conductor of electricity. Examples: silver, copper, aluminium.
• A material which has extremely high electrical resistance is called an insulator of electricity.
• Examples : rubber, wood, glass.
• When we pick up a piece of iron resting on the ground, we don’t get electric shock because that piece does not carry any electric current at that time.

Energy transfer in electric circuit :

→ Electric power : Electric power is the electric work done per unit time or electric energy used per unit time. Its SI unit is the watt (W).

→ Electric power (P)
= P = VI = I2R = V2/R. → Mechanical power (P) → The watt: If one joule of electric work is done per second, the electric power is 1 watt.
1 watt (W) = $$\frac{1 \text { joule }(\mathrm{J})}{1 \text { second }(\mathrm{s})}$$

→  Commercial unit of electric energy : The commercial unit of electric energy is the kilowatt-hour (kWh).
1 kW.h -3.6 × 106 J
It is commonly known as the unit.

→  The unit of electric power 1 W is a very small unit, hence 1000 W or 1 kW is used as a unit to measure electric power, in practice. If 1 kW power is used for 1 hour, it will mean 1 kW × 1h of electric energy is used 1 kW.h -1 kilowatt-hour = 1000 W × 3600 S
= 3.6 × 106 W.s = 3.6 × 106 J

→  Electricity bill : Electricity bill shows the consumption of units (i.e., kW.h) and the cost of using electric energy Check monthly electricity bill received from the Electricity Distribution Co. Ltd. Observe various details and get information about them. The electricity bill specifies the usage in ‘Units’. What is this unit? When 1 kW.h electric energy is used, it is termed as 1 unit of energy. Heating effect of electric current :

→  Joule’s law about heating effect of electric current: The quantity of heat produced (H) in a conductor of resistance R, when a current I flows through it for a time t is directly proportional to

• the square of the current
• the resistance of the conductor
• the time for which the current flows.

→  H = I2 Rt = VIt = $$\frac{V^{2}}{R}$$t, where V is the potential difference (RI) across the conductor. Here, H is expressed in joule, V in volt, I in ampere, R in ohm and t in second. 1 calorie (cal) = 4.18 joules (J). With V in volt, I in ampere, R in ohm and t in second, we have: →  The working of an electric bulb, electric iron, fuse wire, etc., is based on the heating effect of electric current Magnetic effect of electric current :

→  When an electric current is passed through an electric resistor (electric conductor), heat is produced in it. Passage of electric current through a conductor also produces a magnetic field around it. This effect, called magnetic effect of electric current, was discovered by Hans Christian Oersted. The unit of intensity of magnetic field, the oersted, is named after him. →  Oersted’s discovery : Hans Christian Oersted (1777 – 1851), Danish physicist, discovered the magnetic effect of electric current in 1820. He observed the deflection of a compass needle when placed near a wire carrying an electric current. The experiment described on page 51 of the textbook refers to his discovery.

→  Right hand thumb rule : Imagine that you have held a current-carrying straight conductor in your right hand in such a way that your thumb points in the direction of the current. Then turn your fingers around the conductor. The direction of the fingers is the direction of the magnetic lines of force produced by the current.

Magnetic field due to a current-carrying conductor :

• The intensity of the magnetic field produced at a given point is directly proportional to the current passing through the conductor.
• The intensity of the magnetic field produced by a given current in the conductor decreases as the distance from the conductor increases.

→ A magnetic field is produced around a straight current-carrying conductor. If the current is unchanged, this magnetic field reduces as the distance from the wire increases. Therefore, the concentric circles representing the magnetic lines of force are shown bigger and rarefied as conditioners, use of electricity in shops, etc. → As a result, excessive current is drawn from the transformer supplying the electricity, and if the capacity of the transformer is insufficient, its fuse wire melts and the supply gets shut down. Such events occur due to overloading. → These days miniature circuit breaker (MCB) switches are used in homes. When the current in the circuit suddenly increases this switch opens and current stops. Different types of MCBs are in use. For the entire house, however the usual fuse wire is used.  • Lamps, TV, computer, electric fan, electric bell.
• Heating effect, magnetic effect, production of light, conversion of electric energy into mechanical energy and the conversion of that mechanical energy into sound.

## Chemical reactions and equations Science Notes

Importent points :

• Elements are divided into three classes i.e. metals, nonmetals and metalloids. When two or more elements combine chemically in a fixed proportion by weight, a compound is formed.
• The properties of a compound are altogether different from those of the constitutional elements.
• The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency of that element.
• While writing the molecular formulae of different compounds, the symbol of the radicals and their valence should be known.
• The number of the ions is written as subscript on the right of the symbol of the ion. By cross multiplication of valencies chemical formula is obtained Chemical reactions :

→ During chemical reactions composition of the matter changes and that change remains permanent and during physical change only the state of matter changes and this change is often temporary in nature. Identify physical and chemical changes from the phenomena given in the following table.

 Phenomenon Physical change Chemical change 1. Transformation of ice into water ✓ 2. Cooking of food ✓ 3. Ripening of fruit ✓ 4. Milk turned into curd ✓ 5. Evaporation of water ✓ 6. Digestion of food in the stomach 7. Size reduction of naphtha balls exposed to air ✓ 8. Staining of Shahbadi or Kadappa tile by lemon juice ✓ 9. Breaking of a glass object on falling from a height ✓

Chemical reaction : A process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds is called a chemical reaction. Rules writing chemical reaction :

→ Chemical equation : The representation of a chemical reaction in a condensed form using chemical formulae is called as the chemical equation.

Rules used in writing a chemical equation :

• The reactants are written on the left hand side (LHS), while the products are written on the right hand side (RHS).
• Whenever there are two or more reactants, a plus sign (+) is written between each two of them. Similarly, if there are two or more products, a plus sign is written between each two of them.
• Reactant side and product side are connected with an arrow (→) pointing from reactants to products. The arrow represents the direction of the reaction. Heat is to be given from outside to the reaction, it is indicated by the sign ∆ written above the arrow.
• The conditions like temperature, pressure, catalyst, etc., are mentioned above the arrow (→) pointing towards the product side.
• The physical states of the reactants and products are also mentioned in a chemical equation. The notations g. 1, s, and aq are written in brackets as a subscript along with the symbols /formulae of reactants and products.
• The symbols g. 1, s, and aq stand for gaseous, liquid, solid and aqueous respectively. If the product is gaseous, instead of (g) it can be indicated by an arrow↑pointing upwards.
• If the product formed is insoluble solid, then instead of (s) it can be indicated by an arrow↓pointing downwards.
• Special information or names of reactants/ products are written below their formulae.

Balancing a chemical equation :

→ In a chemical reaction, the number of atoms of the elements in the reactants is same as the number of atoms of those elements in the product, such an equation is called a balanced equation.

→ Example: AgNO3 + Nacl → AgCl + NaNO3
In the above reaction, the number of atoms of the elements in the reactants is same as the number of atoms of elements in the products. Types of chemical reactions :

Types of chemical reactions :

• Combination reaction
• Decomposition reaction
• Displacement reaction
• Double displacement reaction.

→ Combination reaction : When two or more reactants combine in a reaction to form a single product, it is called a combination reaction.

→ An example of combination reaction : The ammonia gas reacts with hydrogen chloride gas to form the salt in gaseous state, immediately it condenses at room temperature and gets transformed into the solid state. → Decomposition reaction: The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction.

→ Examples of decomposition : Thermal decomposition: The reaction in which a compound is decomposed by heating it to a high temperature is called thermal decomposition At high temperature, calcium carbonate decomposes into calcium oxide and carbon dioxide. → Electrolytic decomposition : The reaction in which a compound is decomposed by passing an electric current through its solution or molten mass is called an electrolytic decomposition.
When an electric current is passed through acidulated water, it is electrolysed giving hydrogen and oxygen. → It is possible to produce hydrogen by decomposition of water by means of heat, electricity or light.

→ Displacement reaction : The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction. → An example of displacement reaction : When zinc granules are added to the blue coloured copper sulphate solution, the zinc ions formed from zinc atoms take the place of Cu2+ ions in CuSO4, and copper atoms, formed from Cu2+ ions comes out i.e. the more reactive zinc displaces the less reactive Cu from copper sulphate. → Double displacement reaction: The reaction in which the ions in the reactants are exchanged to form a precipitate are called double displacement reaction.

→ An example of double displacement reaction : Solutions of sodium chloride and silver nitrate react with each other forming a precipitate of silver chloride and a solution of sodium nitrate. → White precipitate of AgCl is formed by exchange of ions Ag+ and Cl between the reactants.

→ Exothermic process. The process in which heat is given out is called an exothermic process. When NaOH dissolves in water, there is evolution of heat leading to a rise in temperature.
NaOH(s) + H2Ol → NaOH(aq) + Heat

→ Endothermic process. The reaction in which heat is absorbed is called an endothermic process. When KNO3(s) dissolves in water, there is absorption of heat during the reaction and the temperature of the solution falls.
KNO3(s) + H2O(l) + Heat → KNO3(aq).

→ Rate of chemical reaction : Some reactions are completed in short time, i.e., occur rapidly, while some other require long time for completion, i.e, occur slowly. It means that the rate of different reaction is different.

• One or more chemical reactions take place during every chemical change.
• Strong acid and strong base react instantaneously.
• In our body, enzymes increase the rate of physiological reactions.
• If the rate of the chemical reaction is fast, it is profitable for the chemical factories.
• The rate of chemical reaction is important with respect to environment.
• The ozone layer in the earth’s atmosphere protects the life on earth from the ultraviolet radiation of the sun. The process of depletion or maintenance of this layer depends upon the rate of production or destruction of ozone molecules. Factors affecting the rate of a chemical reaction :

• Nature of reactants
• Size of the particles of reactants
• Concentration of the reactants
• Temperature of the reaction
• Catalyst. → Oxidation : The chemical reaction in which a reactant combines with oxygen or loses hydrogen to form the product is called oxidation reaction. The chemical substances which bring about an oxidation reaction by making oxygen available are called oxidants or oxidizing agents. In the combustion of carbon, oxygen is an oxidant.
Examples : → A variety of oxidants : K2 Cr2O7 / H2SO4, KMnO4 / H2SO4 are the commonly used chemical oxidants. Hydrogen peroxide (H2O2) is used as a mild oxidant. Ozone (O3) is also a chemical oxidant. Nascent oxygen is generated by chemical oxidants and it is used for the oxidation reaction.

O3 → O2 + [O]
H2O2 → H2O + [O]
K2 Cr2 O7 + 4H2SO4 → K2 SO4 + Cr2(SO4)3 + 4H2O + 3[O]
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O]

→ Nascent oxygen is a state prior to the formation of the O2 molecule. It is the reactive form of oxygen and is represented by writing the symbol as [O].

→ Reduction : The chemical reaction in which a reactant gains hydrogen or loses oxygen to form the product is called reduction. The chemical substance that brings about reduction is called a reductant, or a reducing agent.

→ When hydrogen gas is passed over black copper oxide a reddish coloured layer of copper is formed.
CuO + H2 → Cu + H2O → Redox reaction : The reaction which involves simultaneous oxidation and reduction is called an oxidation-reduction or redox reaction.

• In a redox reaction, one reactant gets oxidised while the other gets reduced during a reaction.
• Redox reaction = Reduction + Oxidation
• In redox reaction, the reductant is oxidized by the oxidant and the oxidant is reduced by the reductant.

Examples : MnO2 + 4HCl → MnCl2 + 2H2O + Cl2

→ Corrosion : Due to various components of atmosphere, oxidation of metals takes place, consequently resulting in their damage. This is called ‘corrosion’. Iron rusts and a reddish coloured layer is collected on it. This is corrosion of iron. This is also termed as rusting of iron. Its formula is Fe2 O3.H2O.

→ Prevention of corrosion : Corrosion damages buildings, bridges, automobiles, ships, iron railings and other articles made of iron. It can be prevented by using an anti-rust solution, coating surface by the paint by processes like galvanising and electroplating with other metals.

→ Rancidity : Fats and oils in food, is kept for a long time, gets oxidised, it is found to have foul odour called rancidity.

## Periodic Classification of Element Science Notes

Elements and their classification :

• The types of matter are solid, liquid, gas and plasma.
• The types of elements are metals, nonmetals and metalloids.
• The smallest particles are called atoms.
• Elements contain only one kind of atoms in the free state or combined state.
• An element cannot be decomposed into simple substances by any chemical reaction or simple physical process, e.g. copper, iron, oxygen.
• A compound is produced by a chemical reaction of two or more elements.
• The constituents of a compound can be separated by a chemical process, e.g. salt, water and sugar. • Dobereiner’s Triads : In the year 1817 Dobereiner (a German scientist) proved that the properties of
elements are related to their atomic masses.
• He made groups of three elements each, showing similar chemical properties and called them triads.
• He arranged the three elements in a triad in an increasing order of atomic mass and showed that the atomic mass of the middle element was approximately equal to the mean of the atomic masses of the other two elements. Newlands Law of Octaves :

→ Newlands’ Law of Octaves : In the year 1866, Newlands arranged the elements known at that time in an increasing order of their atomic masses, he found that every eighth element had properties similar to those of the first. For example, sodium is the eighth element from lithium and both have similar properties.

→ Limitations of Newlands’ Octaves :

• Newlands could arrange 56 elements only up to calcium in an increasing order of their atomic masses.
• This arrangement started with the lightest element hydrogen and ended up with thorium.
• Newlands placed the metals Co and Ni under the note Do along with halogens, while Fe having similarity with Co and Ni, away from them along with the nonmetals O and Sunder the note Ti.
• The properties of the new elements discovered later did not fit in the Newlands’ law of octaves. Mendeleev’s Periodic Table :

→ Mendeleev’s Periodic table :

• The most important step in the classification of elements in Mendeleev’s periodic table is the fundamental property of elements, namely, the atomic mass, as standard.
• He arranged 63 elements known at that time in an increasing order of their atomic masses.
• Then he transformed this into the periodic table of elements according to their physical and chemical properties.
• Mendeleev found that the elements with similar physical and chemical properties repeat after a definite
• interval Mendeleev’s periodic law : The elements with physical and chemical properties are a periodic function of their atomic masses.

→ Introduction to scientist : • Dmitri Mendeleev (1834-1907) was a professor in the St. Petersburg University.
• He made a separate card for every known element showing its atomic mass.
• He arranged the cards in accordance with the atomic masses and properties of the elements which resulted in the invention of the periodic table of elements.
• The vertical columns in the periodic table are called groups while the horizontal rows are
called periods.

→ Merits of Mendeleev’s periodic table :

• To give the proper place in the periodic table, atomic masses of some elements were revised in accordance with their properties.
• For example, the previously determined atomic mass of beryllium, 14.09, was changed to the correct value 9.4, and beryllium was placed before boron.
• Mendeleev had kept some vacant places in the periodic table for elements that were yet to be discovered.
• Three of these unknown elements were given the names eka-boron, eka-aluminium and eka-silicon from the known neighbours and their atomic masses were indicated as 44, 68 and 72, respectively.
• Their properties were also predicted. Later on these elements were discovered subsequently and were named as scandium (Sc), gallium (a) and germanium (Ge) respectively.
• The properties of these elements matched well with those predicted by Mendeleev. Due to this success all were convinced about the importance of Mendeleev’s periodic table.
• When noble gases such as helium, neon and argon were discovered, Mendeleev created the ‘zero group’ without disturbing the original periodic table in which the noble gases were placed very well. → Demerits of Mendeleev’s periodic table :

• The elements cobalt (Co) and nickel (Ni) have the same whole number atomic mass. As a result there was an ambiguity regarding their sequence in Mendeleev’s periodic table.
• Isotopes were discovered long time after Mendeleev put forth the periodic table.
• A challenge was posed in placing isotopes in Mendeleev’s periodic table as isotopes have the same chemical properties but different atomic masses.
• The rise in atomic mass does not appear to be uniform when elements are arranged in an increasing order of atomic masses.
• It was not possible, therefore, to predict the number of elements that could be discovered between two heavy elements.
• Position of hydrogen : Hydrogen shows similarity with halogens (group VII). It is difficult to decide the correct position of hydrogen whether it is in the group of alkali metals (group I) or in the group of halogens (group VII).

Modern Periodic Table :

→ Modern Periodic Law : Henry Moseley showed that the atomic number of an element is the most fundamental property and not its atomic mass. Accordingly Mendeleev’s Periodic law was modified into Modern Periodic law and it can be stated as : The chemical and physical properties of elements are a periodic function of their atomic numbers.

→ In the modern periodic table, the elements are arranged in the order of their increasing atomic numbers. There are seven horizontal rows called periods 1 to 7. There are eighteen vertical columns called groups 1 to 18. The arrangement of the periods and groups results into formation of boxes. → Each box corresponds to the place for one element. There are two series of elements placed separately at the bottom of the periodic table. These are called lanthanide series and actinide series. There are 118 boxes in the periodic table including the two series.

→ The elements in the modern periodic table are divided into four blocks : the s-block, the p-block, the d-block and the f-block. The groups, 1 and 2 together with hydrogen form the s-block elements. The groups 13 to 18 form the p-block elements. The groups 3 to 12 together form the d-block elements. The two series (the lanthanides and actinides) at the bottom of the periodic table together form the f-block elements.

→ The d-block elements are called transition elements. A zig-zag line is seen in the p-block of the periodic table. The metalloid elements lie along the border of this zig-zag line. All the metals lie on the left side of zig-zag line while all the nonmetals lie on the right side.

→ Modern Periodic Table and electronic configuration of the elements : The characteristics of the groups and periods in the modern priodic table are because of electronic configuration of the elements.

→ It is the electronic configuration of an element which decides the group and the period in which it is to be placed. The neighbouring elements within a period differ slightly in their properties while distant elements differ widely in their properties. Elements in the same group show similarity and gradation in their properties.

→ Groups and electronic configuration : Characteristics of the Groups and Periods. Various properties of all the elements in a group show similarity and gradation. However, the properties of elements change slowly while going from one end to the other (for example, from left to right) in a particular period.

→ The number of valence electrons in all these elements from the group 1, i.e. the family of alkali. metals, is the same. Similarly, the element from any other group, the number of their valence electrons to be the same.

→ For example, the elements beryllium (Be), magnesium (Mg) and calcium (Ca) belong to the group 2, i.e. the family of alkaline earth metals. There are two electrons in their outermost shell the number of valence electrons are 2.

→ Similarly, there are seven electrons in the outermost shell of the elements such as fluorine (F) and chlorine (Cl) from the group 17, i.e., the family of halogens. While going from top to bottom within any group, one electronic shell is added at a time. From this, the electronic configuration of the outermost shell is characteristics of a particular group.

→ In the modern periodic table :

• Elements are arranged in an increasing order of their atomic numbers.
• Vertical columns are called groups. There are 18 groups. The chemical properties of the elements in the same group show similarity and gradation.
• Uranium has atomic number 92. All the elements beyond uranium (with atomic numbers 93 to 118) are manmade. All these elements are radioactive and unstable, and have a very short life. → Periods and electronic configuration :

• In modern periodic table, there are seven horizontal rows called periods.
• In a period, the change in valency of an element varies with electronic configuration.
• In a period, while going from left to right, the atomic number increases by one at a time and number of valence electrons also increases by one at a time. In a period, there is gradation in properties of elements.
• The elements with the same number of shells occupied by electrons belong to the same period. The elements in the second period, have electrons in the two shells, K and L.
• The elements in the third period have electrons in the three shells; K, L and M.
• The chemical reactivity of an element is determined by the number of valence electrons in it and the shell number of the valence shell.
• Periodic trends in the modern periodic table : When the properties of elements in a period or a group of the modern periodic table are compared, certain regularity is observed in their variations.
• It is called the periodic trends in the modern periodic table.
• Valency, atomic size and metallic-nonmetallic character are some properties which show periodic trends in the modern periodic table.

→ Valency: The valency of an element is determined by the number of electrons present in the outermost shell of its atoms, that is, the valence electrons.

 Shell n 2n2 Electron capacity K 1 2 × 12 2 L 2 2 × 22 8 M 3 2 × 32 18 N 4 2 × 42 32
• For the shell K, the value of n is l. For the shell L, the value of n is 2. For the shell M, the value of n is 3.
• The maximum number of electrons that can be accommodated in a shell is 32 electrons. The formula is 2n2
• For the shell K, the maximum electron capacity is 2 electrons.
• For the shell L, the maximum electron capacity is 8 electrons.
• For the shell M, the maximum electron capacity is 18 electrons. → Atomic size : The size of an atom is indicated by its radius. Atomic radius is the distance between the nucleus of the atom and its outermost shell. Atomic radius is expressed in the unit picometre (pm) which is smaller than nanometre (1 pm = 10 -12 m).

→ Atomic size depends on number of shells of an atom. More the number of shells larger is the atomic size. While going down a group the atomic size goes on increasing. This is because while going down a group a new shell is added. Therefore the distance between the outermost electrons and the nucleus goes on increasing. As a result of this the atomic size increases.

→ While going from left to right within a period atomie radius goes on decreasing and the atomic number increases one by one, that means positive charge on the nucleus increases by one unit at a time. However, the additional electron gets added to the same outermost shell. Due to the increased nuclear charge the electrons are pulled towards the nucleus to a greater extent and thereby the size of the atom decreases.

→ Metallic-Nonmetallic character : It is observed that the metallic elements like sodium, magnesium are towards the left. The nonmetallic elements such as sulphur, chlorine are towards the right. The metalloid element silicon lies in between these two types.

• While going downwards in any group the electropositivity of elements goes on increasing while their electronegativity goes on decreasing
• While going from left to right in any period the electronegativity of elements goes on increasing while their electropositivity goes on decreasing.
• Larger the electropositivity or electronegativity of the element higher the reactivity.
• The periodic trend in the metallic character of elements is clearly understood from their position is the modern periodic table.
• In a group while going down a group a new shell is added, resulting in an increase in the distance between the nucleus and the valence electrons.
• This results in lowering the effective nuclear charge and thereby lowering the attractive force on the valence electrons.
• As a result of this the tendency of the atom to lose electrons increases. Also the penultimate shell becomes the outermost shell on losing valence electrons.
• The penultimate shell is a complete octet. Therefore, the resulting cation attains special stability.
• The metallic character of an atom is its tendency to lose electrons.
• Therefore, the following trend is observed : The metallic character of elements increases while going down the group. While going from left to right within a period the outermost shell remains the same.

→ However, the positive charge on the nucleus goes on increasing while the atomic radius goes on decreasing and thus the effective nuclear charge goes on increasing. Therefore valence electrons are held with greater attractive force. This is called electronegativity.

→ As a result of this the tendency of atom to lose valence electrons decreases within a period from left to right, i.e. electronegativity increases. Thus, non metallic character of elements increases within a period from left to right. → Gradation in Halogen Family : The group 17 contains the members of the halogen family. All of them have the general formula X2. A gradation is observed in their physical state down the group. Thus, fluorine (F2) and chlorine (Cl2) are gases, bromine (Br2) is a liquid while iodine (I2) is a solid.

→ Reaction of alkaline earth metal with water : A general chemical equation indicating the reaction of alkaline earth metals is M + 2H2O + M(OH)2 + H2. While going down the second group as Be → Mg → Ca → Sr → Ba, the gradation in this chemical property of the alkaline earth metals is observed.

→ While going down the second group the reactivity of the alkaline earth metals goes on increasing and thereby the ease with which this reaction takes place also goes on increasing. Thus beryllium (Be) does not react with water. Magnesium (Mg) reacts with steam, while calcium (Ca), strontium (Sr) and barium (Ba) react with water at room temperature with increasing rates.