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
→ 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.
|2 × 12
|2 × 22
|2 × 32
|2 × 42
- 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.