Periodic Properties

The Modern Periodic Table has 18 vertical columns known as ‘groups’ and 7 horizontal rows known as ‘periods’.

You will find that all these elements contain the same number of valence electrons. Similarly, you will find that the elements present in any one group have the same number of valence electrons. For example, the elements fluorine (F) and chlorine (Cl) belong to group 17. Hence, we can say that groups in the Periodic Table signify an identical outer shell electronic configuration. On the other hand, the number of shells increases as we go down the group.

All anomalies of Mendeleev's classification disappear.

• Explanation of the position of Isotopes (same atomic number put at one place in the same group)
• Cobalt with atomic number 27 came first, and Nickel 28 should come later. 
• Unlike atomic masses, an atomic number is always a whole number, so there is no element between hydrogen and helium.
• Atomic number is denoted by ‘Z’ and is equal to the number of protons in the nucleus of an atom.
• Elements with the same number of valance electrons are placed in the same group

example:

Li: 2, 1

Na: 2, 8, 1

K: 2, 8, 8, 1

• Outermost or valance shell in all three contains 1 electron. These elements have been placed in the same group.
• Number of shells increases as we go down the group.
• Elements with the same number of occupied shells are placed in the same period.

For example: Li(2, 1); Be(2, 2); B(2,3); C(2,4) these elements have the same number of shells.

• Each period marks a new electronic shell getting fitted.
• Maximum number of electrons that can be fitted in a shell is given by 2n2, where n is the shell number.

For example,

K Shell – 2 × (1)2 = 2; hence the first period has 2 elements.

L Shell – 2 × (2)2 = 8; hence the second period has 8 elements.

The third, fourth, fifth, sixth and seventh periods have 8, 18, 18, 32 and 32 elements, respectively.

• The positions of the elements in the periodic table tell us their chemical reactivity.
• Valence electrons determine the kind and number of bonds formed by the element.

VALENCY: As you know, the valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.

• Valency of sulphur and magnesium is 2
• On moving from left to right from each period, valency increases from 0 to 4 and then decreases to 0
• The valency remains the same, moving down the groups.

The term atomic size refers to the radius of an atom. The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom.

• Atomic size decreases as we move from left to right in a period because electrons are pulled closer to the nucleus due to a large positive charge on the nucleus, decreasing the atom's size.
• Atomic size increases as we move down the group as new shells are added, increasing the distance between the nucleus and the outermost electron.

METALS:

• Metallic character means the tendency of an atom to lose an electron.
• As the effective nuclear charge acting on the valence shell electrons increases across a period, the tendency to lose electrons will decrease as we move from left to right in a period.
• Metals are electropositive as they tend to lose electrons while forming bonds.
• Metallic character increases as we go down a group as the effective nuclear charge decreases.

NON-METALS:

• Non-metals, on the other hand, are electronegative. They tend to form bonds by gaining electrons.
• Non-metallic character increases across a period due to an increase in effective nuclear charge, which means a tendency to gain electron increases
• Non-metallic character decreases down the group due to a decrease in effective nuclear charge experienced by the valance electrons; therefore, the tendency to gain electrons decreases.
• In the middle of the periodic table, we have semi-metals or metalloids because they exhibit the same properties as that metals and non-metals.
• Oxides of metals are basic, and that of non-metals are acidic in general.