The Sodium and Halogen Group

Do you know the valency of the elements present in the third period?

Valency is defined as the number of electrons that an atom requires to lose, gain, or share in order to complete its valence shell to attain the stable noble gas configuration. Valencies of the elements can also be determined by the number of electrons present in the outermost shell known as the valence shell.

Therefore, on moving across a period, from left to right, the valency first increases from 1 to 4 and then decreases from 4 to 0.

Element	Valence electrons	Valency
Sodium, Na	1	1
Magnesium, Mg	2	2
Aluminium, Al	3	3
Silicon, Si	4	4
Phosphorus, P	5	3, 5
Sulphur, S	6	2
Chlorine, Cl	7	1
Argon, Ar	8	0

Do you know how the valency of an element present in the same group changes on moving down the group?

The number of valence electrons present in an element in any group remains the same. As a result, valency also remains the same. Hence, the valency of group 1 elements is one, group 2 elements is two, and so on.

If you look at the group IA of the periodic table, then you will observe the trends in periodic properties of the alkali metals (Li, Na, K, Rb, and Cs).

For example, the valency of the first three elements of group IA of the periodic table is given as follows. The valency of all these three elements is 1.

Valency of the first three elements of group 1
Element	Electronic configuration				Valency
	K	L	M	N
Lithium	2	1			1
Sodium	2	8	1		1
Potassium	2	8	8	1	1

Similarly, if you look at the group VIIA of the periodic table, then you will observe the trends in periodic properties of the halogen group (F, Cl, Br, I and At).

For example, the valency of the first three elements of group VIIA of the periodic table is given as follows. The valency of all these three elements is 1.

Valency of the first three elements of group 7
Element	Electronic configuration				Valency
	K	L	M	N
Flourine	2	7			1
Chlorine	2	8	7		1
Bromine	2	8	18	7	1

Atomic size is the radius of an atom in its neutral state i.e., the distance of the nucleus from its valence shell in an isolated state.

Changes in atomic size on moving down a group

The atomic radii of the elements present in the third period are given in Pico meters.

Changes in atomic size on moving across a period

The atomic radius of halogen atoms goes on increasing on moving down the group.

Element	Atomic radius (in pm)
Flourine	133
Chlorine	184
Bromine	196
Iodine	220

Do you know that the metallic character of elements can also be determined with the help of the periodic table?

Metals are electropositive in nature as they lose electrons to form positive ions called cations. For example, Na loses one electron to form Na⁺ion.

We have learnt that there is a decrease in the number of valence electrons as well as the atomic size as we move across a period. As a result, it becomes difficult for an atom to lose electrons as we move from left to right across the periodic table. Hence, metallic character decreases on moving across a period from left to right.

On the other hand,atomic size increases when we move down a group. This makes it easier for an element to lose electrons. Hence, metallic character increases on moving down a group.

Therefore, the elements present in the 1^st group are the most metallic in their respective periods as they contain only one electron and have the largest atomic size in their respective periods.

Changes in metallic character on moving down a group

Similar trend is followed for group VIIA i.e., the halogen group.

Iodine possesses the maximum metallic character.

Do you know that with the help of a periodic table, we can also predict the nature of the oxides formed by the various elements? The oxides formed by metals are basic in nature whereas the oxides of non-metals are acidic in nature. Hence, the elements present on the left hand side of the periodic table will form basic oxides whereas the elements present on the right hand side of the periodic table will form acidic oxides.