The modern periodic table organizes elements based on their atomic number revealing recurring properties. Consider the elements of the third period — Sodium (Na), Magnesium (Mg), Aluminium (Al), Silicon (Si), Phosphorus (P), Sulphur (S), Chlorine (Cl) and Argon (Ar).
Elements in a period show gradual changes in their properties on moving from left to right, due to increasing nuclear charge and more electrons added in the same shell, affecting attraction and size.

Answer the following questions :

(a) Why does the size of atoms generally decrease from sodium to chlorine ?

(b) Identify the most metallic element and the most non-metallic element in the third period of the modern periodic table.

(c) Which element in this period acts as a semi-metal (metalloid) showing properties of both metals and non-metals ?

(d) Elements in the same group have similar properties. Give an example of elements from same group and explain why they are similar?

(e) Which element of the third period is a noble gas? Explain why it is unreactive ?

(f) How does the valency change from sodium to chlorine ?

(a) The size of atoms generally decreases from sodium to chlorine because, on moving from left to right in a period, the number of protons increases. This increases the nuclear charge, while the electrons are added to the same shell. Hence, the nuclear pull on the electrons increases and the atomic size decreases.

(b) The most metallic element in the third period is sodium (Na).
The most non-metallic element in the third period is chlorine (Cl).

(c) Silicon (Si) acts as a semi-metal or metalloid in the third period, as it shows properties of both metals and non-metals.

(d) Elements in the same group have similar properties because they have the same number of valence electrons in their outermost shell.
Example: Sodium and potassium belong to Group 1. Both have one electron in their outermost shell. Hence, both show valency 1, form unipositive ions and have similar chemical properties.

(e) Argon (Ar) is the noble gas in the third period. It is unreactive because it has a completely filled outermost shell with 8 electrons, i.e. a stable octet.

(f) From sodium to chlorine, the valency first increases from 1 to 4 and then decreases from 3 to 1.

| Element | Na | Mg | Al | Si | P | S | Cl | | ------- | -: | -: | -: | -: | -: | -: | -: | | Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 |

Thus, the valency changes as: 1, 2, 3, 4, 3, 2, 1.