Question

Write the number of periods the modern periodic table has. How do the valency and metallic character of an element vary on moving from left to right in a period? How do the valency and atomic size of elements vary down a group?

Answer 1

Hint: It has been observed that on moving down the group, effective nuclear charge decreases whereas on moving along a period from left to right, effective nuclear charge tends to increase.

Complete step by step solution:
1. We know that in the modern periodic table, there are 18 vertical columns called groups and 7 horizontal rows called periods.
2. According to modern periodic law, the properties of elements are repeated after regular intervals when all elements are arranged in order of their increasing atomic numbers.
3. The cause of periodicity in properties of elements is due to the repetition of similar electronic configuration after certain regular intervals.
4. Valency is the ability of atoms to form bonds by losing, gaining or sharing electrons. Ongoing from left to right along a period, valency first increases from 1 to 4 and then decreases from 4 to 0. This happens because in periods, nuclear charge increases on going from left to right, so atoms or electrons tend to gain electrons. Whereas on going down in a group all elements will have the same valency because they all will have the same number of electrons in their outermost shell.
5. The metallic character decreases on going from left to right in a period. This is because atomic size decreases and effective nuclear charge increases. Therefore, electrons are not easily released and thus metallic character tends to decrease.
6. As we move down within a group, the atomic size of elements increases with increase in atomic number. This happens because a new energy shell is added at each succeeding element. As a result of this, the attraction of the nucleus for valence electrons decreases and hence the atomic radius increases.


Note: It should be remembered that on moving down in a group, we should look for variation in the atomic radius whereas on moving along a period, we should look for variation in effective nuclear charge.