Question

(a) How does the atomic size vary on going down from top-bottom in a group of the periodic table? Why does it vary this way?
(b) Lithium, sodium and potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements? Explain your answer.

Answer 1

Hint: The distance between the centre of the nucleus and the outermost shell of an atom is known as the atomic size of that atom. The electron cloud which is forming a shell of an atom does not have a fixed shape due to which it becomes a very difficult task to determine the atomic size of an atom. So, practically it is not possible to evaluate the size of an individual atom.

Complete answer:
(a) In the modern periodic table on moving top to bottom in a group, the number of energy levels (\[n\]) i.e., the number of shells increases. Since, the distance between the nucleus and the outermost shells increases due to increase in the value of \[n\]. Therefore, the atomic size of the elements increases on moving down from top to bottom in a group.
(b) In the modern periodic table, elements are arranged in such a way that each group of the periodic table will consist of all the elements with similar properties. The metals Lithium, sodium and potassium belongs to the first group of the periodic table and have following similarities:
They have a general electronic configuration of \[n{s^1}\].
The valence shell consists of one electron only which makes them highly unstable and highly reactive.
All elements of the group 1 are known as alkali metals and have relatively low melting and boiling points as compared to heavy metals like copper.
Hence, alkali metals i.e Lithium, sodium and potassium react with water to liberate hydrogen gas.

Note:
Atomic size of an element is a very important characteristic property to understand various trends like reactivity, ionization enthalpy, effective nuclear charge, screening effect etc. Also, it is important to note that due to presence of only one electron in the outermost shell of the alkali metals, they possess high electropositive character.