Define electropositivity.

OR

The atomic radii of first group elements are given below:

Group-I element	Atomic Radii (pm)
Na	86
K	231
Rb	244
Cs	282

State the reason behind the observed trend in the above elements.

Electropositivity

Electropositivity is the tendency of an atom of an element to lose valence electrons and form positively charged ions (cations). Metals are highly electropositive because they readily lose electrons to achieve a stable octet configuration. For example, alkali metals like sodium ($\text{Na}$) and potassium ($\text{K}$) react vigorously with water to form basic hydroxides by losing their valence electrons:

$$2\text{Na(s)} + 2\text{H}_2\text{O(l)} \rightarrow 2\text{NaOH(aq)} + \text{H}_2\text{(g)} + \text{heat energy}$$

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OR

Reason behind the observed trend in atomic radii

The atomic radius increases down the group from Sodium ($\text{Na}$) to Caesium ($\text{Cs}$). As we move down Group 1, a new electronic shell is added at each successive step. This increases the distance between the outermost valence electrons and the nucleus. Although the nuclear charge (number of protons) also increases down the group, the effect of adding a new electronic shell far outweighs the increased nuclear attraction, resulting in an overall increase in atomic size.