Na, Mg and Al are the elements having one, two and three valence electrons respectively. Which of these elements (i) has the largest atomic radius, (ii) is least reactive ? Justify your answer stating reason for each.

(i) Largest Atomic Radius:

• Element: Sodium ($\text{Na}$)
• Reason: $\text{Na}$, $\text{Mg}$, and $\text{Al}$ belong to the same period (Period 3). As we move from left to right across a period, the atomic number (nuclear charge) increases because protons are added to the nucleus, while electrons are added to the same outermost shell. This increased nuclear charge exerts a stronger electrostatic pull on the electrons, drawing them closer to the nucleus and decreasing the atomic radius. Therefore, sodium ($\text{Na}$), having the lowest nuclear charge among the three, has the largest atomic radius.

(ii) Least Reactive Element:

• Element: Aluminium ($\text{Al}$)
• Reason:

1. Ionisation Tendency: $\text{Na}$ has 1 valence electron, $\text{Mg}$ has 2, and $\text{Al}$ has 3. It is easiest to lose 1 electron ($\text{Na}$) than to lose 2 ($\text{Mg}$) or 3 ($\text{Al}$) electrons due to the increasing effective nuclear charge. Thus, $\text{Al}$ has the least tendency to lose electrons and form ions.
2. Experimental Evidence (NCERT Chapter 3):

• Sodium ($\text{Na}$) is highly reactive and reacts violently with cold water to catch fire:

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

• Magnesium ($\text{Mg}$) does not react with cold water but reacts with hot water.
• Aluminium ($\text{Al}$) is much less reactive; it does not react with cold or hot water, but only reacts with steam:

$$2\text{Al(s)} + 3\text{H}_2\text{O(g)} \rightarrow \text{Al}_2\text{O}_3\text{(s)} + 3\text{H}_2\text{(g)}$$

• According to the reactivity series (Table 3.2), the reactivity order is $\text{Na} > \text{Mg} > \text{Al}$. Hence, Aluminium is the least reactive.