Question

Na, Mg, and Al are the elements in which the numbers of valence electrons are one, two, and three respectively. (a) Which of these elements has the maximum atomic radius? (b) Which of these elements is the least reactive? Answer each with the reason.

Hint:

In general, across a period from left to right, atomic size decreases and ionization energy increases, making elements on the left more reactive and elements on the right less reactive.

Answer 1

(a) Which of these elements has the maximum atomic radius?
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Atomic radius refers to the size of an atom, typically the distance from the nucleus to the outermost electron shell. The atomic radius generally increases as we move down a group in the periodic table, as new electron shells are added. However, the atomic radius decreases across a period as the effective nuclear charge increases, pulling the electrons closer to the nucleus.
- Na (Sodium), Mg (Magnesium), and Al (Aluminum) are all in the same period (Period 3) of the periodic table. Since the atomic radius decreases as we move from left to right across a period, and Na is the farthest left in the period, it has the largest atomic radius. \[ \text{Na}>\text{Mg}>\text{Al} \] Thus,
Na has the maximum atomic radius.

(b) Which of these elements is the least reactive?
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Reactivity in metals generally decreases as we move across a period from left to right because the atomic size decreases and the ionization energy increases. Elements with fewer valence electrons tend to be more reactive as they lose electrons more easily.
- Na has one valence electron, making it highly reactive as it easily loses this electron to form Na\(^+\). Mg has two valence electrons, making it less reactive than Na, but still quite reactive. Al has three valence electrons and is less reactive than both Na and Mg because it holds on to its electrons more tightly. Thus,
Al is the least reactive of the three elements.
Conclusion:
(a) Na has the maximum atomic radius.
(b) Al is the least reactive element.