Question

The reactivity of an atom is directly determined by

Answer 1

Answer: B) The number of electron in its valence shell.

Explanation:

Outermost electrons of the atoms which can participate in the chemical bond formation is known as valence electrons.  

Atoms either transfer or share its valence electrons during chemical bond formation. Atoms transfer or share electrons to attain octet or in other words stability.

Generally, 8 electrons are required in their outermost shell or valence shell in order to attain stability. This rule is known as octet rule.

For example, general electronic configuration of sodium belonging to group 1 is  1s2 2s2 2p6 3s1.

No. of valence electrons in Na = 1

by loosing 1 electron, its outer shell electronic configuration become =  1s2 2s2 2p6 (Octet)

Hence, sodium and other group 1 elements are very reactive.

Similarly, halogens (ns2 np5) by gaining 1 electron attain octet (ns2 np6). Hence halogens are also very reactive.

Carbon atom belonging to group 14 has 4 valence electrons (ns2 np2) and share all 4 valence electrons to attain octet and hence reactive.

Nobel gases have octet in their outermost shell (ns2 np6), hence they are most nonreactive element.