How do electron shells influence chemical reactivity?

Electron shells influence chemical reactivity by determining how readily an atom will form bonds with other atoms.

In more detail, electron shells, also known as energy levels, are the orbits or layers that surround the nucleus of an atom. These shells are where the electrons reside. The arrangement of electrons in these shells is what determines the chemical behaviour of an atom, including its reactivity.

The outermost shell of an atom, known as the valence shell, is particularly important in determining reactivity. This is because atoms strive to have a full valence shell, which usually means having eight electrons in this shell (this is known as the octet rule). If an atom's valence shell is not full, it will try to achieve a full shell by gaining, losing, or sharing electrons with other atoms. This process of gaining, losing, or sharing electrons is essentially what a chemical reaction is.

For example, sodium (Na) has one electron in its outermost shell. It's more energetically favourable for sodium to lose this electron and have a full outer shell, even though it's the next one in. Chlorine (Cl), on the other hand, has seven electrons in its outermost shell and needs one more to have a full shell. Therefore, sodium can lose its electron to chlorine, forming a bond and creating sodium chloride (NaCl), which is table salt.

In contrast, noble gases like helium (He), neon (Ne), and argon (Ar) have full outer shells and are therefore very stable and unreactive. They rarely participate in chemical reactions because they already have a full valence shell and don't need to gain, lose, or share electrons.

In summary, the arrangement of electrons in an atom's shells, particularly the outermost shell, plays a crucial role in determining the atom's reactivity. Atoms with incomplete outer shells tend to be more reactive as they seek to achieve a stable, full outer shell through chemical reactions.