IGCSE Chemistry 0620: Metallic to Non-Metallic Character Across Period 3

Period 3 of the Periodic Table contains eight elements, arranged left to right in order of increasing proton number: sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl) and argon (Ar).

(a) State how metallic character changes going across Period 3, from sodium to chlorine. [1]

(b) Sodium (11 protons) has the electron configuration 2,8,1 and chlorine (17 protons) has the electron configuration 2,8,7. Using the number of protons and the number of outer-shell electrons, explain why sodium readily loses its outer electron to form an ion, while chlorine readily gains one electron rather than losing seven. [3]

(c) Explain, in terms of electron configuration, why chlorine shows much stronger non-metallic character than sodium. [2]

(d) Argon, the final element of Period 3, does not readily form ions and is neither classed as a metal nor a non-metal in the usual reactive sense. Suggest why, in terms of its electron configuration. [2]

Part (a): The trend across Period 3

Reading left to right from sodium to chlorine, elements go from a soft reactive metal (sodium) to a strongly reactive non-metal (chlorine). Metallic character decreases going across Period 3, from sodium to chlorine.

Part (b): Why sodium loses an electron but chlorine gains one

Sodium (2,8,1): its outer shell holds only 1 electron. Losing this single electron leaves behind a full, stable outer shell (2,8). This is the shortest possible route to a stable electron arrangement, so sodium readily loses its outer electron to form a $1+$ ion.

Chlorine (2,8,7): its outer shell already holds 7 electrons, one short of a full shell. Gaining just 1 more electron completes a full, stable outer shell (2,8,8), a far shorter route to stability than losing all 7 electrons, so chlorine gains an electron to form a $1-$ ion instead.

Chlorine’s much larger proton number (17, compared with sodium’s 11) also means its nucleus attracts additional electrons strongly, reinforcing why chlorine gains an electron rather than losing several.

Part (c): Why chlorine is more non-metallic than sodium

Non-metallic character is linked to an atom’s tendency to gain electrons (or share them in covalent bonds) rather than lose them.

Chlorine’s outer shell already holds 7 of 8 possible electrons, so it strongly attracts one more electron to complete a stable shell. This strong pull for extra electrons is what gives chlorine its strong non-metallic character.

Sodium’s outer shell holds only 1 electron, nowhere near full, so instead of trying to gain 7 more electrons, it loses its single electron. This tendency to lose electrons is what gives sodium its strong metallic character.

Part (d): Why argon is unreactive

Argon has the electron configuration 2,8,8, a full, stable outer shell. Because its outer shell is already complete, argon has no tendency to lose, gain or share electrons, so it does not readily form ions or chemical bonds. This is why argon behaves as an unreactive noble gas, rather than showing typical metallic or non-metallic reactivity.

Final answers

(a) Metallic character decreases across Period 3 from sodium to chlorine.
(b) Sodium loses 1 electron (shortest route to a full shell); chlorine gains 1 electron (shortest route to a full shell), helped by its higher proton number.
(c) Chlorine’s nearly-full outer shell strongly attracts electrons (non-metallic); sodium’s almost-empty outer shell readily loses its electron (metallic).
(d) Argon’s outer shell (2,8,8) is already full, so it has no tendency to react.