What is the transfer of electrons in Al + Cl = AlCl3

3 e⁻ transfer has occurred.

Explanation

This is a redox reaction.

Oxidation (loss of electrons or increase in the oxidation state of entity) Reduction (gain of electrons or decrease in the oxidation state of the entity) An element undergoes oxidation or reduction in order to achieve a stable configuration. It can be an octet or duplet configuration. An octet configuration is that of outer shell configuration of noble gas. [Ne] = (1s²) (2s² 2p⁶)

A combination of both the reactions (Half-reactions) leads to a redox reaction.

Let us look at initial configurations of Al and Cl

[Al] = 1s² 2s² 2p⁶ 3s² 3p¹

[Cl] = 1s² 2s² 2p⁶ 3s² 3p⁵

Hence, Al can lose 3 electrons to achieve octet config.

and, Cl can gain 1e to achieve nearest noble gas config. [Ar]

This reaction can be rewritten, by clearly mentioning the oxidation states of all the entities involved.

Al⁰ + Cl⁰ → (Al⁺³) (Cl⁻) ₃

Here, Aluminum is undergoing an oxidation (i. e loss of electrons) from: 0→ (+3)

Chlorine undergoes a reduction half reaction (i. e gain of electrons) from: 0→ (-1). There are 3 such chlorine atoms, hence 3 e⁻ transfer has occurred.