The Valency of magnesium and aluminium is

 2 & 3 respectively

The valency of magnesium and aluminium is 2 and 3, respectively. This is because:

Magnesium (Mg):

• Atomic number: 12
• Electron configuration: 2, 8, 2
• Valence electrons: 2
• Valency: 2

Aluminium (Al):

• Atomic number: 13
• Electron configuration: 2, 8, 3
• Valence electrons: 3
• Valency: 3

Valency refers to the combining capacity of an element, determined by the number of electrons in its outermost shell. Elements tend to lose, gain, or share electrons to achieve a stable electron configuration, often resembling the nearest noble gas.

In the case of magnesium, it has two electrons in its outermost shell. To attain stability, magnesium tends to lose these two electrons, resulting in a valency of 2. This means magnesium can form bonds by donating two electrons.

Similarly, aluminium has three electrons in its outermost shell. To achieve a stable configuration, aluminium tends to lose these three electrons, giving it a valency of 3. This indicates that aluminium can form bonds by donating three electrons.