International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

What is the charge on the ion formed from a magnesium atom with the electron configuration 2.8.2?

1. 1+

2. 2+

3. 3+

4. 0

The correct answer is: 2+

The ion formed from a magnesium atom, which has the electron configuration of 2.8.2, will have a charge of 2+. In a neutral atom, the number of protons equals the number of electrons. Magnesium has 12 electrons and thus 12 protons, leading to a balanced neutral state. However, when a magnesium atom forms an ion, it tends to lose electrons to achieve a more stable electron configuration, resembling that of noble gases. In the case of magnesium, it has two electrons in its outermost shell (the third shell, which is the 2 in the 2.8.2 configuration). By losing these two outer electrons, magnesium achieves a stable electron configuration of 2.8, which is the same as neon. When magnesium loses these two electrons, it becomes positively charged since it now has more protons than electrons. The loss of two electrons results in a charge of 2+, confirming that the magnesium ion is Mg²⁺. This tendency to lose two electrons is characteristic of alkaline earth metals like magnesium, which typically form ions with a +2 charge.