Question

Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.

Answer 1

1. Number of protons equals the atomic number, so a magnesium atom has 12 protons. 
2. Number of neutrons = mass number − atomic number = 24 − 12 = 12 neutrons. 
3. In a neutral atom, the number of electrons equals the number of protons, so there are 12 electrons. Arrangement of electrons: K-shell (n = 1) holds 2 electrons, L-shell (n = 2) holds 8 electrons, and the remaining 2 electrons go to the M-shell (n = 3). So the electronic configuration is K = 2, L = 8, M = 2, i.e., 2, 8, 2. In a 2-D diagram, draw the nucleus with 12 protons and 12 neutrons at the centre, then circles for the K, L and M shells holding 2, 8 and 2 electrons respectively.