O-Level The Particulate Nature of Matter: worked solution

Question

Complete the table for the ions 39/19 K+ and 16/8 O2-, giving the number of protons, neutrons and electrons in each.

Step 1 - read what the two numbers mean. In 39/19 K the lower number (19) is the proton number and the upper number (39) is the nucleon (mass) number, which counts protons + neutrons together. So neutrons = nucleon number - proton number. Also remember that a charge only ever changes the electrons; the nucleus is untouched. Step 2 - the potassium ion, 39/19 K+. Protons = 19 (this is fixed - it is what makes the atom potassium). Neutrons = 39 - 19 = 20. A neutral K atom has 19 electrons. The + charge means one electron has been lost, so electrons = 19 - 1 = 18. Step 3 - the oxide ion, 16/8 O2-. Protons = 8. Neutrons = 16 - 8 = 8. A neutral O atom has 8 electrons. The 2- charge means two electrons have been gained, so electrons = 8 + 2 = 10.