 1. Atoms are made up of three different particles. Protons, electrons and neutrons. Part A. Complete the table showing the number of each particle for these elements. Let's write down what we know. The atomic number is the number of protons in an atom. Therefore the proton number for potassium, K, is 19. And the proton number for argon, AR, is 18. The number of protons equals the number of electrons in an atom. Therefore the number of electrons in potassium, K, is 19. And the number of electrons for argon, AR, is 18. Mass number equals the number of protons plus the number of neutrons. We can rearrange this formula to give us neutrons equals mass number minus protons. Potassium has a mass number of 39 and has 19 protons. 39 minus 19 equals 20. Potassium therefore has 20 neutrons. Argon has a mass number of 40 and has 18 protons. 40 minus 18 equals 22. Argon therefore has 22 neutrons. Part B. Draw the dot and cross diagram to show the electronic configuration of these elements. So what do we know? The first shell in an atom can hold a maximum of two electrons. And the next shells in an atom can hold a maximum of eight electrons. Potassium has 19 electrons. Two in the first shell, eight in the second shell, eight in the third shell, and one in the fourth shell. Argon has 18 electrons. Two in the first shell, eight in the second shell, and eight in the third shell.