Atomic Number, Mass Number and Electron Arrangement

<h3>Counting the particles</h3><p>The <strong>atomic number (Z)</strong> is the number of protons in an atom. It is unique to each element &mdash; carbon is always 6, oxygen is always 8. In a neutral atom, Z also equals the number of electrons.</p><p>The <strong>mass number (A)</strong> is the total number of protons and neutrons. To find the number of neutrons, use:</p><p><code>Number of neutrons = A &minus; Z</code></p><p>For example, a chlorine atom with A = 35 and Z = 17 has <code>35 &minus; 17 = 18</code> neutrons.</p><h3>Electron arrangement</h3><p>Electrons occupy regions called <strong>shells</strong>. The first shell fills first (maximum 2 electrons), then the second and third shells (maximum 8 each for the first twenty elements). We write the arrangement as numbers separated by dots:</p><ul><li>Hydrogen (Z=1): <code>1</code></li><li>Carbon (Z=6): <code>2.4</code></li><li>Sodium (Z=11): <code>2.8.1</code></li><li>Calcium (Z=20): <code>2.8.8.2</code></li></ul><div class="callout"><strong>Kenyan context:</strong> Table salt sold in dukas across Kenya is sodium chloride. Sodium has the arrangement 2.8.1 and chlorine 2.8.7. The single outer electron of sodium transfers to chlorine, and that is exactly why salt forms &mdash; chemistry you can taste!</div>