You find the valency of chlorine, sulphur and magnesium by looking at their outermost (valence) electrons and seeing how many they lose or gain to get 8 electrons in that shell (octet rule).

Step 1: Recall atomic numbers and electronic configuration

  • Chlorine (Cl): atomic number 17 → electronic configuration = 2, 8, 7
  • Sulphur (S): atomic number 16 → electronic configuration = 2, 8, 6
  • Magnesium (Mg): atomic number 12 → electronic configuration = 2, 8, 2

The valence shell is the outermost shell (the last number in each configuration).

Step 2: Use the octet rule

Atoms tend to complete 8 electrons in the valence shell either by losing or gaining electrons.

  • If an atom has 1, 2, or 3 electrons in the outer shell, it usually loses them (valency = that number).
  • If it has 5, 6, or 7 electrons in the outer shell, it usually gains electrons to reach 8 (valency = 8 − that number).

This is the basic school-level rule for valency.

Step 3: Apply to each element

1. Valency of chlorine

  • Electronic configuration: 2, 8, 7 → 7 electrons in valence shell.
  • To complete 8, chlorine gains 1 electron.

So, valency of chlorine = 1.

2. Valency of sulphur

  • Electronic configuration: 2, 8, 6 → 6 electrons in valence shell.
  • To complete 8, sulphur gains 2 electrons.

So, valency of sulphur = 2.

3. Valency of magnesium

  • Electronic configuration: 2, 8, 2 → 2 electrons in valence shell.
  • It is easier for magnesium to lose 2 electrons than to gain 6.

So, valency of magnesium = 2.

Mini “quick method” you can memorize

For main-group elements (like these):

  • Valency ≈ number of valence electrons, if that number ≤ 4.
  • Valency ≈ 8 − number of valence electrons, if that number > 4.

So:

  • Chlorine: valence e⁻ = 7 → valency = 8 − 7 = 1.
  • Sulphur: valence e⁻ = 6 → valency = 8 − 6 = 2.
  • Magnesium: valence e⁻ = 2 → valency = 2 (it loses 2).

Simple HTML table (as requested)

html

<table>
  <tr>
    <th>Element</th>
    <th>Atomic number</th>
    <th>Electronic configuration</th>
    <th>Valence electrons</th>
    <th>How it gets octet</th>
    <th>Valency</th>
  </tr>
  <tr>
    <td>Chlorine (Cl)</td>
    <td>17</td>
    <td>2, 8, 7 [web:8]</td>
    <td>7</td>
    <td>Gains 1 electron</td>
    <td>1 [web:8]</td>
  </tr>
  <tr>
    <td>Sulphur (S)</td>
    <td>16</td>
    <td>2, 8, 6 [web:3][web:8]</td>
    <td>6</td>
    <td>Gains 2 electrons</td>
    <td>2 [web:3][web:8]</td>
  </tr>
  <tr>
    <td>Magnesium (Mg)</td>
    <td>12</td>
    <td>2, 8, 2 [web:1][web:3][web:8]</td>
    <td>2</td>
    <td>Loses 2 electrons</td>
    <td>2 [web:1][web:3][web:5][web:7][web:8]</td>
  </tr>
</table>

One-line TL;DR

Use electronic configuration and the octet rule: Cl has valency 1, S has valency 2, and Mg has valency 2.