A chlorine atom contains 17 electrons, 18 neutrons and 17 protons - Edexcel - GCSE Chemistry - Question 2 - 2018 - Paper 1

The electronic configuration of the chlorine atom (which has 17 electrons) can be stated as:

$1s^2 2s^2 2p^6 3s^2 3p^5$

This indicates that chlorine has 2 electrons in the first energy level, 8 electrons in the second energy level, and 7 electrons in the third energy level.

When damp, blue litmus paper is placed into chlorine gas, the following observations can be made:

• Initially, the blue litmus paper will turn red due to the acidic nature of the chlorine gas.
• Over time, the color may change as the chlorine bleaches the litmus paper, eventually causing it to turn white.

The litmus paper turns red because the solution formed by hydrogen chloride (HCl) when dissolved in water is acidic. The presence of hydrogen ions (H⁺) in the solution is responsible for this acidity, which causes the litmus paper to change color.