Question

9. In a period, do the number of shielding electrons change? Why do you think

or shielding electrons change? Why do you think atomic radius decreases across a
period?

Answer 1

The nuclear charge increases, but the number of inner shielding electrons stays the same.

Step-by-step explanation:

Their shielding does not change, so the effective nuclear charge — the charge felt by a valence electron — increases.

The valence electrons are pulled closer to the nucleus, decreasing the atomic radius.

For example, consider the elements of Period 3.

`\begin{array}{ccccc}& & \textbf{Number of} & \textbf{Inner} & \textbf{Valence}\\\textbf{At. No.} & \textbf{Symbol} & \textbf{Protons} & \textbf{Electrons} & \textbf{Electrons}\\11& \text{Na} & 11 & 10 & 1\\12& \text{Mg} &12 & 10 & 2\\13& \text{Al} & 13 & 10 & 3 \\14& \text{Si} & 14 & 10 & 4 \\15& \text{P} & 15 & 10 & 5\\16& \text{S} & 16 & 10 & 6\\17& \text{Cl} & 17 & 10 & 7\\18& \text{Ar} & 18 & 10 & 8 \\\end{array}`

The number of protons increases as you go from one element to the next, but the number of inner electrons is constant.