What are the ions in NaHCO3, and what is the balanced equation for its decomposition? Write the electron configuration for Silicon.

There are 2 Na atoms on the right and 1 on the left. Add a coefficient of 2 to $\text{NaHCO}_3$.

Left side: Na = 2, H = 2, C = 2, O = 6
Right side: Na = 2, C = 1 (in $\text{Na}_2\text{CO}_3$) + 1 (in $\text{CO}_2$) = 2, H = 2, O = 3 (in $\text{Na}_2\text{CO}_3$) + 1 (in $\text{H}_2\text{O}$) + 2 (in $\text{CO}_2$) = 6
All atoms are balanced.
The balanced chemical equation is:

21. a)
Silicon (Si) has an atomic number (Z) of 14, meaning it has 14 electrons.
The electron configuration for a neutral Silicon atom is:

21. b)

Silicon has 2 electrons in its p-orbital ($3p^2$). The doping element has "exactly one more electron in its p-orbital than Silicon", so it has 3 electrons in its p-orbital ($3p^3$).

The electron configuration for this element would be $1s^2 2s^2 2p^6 3s^2 3p^3$.
Total electrons = $2+2+6+2+3 = 15$.

The element with atomic number 15 is Phosphorus (P).

This refers to the valence s and p orbitals.
The s, p configuration for Phosphorus is: