Identify the ions in NaHCO3 and write the balanced chemical equation for its thermal decomposition.

Sodium Hydrogen Carbonate ($\text{NaHCO}_3$) decomposes into Sodium Carbonate ($\text{Na}_2\text{CO}_3$), Water ($\text{H}_2\text{O}$), and Carbon (IV) oxide ($\text{CO}_2$).

Balance Sodium (Na) atoms: There are 2 Na atoms on the right in $\text{Na}_2\text{CO}_3$. Place a coefficient of 2 in front of $\text{NaHCO}_3$ on the left.

Now, check other atoms:
Hydrogen (H): $2 \times 1 = 2$ H on the left. 2 H on the right in $\text{H}_2\text{O}$. (Balanced)
Carbon (C): $2 \times 1 = 2$ C on the left. 1 C in $\text{Na}_2\text{CO}_3$ and 1 C in $\text{CO}_2$ (total 2 C) on the right. (Balanced)
Oxygen (O): $2 \times 3 = 6$ O on the left. 3 O in $\text{Na}_2\text{CO}_3$, 1 O in $\text{H}_2\text{O}$, and 2 O in $\text{CO}_2$ (total 6 O) on the right. (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 $1s^2 2s^2 2p^6 3s^2 3p^2$.
The configuration for a neutral Silicon atom is $\boxed{1s^2 2s^2 2p^6 3s^2 3p^2}$.

21.b)

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

An element with $3p^3$ as its outermost p-orbital configuration is in Period 3, Group 15 of the periodic table. This element is Phosphorus (P).

Phosphorus (P) has an atomic number (Z) of 15, meaning it has 15 electrons.
Its electron configuration is $1s^2 2s^2 2p^6 3s^2 3p^3$.
The element used for doping is Phosphorus (P), and its s, p configuration is $\boxed{1s^2 2s^2 2p^6 3s^2 3p^3}$.