Đáp án + giải thích các bước giải:
`a.` $^{40}_{20}Ca$
`Z=20: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2`
\begin{array}{|c|c|c|}\hline \text{Lớp}&\text{n}&\text{Phân lớp}\\\hline \text{K}&\text{1}&\text{}1s^2\\\hline \text{L}&\text{2}&\text{}2s^2 2p^6\\\hline \text{M}&\text{3}&\text{}3s^2 3p^6\\\hline \text{N}&\text{4}&\text{}4s^2\\\hline\end{array}
`b.` $^{27}_{13}Al$
`Z=13:1s^2 2s^2 2p^6 3s^2 3p^1`
\begin{array}{|c|c|c|}\hline \text{Lớp}&\text{n}&\text{Phân lớp}\\\hline \text{K}&\text{1}&\text{}1s^2\\\hline \text{L}&\text{2}&\text{}2s^2 2p^6\\\hline\text{M}&\text{3}&\text{}3s^2 3p^1\\\hline\end{array}
`c.` $^{56}_{26}Fe$
`Z=26: 1s^2 2s^2 2p^6 3s^2 3p^6 3d^6 4s^2`
\begin{array}{|c|c|c|}\hline \text{Lớp}&\text{n}&\text{Phân lớp}\\\hline \text{K}&\text{1}&\text{}1s^2\\\hline \text{L}&\text{2}&\text{}2s^2 2p^6\\\hline \text{M}&\text{3}&\text{}3s^2 3p^6 3d^6\\\hline \text{N}&\text{4}&\text{}4s^2\\\hline\end{array}