Question

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Answer 1

Answer:

$1s^2\, 2s^2$.

Explanation:

Electron orbitals in an atom (e.g., $1s$) are denoted with:

• A number, denoting the shell (principal energy level) of this orbital, and
• A letter, denoting the shape of this orbital ($s$, $p$, $d$, etc.)

There are two aspects to consider when finding the electron configuration of an atom:

• The number of electrons that each type of orbitals could hold, and
• The order in which the orbitals are filled.

The $s$ orbital in each shell could hold up to $2 \times 1 = 2$ electrons (one $s\!$ orbital per shell, with up to two electrons.)

The $p$ orbitals in each shell could hold up to $2 \times 3 = 6$ electrons (three $p\!$ orbitals per shell, with up to two electrons in each orbital.)

The $d$ orbitals in each main shell could hold up to $2 \times 5 = 10$ electrons (five $d\!$ orbitals per shell, with up to two electrons in each orbital.)

Refer to the order in which the orbitals are filled (Aufbau principle.)

• The first orbital to be filled would be $1s$ (the $s$ orbital of the first shell,) accommodating up to $2$ electrons.
• The second orbital to be filled would be $2s$ (the $s$ orbital of the second shell,) accommodating up to $2$ electrons.

All four electrons of Beryllium are thus assigned to the $1s$ and $2s$ orbitals. In a ground-state Beryllium atom, orbitals $2p$ and beyond would contain no electrons.

Notation:

• Two electrons in the $1s$ orbital: $1s^{2}$ (the superscript denotes the number of electrons in this orbital (or group of orbitals).)
• Two electrons in the $2s$ orbital: $2s^2$.

Write the non-empty orbitals in the order by which they are filled:

$1s^2\, 2s^2$.