Question

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- Electrons always fill orbitals in the same order. Each s orbital holds 2 electrons, each set of p orbitals holds 6 electrons, each set of d orbitals holds 10 electrons, and each set of f orbitals holds 14 electrons. The order in which orbitals are filled, from first to last, is:

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p

Beryllium has 4 electrons. What is the electron configuration of beryllium?

A.
2s^22p^2
B.
1s^22s^2
C.
2s^4
D.
4p^1

Answer 1

`1s^2\, 2s^2`.

Step-by-step 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 * 1 = 2` electrons (one
`s\!` orbital per shell, with up to two electrons.)

The
`p` orbitals in each shell could hold up to
`2 * 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 * 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`.