Question

Select the correct answer. What is the electron configuration of an element with atomic number 15? A. B. C. D. E. 15² 25² 2p6 1s² 2s² 2p 3s² 3p5 1s² 2s²2p6 2p 3s² 3p³ 1s² 25² 2p 3s² 1s² 2s² 2p 3p³ 3s² 3p¹​

Answer 1

`1s^(2)\, 2s^(2)\, 2p^(6)\, 3s^(2)\, 3p^(3)`.

Step-by-step explanation:

In an atom of a given element, the number of electrons should be the same as the atomic number of that element.

In this question, the atom with atomic number
`15` will have
`15\!` electrons.

In electron configurations, the number and the letter (e.g.,
`2p`) denote atomic orbitals. The superscript (e.g., the "
`6`" in
`2p^(6)`) denote the number of electrons in those orbitals.

The
`s` orbital of each shell (
`1s`,
`2s`, etc.) can hold up to
`1 * 2 = 2` electrons. The
`p` orbitals of each shell (e.g.,
`2p`,
`3p`) can hold up to
`3 * 2 = 6` electrons.

The electrons will fill atomic orbitals with the least amount of energy before going to the next (the Aufbau Principle.) The first few orbitals are filled in the order of
`1s`,
`2s`,
`2p`,
`3s`,
`3p`,
`4s`,
`3d`.

For the atom in this question (from orbitals of lower energy to higher energy):

• 
  `1s` orbital is completely filled
  `(2/2)`.
  (There is no
  `p` orbital in the first shell)
• 
  `2s` orbitals are completely filled
  `(2/2)`.
• 
  `2p` orbitals are completely filled
  `(6/6)`.
• 
  `3s` orbitals are completely filled
  `(2/2)`.
• 
  `3p` orbitals are only partially filled
  `(3/6)` since there are exactly
  `2 + (2 + 6) + (2 + 3) = 15` electrons in this atom.

Therefore, the electron configuration of this atom would be
`1s^(2)\, 2s^(2)\, 2p^(6)\, 3s^(2)\, 3p^(3)`.