Writing Electron Configurations
This topic covers how to determine the arrangement of electrons in shells for the first 20 elements (Hydrogen to Calcium) and their simple ions. This is a foundational concept for understanding an element's chemical reactivity and position in the Periodic Table.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- For a neutral atom, the number of electrons is equal to its atomic number (Z).
- Electrons fill shells in order of increasing energy, starting with the shell closest to the nucleus.
- For the first 20 elements, the shell capacities are: the first shell holds up to 2 electrons, the second holds up to 8, and the third also holds up to 8.
- The electron configuration is written as a sequence of numbers separated by commas, representing the electrons in each shell.e.g., Chlorine, Z=17, is 2,8,7
- For ions, first calculate the total number of electrons. For positive ions (cations), subtract the charge from the atomic number. For negative ions (anions), add the magnitude of the charge to the atomic number.
- The sum of the numbers in an electron configuration gives the total number of electrons in the atom or ion.
Diagram
Formulae
Total electrons (ion) = Atomic Number (Z) - charge Use to find the number of electrons in an ion. For example, for Mg²⁺ (Z=12), electrons = 12 - (+2) = 10. For O²⁻ (Z=8), electrons = 8 - (-2) = 10.
Definitions
- Atomic Number (Z)
- The number of protons in an atom's nucleus. In a neutral atom, this is also the number of electrons.
- Electron Configuration
- A representation of how electrons are distributed among the available energy shells in an atom or ion.
- Shell
- A principal energy level surrounding the nucleus that can be occupied by electrons. Shells further from the nucleus are at a higher energy level.
Worked example
What is the electron configuration of a phosphide ion, P³⁻? (The atomic number of phosphorus is 15).
- 1
Identify the number of electrons in a neutral phosphorus atom.
This is equal to its atomic number, so there are 15 electrons.
- 2
Calculate the number of electrons in the P³⁻ ion.
The 3- charge indicates that the atom has gained 3 electrons.
Total electrons = 15 + 3 = 18.
- 3
Distribute these 18 electrons into shells according to the rules.
- 4
The first shell can hold a maximum of 2 electrons.
(18 - 2 = 16 electrons remaining) - 5
The second shell can hold a maximum of 8 electrons.
(16 - 8 = 8 electrons remaining) - 6
The third shell holds the remaining 8 electrons.
- 7
Combine the numbers for each shell in order, separated by commas.
Answer: 2,8,8
Common mistakes
- ×Confusing the charge sign when calculating electrons for ions: remember negative ions have *gained* electrons (add to Z), and positive ions have *lost* electrons (subtract from Z).
- ×Incorrectly filling the third shell for elements 19 (K) and 20 (Ca). The third shell holds a maximum of 8 electrons before the fourth shell begins to fill.
- ×Forgetting to check your work. The sum of the numbers in the configuration must equal the total number of electrons you calculated.
No-calculator tips
- ✓To quickly check your answer for a neutral atom, add up the numbers in your configuration. The sum must equal the atomic number.
- ✓For ions, think of the configuration of the nearest noble gas. For example, a sulfide ion (S²⁻, 16+2=18 electrons) will have the same configuration as Argon (Ar, Z=18), which is 2,8,8.