Electron configurations of atoms

When electrons fill the energy levels, it fills principal energy levels, sublevels, atomic orbitals from lowest energy first.

See that:

1. some 4 sublevel is lower in energy than a 3 sublevel (i.e. 4s is lower in energy than 3d; denoted by * in the energy diagram)
2. some 5 or 6 sublevel is lower in energy than a 4 sublevel (i.e. 5p and 6s are lower in energy than 4f; denoted by arrows in the energy diagram )

At first glance it appears that the sequence for electrons to fill the atomic orbitals are of random order.

Filling order of the sublevels

How do we go about remembering the sequence in which electrons fill the sublevels? The order in which electrons fill the sublevels is easy to remember if you follow these steps:

1. Write the principal energy levels and their sublevels on separate lines.
2. Draw arrows over the sublevels (see the red diagonal lines on the diagram).
3. Join the diagonal lines from end to end (see how the red diagonal lines have been joined).
4. Find the "start here" arrow and follow the arrows. The sublevels are magically arranged in the correct sequence from lowest energy (i.e. compare the order of filling sublevel sequence with the energy diagram of the sublevels).

Notation

There is a way to represent precisely the electron arrangement in atoms. Let's take a look at the simplest atom, hydrogen.

A hydrogen atom has 1 electron. That electron will occupy the lowest principal energy level, n = 1, and the only sublevel, s. We denote the electron configuration of hydrogen as

Similarly,

• Helium has 2 electrons; the 2 electrons both occupy the s sublevel in principal energy level 1.
  • Helium's electron configuration is 1s²
• Lithium has 3 electrons; 2 of the 3 electrons occupy the s sublevel in principal energy level 1. The 3rd electron must go in the next available sublevel, 2s.
  • Lithium's electron configuration is 1s² 2s¹
• Beryllium has 4 electrons; 2 of the 3 electrons occupy the s sublevel in principal energy level 1. The 3rd and 4th electrons must go in the next available sublevel, 2s.
  • Beryllium's electron configuration is 1s² 2s²

Elements

When the electrons are arranged in their lowest energy state, the atom is in the ground state. The following table summarizes the ground state electron configuration of the first 20 elements on the periodic table.

NB: the superscripts add up to the atomic number of the atom.

Name	Atomic Number	Electron Configuration
Period 1
Hydrogen	1	1s1
Helium	2	1s2
Period 2
Lithium	3	1s2 2s1
Beryllium	4	1s2 2s2
Boron	5	1s2 2s22p1
Carbon	6	1s2 2s22p2
Nitrogen	7	1s2 2s22p3
Oxygen	8	1s2 2s22p4
Fluorine	9	1s2 2s22p5
Neon	10	1s2 2s22p6
Period 3
Sodium	11	1s2 2s2 2p6 3s1
Magnesium	12	1s2 2s22p63s2
Aluminum	13	1s2 2s22p63s23p1
Silicon	14	1s2 2s22p63s23p2
Phosphorus	15	1s2 2s22p63s23p3
Sulfur	16	1s2 2s22p63s23p4
Chlorine	17	1s2 2s22p63s23p5
Argon	18	1s2 2s22p63s23p6
Period 4
Potassium	19	1s2 2s22p63s23p64s1
Calcium	20	1s2 2s22p63s23p64s2

Periodic table

The periodic table in terms of electron configurations.

There is a pattern between the electron configuration for the elements and their positions on the periodic table. Compare the electron configuration of an element and its position on the periodic table.

• Elements belonging in Group IA (eg - H, Li, Na, K) all have electron configuration ending in ns¹

  (the superscript of '1' indicates there is 1 valence electron for elements belonging to Group IA).

• Elements belonging in Group IIA (eg - Be, Mg, Ca) all have electron configuration ending in 'ns²

  (the superscript of '2' indicates there are 2 valence electrons for elements belonging to Group IIA).

• Elements belonging in Group IIIA (eg - B, Al) all have electron configuration ending in 'ns²np¹

  (the superscripts total to '3' indicates there are 3 valence electrons for elements belonging to Group IIIA).

• Elements belonging in Group IVA (eg - C, Si) all have electron configuration ending in ns²np²

  (the superscripts total to '4' indicates there are 4 valence electrons for elements belonging to Group IVA).

• Elements belonging in Group VA (eg - N, P) all have electron configuration ending in ns²np³

  (the superscripts total to '5' indicates there are 5 valence electrons for elements belonging to Group VA).

• Elements belonging in Group VIA (eg - O, S) all have electron configuration ending in ns²np⁴

  (the superscripts total to '6' indicates there are 6 valence electrons for elements belonging to Group VIA).

• Elements belonging in Group VIIA (eg - F, Cl) all have electron configuration ending in ns²np⁵

  (the superscripts total to '7' indicates there are 7 valence electrons for elements belonging to Group VIIA).

• Elements belonging in Group VIIIA (eg - He, Ne, Ar) all have electron configuration ending in ns²np⁶

  (the superscripts total to '8' indicates there are 8 valence electrons for elements belonging to Group VIIIA).