Periods 1 to 3: Subshell Electronic Configuration

Key Concepts

• The electrons in an atom can be arranged in shells (or energy levels).
• The number of the period of the periodic table tells us which shell (energy level) is being filled.
  

  Period 1: K shell (1^st energy level) filling
  Period 2: L shell (2^nd energy level) filling
  Period 3: M shell (3^rd energy level) filling

• Lower energy levels are filled before higher energy levels.
• Each shell (energy level) can be subdivided into subshells (or sublevels):
  

  s subshell: maximum of 2 electrons
  p subshell: maximum of 6 electrons

• The group of the periodic table that an element belongs to tells us which subshell is being filled:
  

  s subshell: Group 1, Group 2, hydrogen and helium
  p subshell: Groups 13, 14, 15, 16, 17, 18

• Within a particular energy level (shell), the s subshell is filled with electrons before the p subshell.
• The electronic configuration (electron configuration) given in subshell notation indicates how many electrons are in the subshell of each shell (in the energy sublevels of each energy level)
  

  Example of a subshell electronic configuration: 1s² 2s² 2p⁴
  2 electrons in the 1s subshell, 2 electrons in the 2s subshell, 4 electrons in the 2p subshell.

• The subshell electronic configuration of an atom can also be given in an abbreviated form known as a "condensed electron configuration" in which the symbol for the preceding noble gas element is placed in square brackets, followed by the subshell notation for electrons in the highest energy levels:
  

  Example of a condensed subshell electronic configuration: [He] 2s² 2p⁴

Electron Subshell Concept

An s subshell can contain a maximum of 2 electrons.
A p subshell can contain a maximum of 6 electrons.

The s subshell of the 1^st energy level (K shell) is referred to as the 1s subshell.

The s subshell of the 2^nd energy level is referred to as the 2s subshell.
The p subshell of the 2^nd energy level is referred to as the 2p subshell. `

The s subshell of the 3^rd energy level is referred to as the 3s subshell.
The p subshell of the 3^rd energy level is referred to as the 3p subshell. `

In order for an atom to be in its lowest energy state, the electrons occupy the lowest energy sublevels (subshells) first, so electrons fill subshells in this order:

1s then 2s then 2p then 3s then 3p

When we write an electron configuration (electronic configuration) for an atom using subshell notation, we need to identify the number of the energy level first, followed by the symbol for the subshell containing the electrons, and then we indicate that number of electrons in that subshell by a superscript number.

Example: Electron (electronic) configuration of a particular atom is 1s² 2s² 2p³
There are 2 electrons in the s subshell of the 1^st energy level. (1s²)
There are 2 electrons in the s subshell of the 2^nd energy level. (2s²)
There are 3 electrons in the p subshell of the 2^nd energy level. (2p³)

The number of electrons in a given shell (energy level) is the total of the number of electrons in the s and p subshells for that energy level.

Example: Electron (electronic) configuration of a particular atom is 1s² 2s² 2p³
There are 2 electrons in the 1^st energy level. (1s²)
There are 5 electrons in the 2^nd energy level.
(2 electrons in the 2s subshell of the second energy level PLUS 3 electrons in the 2p subshell of the 2^nd energy level.)

The total number of electrons in an atom is therefore the sum of all the electrons in all the subshells of all the energy levels (shells):

Example: Electron (electronic) configuration of a particular atom is 1s² 2s² 2p³
There are 7 electrons in total:
2 electrons in the s subshell of the 1^st energy level PLUS 2 electrons in the s subshell of the 2^nd energy level PLUS 3 electrons in the p subshell of the 2^nd energy level

And, for a neutral atom, the total number of electrons equals the number of protons in the nucleus which is given by the atomic number (Z) of the atom.

Example: Electron (electronic) configuration of a particular atom is 1s² 2s² 2p³
There are 7 electrons in total.
The nucleus of the atom must contain 7 protons.
Atomic number of the atom (Z) = 7
This must be an atom of nitrogen because the atomic number of nitrogen is 7.

Period 1 Subshell Electronic Configuration

The first energy level (K shell) has only one subshell (sublevel), the 1s subshell.

Period 2 Subshell Electronic Configuration

Atoms of period 2 elements have the first energy level, 1s², full .
Each subsequent electron begins filling the second energy level.

2s subshell must be filled with 2 electrons before electrons are added to the 2p subshell.

Because all the period 2 elements have the first energy level (K shell) filled, Chemists often used a short-hand notation to indicate this in which the electron configuration of the filled energy level is represented by the symbol of its Noble Gas in square brackets []. This is known as a condensed electron configuration, or, condensed electronic configuration.
All the period 2 elements have an electronic structure which starts with that of the Noble Gas helium, so this is represented as [He]:

Period 3

Atoms of period 3 elements begin filling the 3^rd energy level (M shell).

3s subshell must be filled with 2 electrons before electrons can be added to the 3p subshell.

Note that the electronic configuration of all period 3 elements begins the same, 1s² 2s² 2p⁶ , which is the electronic configuration of neon, so we use this, [Ne], to write a shorthand version of the electronic configuration, the condensed electronic configuration, of each period 3 element:

Summary

period 1: 1^st energy level (K shell) being filled
period 2: 2^nd energy level (L shell) being filled
period 3: 3^rd energy level (M shell) being filled

The Periodic Table can be divided up into s and p blocks showing us which atoms are having their s and p subshells filled.

s block: Group 1, Group 2, hydogen (H) and helium (He)
p block: Groups 13 to 18