Periods 1 to 3: Subshell Electronic Configuration
- 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 (1st energy level) filling
Period 2: L shell (2nd energy level) filling
Period 3: M shell (3rd 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