Electron Configurations

Electron Configurations

1. The arrangement of electrons in an atom is known as the atom's electron configuration

   • because atoms of different elements have different numbers of electrons, a unique electron configuration exists for the atoms of each element

2. Electrons fill in the lowest-energy orbitals first

   • to build up electron configurations for the ground state of any particular atom, first the energy levels of the orbitals are determined. then electrons are added to the orbitals, one by one (according to three rules.)
     • The 1st rule : shows the order in which electrons occupy orbitals. According to the Aufbau principle, an electron occupies the lowest-energy orbital that can receive it.
     • The 2nd rule: reflects the importance of the spin quantum number. According to the Pauli exclusion principle, no two electrons in the same atom can have the same set of four quantum numbers
     • The 3rd rule: requires placing as many unpaired electrons as possible in separate orbitals in the sublevel. In this way, electron-electron repulsion is minimized so that the electron arrangements have the lowest energy possible. According to Hand's rule, orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all electrons in singly occupied orbitals must have the same spin state.

3. ground-state electron configuration is the lowest-energy arrangement of the electrons for each element

4. Orbital Notation

   • In orbital notation, an unoccupied orbital is represented by a line, _____, with the orbital's name written underneath the line. An orbital containing one electron is represented with a line and an arrow pointing upwards. An orbital containing two electrons is represented with a line and two arrows; one pointing upwards and one downwards.
     • The lines are labeled with the principal quantum number and sublevel letter.

5. Electron-Configuration Notation

   • eliminates the lines and arrows of orbital notation. Instead, the number of electrons in a sublevel is shown by adding a superscript to the sublevel designation. Eg. 1s^1. The superscript indicates that one electron is present in it's 1s orbital.

6. No electrons can occupy a higher-energy sublevel until the energy sublevel below it is filled

   • In the first period elements, hydrogen and helium, electrons occupy the orbital of the first main energy level.
   • According to the Aufbau principle, after the 1s orbital is filled, the next electron occupies the s sublevel in the second main energy level. Thus, lithium has a configuration of 1s^2 2s^1.

7. Elements of the Third Period

   • After the outer octet is filled in neon, the next electron enters the s sublevel in the n = 3 main energy level. Thus, atoms of sodium, have the configuration of 1s^2 2s^2 2p^6 3s^1.

8. Elements of the Fourth Period