ELECTRONS IN ATOMS

Electron Configuration

Steps for determining electron configurations:

1. Identify how many electrons are in the atom.

Example: Carbon has 6 electrons

1. Determine the principal energy level (n) of the atom

n = 2 for carbon (it is in the second period)

1. Determine the number of sublevels.

· In principal energy level 1, there is 1 (1s).

· In principal energy level 2, there are 4 (2s, 2p, 2p, 2p)

· So for carbon we are working with 5 potential sublevels (they might not all be used).

1. Assign electrons to the sublevels following the three rules (Aufbau, Pauli Exclusion, and Hund’s).

· 1s will be filled first, with the maximum of 2 electrons. You still have four electrons left.

· 2s will be filled next, with the maximum of 2 electrons. You still have two electrons left.

· 2p will be filled next, with the maximum of 2 electrons. You don’t have any electrons left now.

1. Write the complete electron configuration.
2. 1s22s22p2

Notice that the superscripts (the electrons in each sublevel) add up to the total number of electrons in the atom. 2 + 2 + 2 = 6

1. To write the abbreviated electron configuration, determine what the previous noble gas is from looking at the periodic table.

The noble gas which comes before carbon is helium.

1. Identify the portion of carbon’s electron configuration that is the same as helium’s electron configuration.

Helium, which has 2 electrons, has an electron configuration of 1s2.

1. Substitute the chemical symbol of the noble gas surrounded by brackets into the original electron configuration to make the abbreviated electron configuration.

[He]2s22p2

Orbital Notation

Follow 3 rules: Aufbau, Hunds , Pauli

• Arrows represent electron (up/down)
• Orbitals are line bellow arrows
• Primary energy level and sub level under orbital (like a fraction)
• Fill in lowest energy to highest energy first (Aufbau principa, diagonal rule)
• Individually fill in orbitals of sub level first before doubling up (Hunds rule)
• If two electors in the same orbital have one arrow point up and the other point down to show difference in spin. ( Pauli's exclusion principal)