Electron Configurations Section 5.3 Electron Configurations

Objectives Apply the Pauli exclusion principle, the Aufbau principle, and Hund’s rule to write electron configurations using orbital diagrams and electron configuration notation. Define valence electrons and draw electron-dot structures representing an atom’s valence electrons. 2

Electron Configurations Energy and stability play an important role in determining how electrons are arranged in atoms. In the atom, electrons and the nucleus interact to make the most stable arrangement possible. Electrons are “arranged” so that the atom is in its lowest possible energy state. 3

Electron Configurations The ways in which electrons are arranged into various orbitals around the nuclei of atoms are called electron configurations. 3 rules or principles define how electrons can be arranged in an atom’s orbitals. 4

Electron Configurations 1. The Aufbau Principle states that electrons occupy the orbitals of lowest possible energy. Each box represents an atomic orbital. The various orbitals for any sublevel of a principal energy level are always of equal energy. Within a principal energy level the s orbital is always the lowest energy sublevel. This diagram is called an orbital or Aufbau diagram. Each box represents an atomic orbital. 5

The Aufbau Principle Since orbitals for any sublevel of a principal energy level have an equal energy status (all 3 p orbitals have equal energy, for example) AND since within an energy level the energy increases from the s to the p’s to the d’s to the f’s, filling the atomic orbitals with electrons proceeds from the bottom to the top in the Aufbau diagram.

The Aufbau Principle Notice in Fig. 18 (pg. 156) in your text, energy levels begin to overlap at about n=4. We cannot, therefore, follow a simple pattern when placing electrons in orbitals. We need to follow an Aufbau diagram!

Electron Configurations 2. The Pauli Exclusion Principle states that a maximum of 2 electrons may occupy a single atomic orbital. To occupy the same orbital, the electrons must have opposite spin. To occupy the same orbital the electrons must have opposite spins. The electron spins must be paired. A vertical arrow indicates an electron and its direction of spin. 8

Electron Configurations 3. Hund’s rule states that single electrons with the same spin must occupy each equal-energy orbital before additional electrons with opposite spins can occupy the same orbital. Carbon has 6 electrons: the first 2 go into the 1s orbital; the next 2 go into the 2s orbital; the last 2 go into separate 2p orbitals. 9

Practice Problems Use an Aufbau diagram to determine electron placement for: 1. nitrogen 2. oxygen 3. fluorine 4. neon

Electron Configurations 11

Electron Configurations A shorthand method for indicating an electron configuration involves 1. writing the energy level and the symbol for every sublevel occupied by an electron; 2. then, indicating the number of electrons occupying that sublevel with a superscript. Examples: Hydrogen: 1s1 Oxygen: 1s2 2s2 2p4 (Note: the sum of the superscripts equals the number of electrons in the atom.) Just like your home address consists of a state, a city and a street name within the city, the “address” of an electron consists of its principal energy level, its sublevel and its orbital within that sublevel. 12

Electron Configurations An even shorter method for writing electron configuration involves noble gases. Noble gas notation uses noble gas symbols in brackets to shorten inner electron configurations of other elements.

Electron Configurations Write the electron configurations and noble-gas configurations for each atom. 1. hydrogen 2. lithium 3. boron 4. nitrogen 5. fluorine Carbon: 1s2 2s2 2p2 Argon: 1s2 2s2 2p6 3s2 3p6 Nickel: 1s2 2s2 2p6 3s2 3p6 3d8 4s2 14

Exceptions Chromium and copper are 2 examples of exceptions to the Aufbau Principle. These exceptions are due to subtle electron-electron interactions in orbitals with similar energies. Cr 1s2 2s2 2p6 3s2 3p6 4s1 3d5 Cu 1s2 2s2 2p6 3s2 3p6 4s1 3d10

Valence Electrons Valence electrons are defined as electrons in the atom’s outermost orbitals. These are the electrons in the atom’s highest principal energy level. Valence electrons DETERMINE THE PROPERTIES of an element. They are easy to identify in electron configuration : S 1s2 2s2 2p6 3s2 3p4 There are 6 valence electrons, identified as 3s2 3p4

Electron-dot (or Lewis) Structures Since valence electrons are involved in bond formation, scientist use a visual shorthand to represent them. The element’s symbol is written. It represents the nucleus and all inner-level electrons of the atom. Dots are drawn to represent the valence electrons. Proper placement of dots is important. They are placed 1 at a time on the 4 “sides” of the symbol and then they are paired up until all are used.

Electron-dot (or Lewis) Structures

Practice Problems Draw the electron-dot structures for the following: 1. Phosphorus 2. Argon 3. Nickel 4. Bromine