Electron Configurations Section 5.3

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

Electron configuration: the arrangement of electrons in an atom Electrons tend to assume an arrangement that gives the atom the lowest possible energy. Why? Electrons tend to assume an arrangement that gives the atom the lowest possible energy. Why? This arrangement is the ground-state electron configuration. This arrangement is the ground-state electron configuration. Three (3) rules define how electrons can be arranged in an atom’s orbitals. Three (3) rules define how electrons can be arranged in an atom’s orbitals.

Rule #1 The Aufbau Principle states that each electron occupies the lowest energy orbital available. The Aufbau Principle states that each electron occupies the lowest energy orbital available. An aufbau diagram show the sequence of orbitals from lowest to highest energy. An aufbau diagram show the sequence of orbitals from lowest to highest energy.

Features of the Aufbau Diagram Each box or circle represents an orbital. Each box or circle represents an orbital. All orbitals in the same sublevel have equal energy values. All orbitals in the same sublevel have equal energy values. Sublevels within an energy level have different energies - in order of increasing energy are s, p, d, & f. Sublevels within an energy level have different energies - in order of increasing energy are s, p, d, & f. Orbitals of one energy level CAN overlap with orbitals of another energy level - 4s has a lower energy than any orbital of 3d. Orbitals of one energy level CAN overlap with orbitals of another energy level - 4s has a lower energy than any orbital of 3d.

Electron Spin Electrons behave as though they were spinning on their own axis. Electrons behave as though they were spinning on their own axis. The spin can be either: The spin can be either: Clockwise - represented by Clockwise - represented by Counterclockwise - represented by Counterclockwise - represented by

Rule #2 The Pauli Exclusion Principle States that a maximum of 2 electrons may occupy a single atomic orbital, but only if the electrons have opposite spins. The Pauli Exclusion Principle States that a maximum of 2 electrons may occupy a single atomic orbital, but only if the electrons have opposite spins.

Rule #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. 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.

Representing Electron Configuration Aufbau or orbital diagrams can be used. Aufbau or orbital diagrams can be used. Carbon C 1s 2s 2p

Practice Problems Use an orbital diagram to represent the electron configurations for the following atoms: Use an orbital diagram to represent the electron configurations for the following atoms: 1. Ge 1. Ge 2. Mg 2. Mg 3. Ti 3. Ti

Representing Electron Configuration Electron configuration notation can be used. Electron configuration notation can be used. The energy level is written first. The energy level is written first. The sublevel is written next to the energy level. The sublevel is written next to the energy level. A superscript is used to represent the number of electrons in all the orbitals of the sublevel. A superscript is used to represent the number of electrons in all the orbitals of the sublevel. For example, the electron configuration notation for carbon is 1s 2 2s 2 2p 2. For example, the electron configuration notation for carbon is 1s 2 2s 2 2p 2.

Practice Problems Use the orbital diagrams already done to write the electron configuration for: Use the orbital diagrams already done to write the electron configuration for: 1. Ge 2. Mg 3. Ti

Representing Electron Configuration Noble-gas notation can also be used. Noble-gas notation can also be used. A bracket around a noble gas symbol is used to represent the inner level electrons. A bracket around a noble gas symbol is used to represent the inner level electrons. [He] represents 1s 2 [He] represents 1s 2 [Ne] represents 1s 2 2s 2 2p 6 [Ne] represents 1s 2 2s 2 2p 6 The remaining electrons are represented with electron configuration notation. The remaining electrons are represented with electron configuration notation. Carbon could then be written as [He] 2s 2 2p 2 Carbon could then be written as [He] 2s 2 2p 2 Sodium could be written as [Ne] 3s 1 Sodium could be written as [Ne] 3s 1

Valence electrons Valence electrons are the electrons in an atom’s outermost orbitals. In other words, they are the electrons in the highest principal energy level. Valence electrons are the electrons in an atom’s outermost orbitals. In other words, they are the electrons in the highest principal energy level. Valence electrons determine the chemical properties of an element. Valence electrons determine the chemical properties of an element. They are easy to identify in electron configuration or noble -gas notation: They are easy to identify in electron configuration or noble -gas notation: S [Ne] 3s 2 3p 4 or 1s 2 2s 2 2p 6 3s 2 3p 4 S [Ne] 3s 2 3p 4 or 1s 2 2s 2 2p 6 3s 2 3p 4 Sulfur has 6 valence electrons, identified as Sulfur has 6 valence electrons, identified as 3s 2 3p 4 3s 2 3p 4

Electron-dot (or Lewis) Structures Since valence electrons are involved in bond formation, scientist use a visual shorthand to represent them. 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. 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. 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. 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: Draw the electron-dot structures for the following: 1. Tin 1. Tin 2. Bromine 2. Bromine 3. Rubidium 3. Rubidium