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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.
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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. • 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? • This arrangement is the ground-state electron configuration. • 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. • 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. • 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. • 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. • The spin can be either: • Clockwise - 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.
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.
Representing Electron Configuration • 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: 1. Ge 2. Mg 3. Ti
Representing Electron Configuration • Electron configuration notation can be used. • The energy level is written first. • 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. • For example, the electron configuration notation for carbon is 1s2 2s2 2p2.
Practice Problems • Use the orbital diagrams already done to write the electron configuration for: • Ge • Mg • Ti
Representing Electron Configuration • Noble-gas notation can also be used. • A bracket around a noble gas symbol is used to represent the inner level electrons. • [He] represents 1s2 • [Ne] represents 1s2 2s2 2p6 • The remaining electrons are represented with electron configuration notation. • Carbon could then be written as [He] 2s2 2p2 • Sodium could be written as [Ne] 3s1
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 determine the chemical properties of an element. • They are easy to identify in electron configuration or noble-gas notation: • S [Ne] 3s2 3p4 or 1s2 2s2 2p6 3s2 3p4 • Sulfur has 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.
Practice Problems • Draw the electron-dot structures for the following: 1. Tin 2. Bromine 3. Rubidium
