I. Electron Configurations A.Principal energy levels B.Energy sublevels C.Orbitals

A. Principal Energy Levels (n) n = 1 to 7 Row # on the periodic table The electron’s principal energy level is based on its location around the nucleus. Electrons closer to the nucleus are at a lower energy level and have lower energy than those farther away from the nucleus

Energy levels are like rungs of a ladder. You cannot be in between a rung Energy levels in an atom’s electron are unequally spaced. The higher energy levels are closer together.

B. Energy Sublevels (s,p,d,f) assigned letters s, p, d or f (smart people do fine) Energy sublevels correspond to a shape where the electron is likely to be found Sometimes also called “orbitals”

C. Orbitals describes the electron’s location orbitals are not necessarily spherical maximum of 2 electrons per orbital – s sublevel has 1 orbital (2 electrons total) – p sublevel has 3 orbitals (6 electrons total) – d sublevel has 5 orbitals (10 electrons total ) – f sublevel has 7 orbitals (14 electrons total)

s orbital (1) - spherical

p orbitals (3) – dumb-bell shaped

d orbitals (5)

f-orbitals (7)

II. Electron Configuration Rules A. Electron configuration – the arrangement of electrons in an atom. Example Sodium (Na) – 1s 2 2s 2 2p 6 3s 1 B. Three rules determine electron configurations 1. the Aufbau Principle, 2. the Pauli Exclusion Principle 3. Hund’s rule

1. The Aufbau Principle Each electron occupies the lowest energy orbital available Like filling the hotel from the bottom up

Aufbau Diagram Which has a lower energy level 4s or 3d orbitals?

Color in the s,p,d, f orbitals in the Periodic Table

2. Pauli Exclusion Principle A maximum of two electrons may occupy a single orbital Like only two people sharing one bed

3. Hund’s Rule If two or more orbitals of equal energy are available, electrons will occupy them singly with the same spin, before filling them in pairs with opposite spins A spin is denoted with an up  and down  arrow to fill orbitals or ↿⇂ This is like trying to find your own bed in the same suite before having to share a bed with someone else

III. Exceptions to the Rules Copper (Cu) and chromium (Cr) are exceptions to the Aufbau principle Some configurations violate the Aufbau Principle because half-filled sublevels are not as stable as filled sublevels

IV. Valence electrons A. Valence electrons are electrons in the outermost s and p orbitals For group A elements, the group number corresponds to number of valence electrons. B. (Lewis) electron dot structures – element’s symbol surrounded by dots representing the valence electrons 8A

V. Using Orbital Diagrams to make Electron Configurations Aufbau diagram for sodium (Na) which has 11 electrons Na electron configuration1s 2 2s 2 2p 6 3s 1

Question Time What two elements are exceptions to the Aufbau rule for electron configurations? What are valence electrons? Orbital Diagrams