Dr. Chirie Sumanasekera Chapter 5: Electrons in Atoms Today we will learn: Review of the Q.M. MODEL OF THE ATOM Principles that govern Electron arrangement inside atoms: Aufbau principle Pauli exclusion principle Hund’s rule Writing Electron configurations Chemistry _ Notes Dr. Chirie Sumanasekera 10 /24/ 2017

Review of the Q.M. MODEL OF THE ATOM

Quantum mechanical model of the Atom: key points Electrons have energy levels (Principal energy levels 1,2,3,4,5…). These are also called Principal Quantum Numbers (n) = 1, 2, 3, 4, 5…. As the numerical value increases, so does the energy of the Principal Energy Level. Each energy level has specified sub levels (orbitals) An orbital = a region in space, around the nucleus, with a 90% probability of finding an electron Regions outside the orbitals show areas with a 10% or less probability of finding an electron Four different types of orbital based on shapes: s, p, d and f s, p, d, and f orbitals represent the 3D-shapes of electron clouds and they all have fuzzy boundaries

Shapes and Numbers of Atomic orbitals: s, p & f p orbitals – Set of three – each holding a maximum of 2 electrons d orbitals – Set of five – each holding a maximum of 2 electrons s orbital – One holding a maximum of 2 electrons f orbitals – Set of Seven – each holding a maximum of 2 electrons * Each orbital, s, p, d or f holds a maximum of 2 electrons

Distribution of orbital-electrons in energy levels

Orbitals become larger as the energy level increases Energy increase Electrons filling order : 1s, 2s 2p, 3s Orbitals arranged in order of increasing energy: 1s, 2s 2p, 3s Orbitals arranged in increasing size: (smallest) 1s <2s <2p < 3s (largest)

Pauli exclusion principle Hund’s rule 2. Principles that govern electron arrangements (configurations) inside atoms: Electron configuration = arrangement of electrons in various orbitals Electrons arrange inside orbitals to make the most stable energy state possible. There are 3 rules that explain electron configurations (* with a few exceptions): Aufbau principle Pauli exclusion principle Hund’s rule

1) Aufbau principle Aufbau principle : Electrons occupy orbitals of lowest energy first. Ex: 1S orbitals fill first, then 2s, 2p and then 3s, 3p, and 3d……. **Note: The number before the orbital = principle quantum/energy level Ex: 2s means the s orbital at principal energy level, n=2 2s2, 2p5 means; n=2, 2S has 2 electrons and 2p has 5 electrons

Aufbau Electron filling order Energy level (principal quantum number) Orbitals Energy level (principal quantum number) s p d f 1 1s 2 2s 2p 3 3s 3p 3d 4 4s 4p 4d 4f 5 5s 5p 5d 5f 6 6s 6p 6d 6f 7 7s 7p 7d 7f Energy increase *Sub-levels (orbitals) with less energy fill with electrons first: *Filling Order: 1s, 2s 2p, 3s 3p, 4s 3d 4p, 5s 4d 5p, 6s 4f 5d 6p ,7s 4f 6d 7p 1s2, 2s2 2p6, 3s2 3p6, 4s2 3d10 4p6, 5s2 4d10 5p6, 6s2 4f14 5d10 6p6,7s2 4f14 6d10 7p6

2) Pauli exclusion principle Electrons have a quantum mechanical property called the “spin.” The spin indicates the direction of the spin (clock-wise or counter-clock-wise). Pauli exclusion principle :For two electrons to occupy the same orbital, they must have opposite spins. (FYI: Think of the spin to be like magnetic poles where like repels and opposites attract) Ex: 1s2 2s2 2p6 Correct spin Incorrect spin 1s2 2s2 2p6

3) Hund’s Rule Hund’s Rule: Every orbital in a sub-level is singly occupied with one electron before any one orbital is doubly occupied. Ex: 1S orbitals fill first, then 2s, then each of the three 2p orbitals fill with one electron before they fill with another to make 2p6, then the electrons fill 3s…. Animation: 1s2 2s2 2p6

3. Writing electron Configurations

Electron configurations of some elements