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Why Should You Care About Electrons? An atom’s electrons tell us about how the atom will behave physically and chemically. Kernel- All of an atom’s electrons except for the ones in the outermost energy level Valence- the electrons in the outermost energy level. 2

1. Basic Electron Configuration In a basic configuration, we know how many electrons are in each of the atom’s principal energy levels (PEL’s – aka “shells”). Each PEL has a a specific number of electrons it can fit: PEL 1 – maximum of 2 electrons PEL 2 – maximum of 8 electrons PEL 3 – maximum of 18 electrons PEL 4 – maximum of 32 electrons The basic electron configuration is listed on the periodic table 3

Look, it’s Calcium! 4 How many electrons are in each of calcium’s PEL’s? How many of calcium’s PEL’s are full? How many valence electrons does calcium have? Bohr model of calcium

PEL’s and The Periodic Table As you can probably see, elements in the same row (aka “period”) of the periodic table all have the same number of electron shells. The number of PEL’s increases by one as you move to higher (lower, really) periods of the table. Q: What do all of the members of a column (aka “group”) of the periodic table have in common? A: They all have the same number of valence electrons! 5

2. Expanded Electron Configuration Each PEL has sublevel’s. The expanded electron configuration tells you the PEL’s and sublevels of an atom that are filled, and how many electrons are in each sublevel. Sub-levels are designated as s,p,d, and f. We use superscript numbers to denote how many electrons are in a particular sublevel. 6

Writing Expanded Configuration Lithium has the basic configuration of 2-1 Its expanded configuration is 1s 2 2s 1 Nitrogen has the basic configuration of 2-5 Its expanded configuration is 1s 2 2s 2 2p 3 Magnesium has the basic configuration of It’s expanded configuration is 1s 2 2s 2 2p 6 3s 2 Chlorine has the basic configuration of It’s expanded configuration is 1s 2 2s 2 2p 6 3s 2 3p 5 7

Filling can get crazy The basic rule for filling is that electrons will always go in the lowest energy sublevel possible. The only problem with this, is that some sublevels are at a higher energy than other sublevels of higher shells: – The d sublevel of any shell that has one is at a higher energy than the s sublevel of the next shell. – The f sublevel of any shell that has one is at a higher energy than the s and p sublevels of the next shell 8

Damn, that’s confusing I know, and I’m sorry. The German’s who figured this stuff out came up with a filling order diagram to help us keep it all straight. It’s called the “Aufbau”: Follow the arrows! You can always make your own!!! 9

Use the Aufbau What is the electron configuration of Potassium? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 What is the electron configuration of Iron? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 What is the electron configuration of Bromine? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5 What is the electron configuration of Gold? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 4f 14 5d 9 10

There has to be a lazier way! Of course there is. When the electron configuration get’s too funky, we can say that our atom has the configuration of the noble gas (group 18) atom in the previous period by putting that atom’s symbol in square brackets, and then list the remaining expanded notation following that gas. e.g. Gold: [Xe] 6s 2 4f 14 5d 9 11

3. Orbital (Box) Diagrams In a box diagram we show how many electrons are in each orbital of the sublevel and what the spin of the electron is. Each sublevel has a different number of orbitals: s has 1 orbital p has 3 orbitals d has 5 orbitals f has 7 orbitals Each orbital can fit a maximum of two electrons. These electrons have opposing spins, which we call “up and “down”. Box notation shows all of this 12

The Pauli Exclusion Principle An orbital can hold 0, 1, or 2 electrons and if there are two electrons in an orbital, they must have opposite spins. 13 Hund’s Rule Electrons in orbitals of the same sub-level will always occupy empty orbitals before they will pair up.

Fun with Box Diagrams Lithium’s Box Diagram: Nitrogen’s Box Diagram: 14

Magnesium’s Box Diagram: Chlorine’s Box Diagram: 15

Why should we care about Boxes? Box notation is useful for helping us see the unpaired electrons in an atom. Unpaired electrons are involved in chemical bonds. Unpaired electrons are always found only in a ground-state atom’s valence. 16

Shells, Sublevels, and Orbitals These are three concepts that are related, but can be confusing. Think about it like this: “If electrons were people, they would live in towns called shells (PEL’s), on streets called sublevels and in houses called orbitals. They prefer to live alone, but will live together if they have to. Two electrons can live in each orbital house, as long as they spin in opposite directions.” Different town’s have different numbers of streets and different streets have different numbers of houses. 17

4. Lewis Dot Diagrams Named after Gilbert Newton Lewis. Only show an atom’s valence electrons, surrounding its chemical symbol. Since the valence electrons are involved in bonding, that’s usually the only part of the electron configuration we care about. 18 Gilbert Newton Lewis (1875 – 1946)

Lewis Diagrams LiBe BC NO FNe 19

More about Valence Electrons The number of valence electrons is very important for most of the rest of chemistry. Atom’s do what they do because of their valence electrons. Every atom is most stable when it has a full valence shell. The maximum number of valence electrons an atom can have is 8 (except Hydrogen and Helium). Once an atom has 8 valence electrons, it has a “stable octet” configuration. Chemical Bonding is the way that atoms achieve a stable octet. 20

21 What now? Any Questions?