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**Electron Configurations**

Chapter 5 Section 3

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**Vocabulary electron configuration aufbau principle**

Pauli exclusion principle Hund’s rule Valence electron Electron-dot structure

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Review Each principal energy level can have the same number of sublevels as the level number Each sublevel orbital has a different shape Each orbital can have only 2 electrons What does this mean?

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**Electron Arraignment follows Rules**

Low energy level systems are more stable than high-energy systems Atoms will assume the electron arrangement that gives the atom the lowest energy Most stable is the “ground state” (lowest energy) Three rules/principles for arranging electrons Aufbau Pauli Exclusion principle Hund’s Rule

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Aufbau Principle Each electron occupies the lowest energy level available Learn the sequence of atomic orbitals from lowest to highest:

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Aufbau Diagram #4 – also do electron DOT diagram, #7 & 8 – SHOW YOUR WORK

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Aufbau Expanded

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Using Aufbau All orbitals related to the same energy level are of equal energy All 2p orbitals have the same energy In a multi-electron atom, the energy sublevels within a principal energy level have different energies: 2p orbitals are higher energy than 2s orbitals The sequence of sublevels within a principle level in increasing energy is: s, p, d, and f Orbitals related to energy sublevels within one principle energy level can overlap orbitals related to another principal level Notice: 4s is lower than 3d

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**Pauli Exclusion Principle**

Each electron has an associated spin, like a top Can spin on bottom or top Each orbital can hold AT MOST 2 electrons, but only if they have opposite spin Designated as:

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Hund’s Rule Because negatively charged electrons repel each other, they try to get as far away from each other as possible, therefore: Single electrons with the same spin will occupy each equal energy orbital before additional electrons with opposite spins occupy the same orbitals. WHAT?

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Hund’s Rule 2 1 4 3

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**Representing Atom’s Electrons**

Orbital Diagrams Electron Configuration Notations

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**Orbital Diagrams Draw one box for each orbital Example:**

Empty box means no electrons Box with single up arrow means orbital with one electron Box with two arrows (up/down) means orbital with two electrons Each box is labeled with the principle quantum number and the sublevel Example:

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**Example Orbital Diagram - N**

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**Example Orbital Diagram – C & O**

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**Electron Configuration Notation**

Specify principal energy level and energy sublevel Use a superscript to represent the number of electrons Use the Aufbau diagram to help you remember the level/sublevel filling order Examples:

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**Electron Configuration Notation**

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**Electron Configuration Notation**

Element ECN Hydrogen 1s1 Carbon 1s2 2s2 2p2 Helium 1s2 Nitrogen 1s2 2s2 2p3 Lithium 1s2 2s1 Oxygen 1s2 2s2 2p4 Beryllium 1s2 2s2 Fluorine 1s2 2s2 2p5 Boron 1s2 2s2 2p1 neon 1s2 2s2 2p6

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**A Variation – Noble Gas Notation**

Similar to Electron Configuration Notation, but: Start with the previous noble gas, and put that symbol in brackets Add electrons to the noble gas Example: Sodium (Na) = [Ne] 3s1 Ne = 1s2 2s2 2p6 Na = 1s2 2s2 2p6 3s1

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**Exceptions to Aufbau Cr (Chromium) = [Ar] 4s1 3d5**

Cu (Copper) = [Ar] 4s1 3d10

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Valence Electrons Only Valence Electrons contribute to chemical bonding Valence Electrons = electrons in outer most principal energy level Generally highest energy Examples: Carbon: [He] 2s2 2p2 has 4 valence electrons Sulfur: [Ne] 3s2 3p4 has 6 valence electrons

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Electron Dot Diagrams Also called Lewis Electron Dot Diagrams or Lewis Electron Dot Structure or Electron Dot Structure Shows atomic symbol and ONLY valence electrons

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Electron Dot Diagrams

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Periodic Table

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Electron Configuration

Electron Configuration

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