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UNIT 2 PART 1: ELECTRONS

Electrons in an Atom

•Bohr’s Model: Electrons resided in an “allowed” orbit.

•Quantum Mechanics Model: Probability of finding an electron in an area around the nucleus.

•This area around the nucleus is called an orbital.

Energy of Orbitals•Orbitals can be described using a few important symbold.

•1. n, “principal quantum number” relative size and energy of atomic orbitals.

• Bigger n = bigger orbital.

• The principal quantum number specifies the atom’s major energy levels, which are called principal energy levels.

• The lowest principal energy level is assigned a nof 1.

Energy of Orbitals

•2. Sublevels• Principal energy levels contain energy sublevels.

• Each principal energy level contains a set number of sublevels.

• The sublevels are labeled as s, p, d or f.

• Each sublevel has a different shape.

• Each sublevel is related to a specific number of orbitals.

Energy of Orbitals

•2. Sublevels

• Sublevels have different energies.

• s(lowest energy) < p < d < f (highest energy)

• Generally, electrons prefer to be in the lowest

energy level, sublevel, and orbital as

possible.

3. Orbitals• In each sublevel, there are a certain number of

orbitals.

• Orbitals are where electrons can be found.

• s sublevel: ___ orbital

• p sublevel: ___ orbitals

• d sublevel: ___ orbitals

• f sublevel: ___ orbitals

• Each orbital in the same subshell has the same energy.

Each orbitalcan hold only up to two electrons.

Orbitals• Electrons “occupy” a certain orbital at any given instant.

• When an electron occupies the lowest energy orbitals, the electron is said to be in its ground state.

• Electrons can be excited to a higher-energy orbital by absorption of a photon (= a packet of light) of appropriate energy.

Energy of Orbitals•Summary: Fill out table

Principal energy level

Number of sublevels

Sublevel name

Number of orbitals in each sublevel

Energy of Orbitals•Summary: Fill out table

Principal energy level

Number of sublevels

Sublevel name Number of orbitals in each sublevel

n=1 1 1s 1s: one orbital

n=2 2 2s, 2p 2s: one orbital2p: three orbitals

n=3 3 3s, 3p, 3d 3s: one orbital3p: three orbitals3d: five orbitals

n=4 4 4s, 4p, 4d, 4f 4s: one orbital4p: three orbitals4d: five orbitals4f: seven orbitals

Shells and Subshells• All orbitals in the same main energy level (ie. same n) is said to be in the same electron shell.

• Eg.

• The set of orbitals in the same main energy level (ie. same n) AND in the same sublevel (ie. the same shape of orbital) is called a subshell.

• Eg.

Shapes of Orbitals

•s orbital

Shapes of Orbitals

•p orbitals

Shapes of Orbitals

•d orbitals

Shapes of Orbitals•f orbitals

Orbitals

Electron Configurations•The arrangement of electrons around a nucleus for a particular atom is called the atom’s electron configuration.

•There are three rules you must follow when you write an electron configuration for any atom.

• 1. Aufbau Principle

• 2. Pauli Exclusion Principle

• 3. Hund’s Rule

Electron Configurations• 1. Aufbau Principle:Electrons occupy the lowest energy orbital available first.

• 2. Pauli Exclusion Principle:Only two electrons can occupy any orbital at a time.

• 3. Hund’s rule:Electrons occupy all the unoccupied orbitals of the same energy subshell first, before pairingup and filling the orbitals.

• Eg. Carbon, Nitrogen, Oxygen

Electron Configurations•There are two ways you can write the electron configuration for any atom.

•1. Orbital Diagrameg.

•2. Electron Configuration Notationeg.

Electron Configurations•Noble gas configurations/Condensed electron configuration:

•Shortened form. Noble gas of the closest lower atomic number given in square brackets:

• Eg. NeonSodiumMagnesiumArgonPotassium

Electron Configurations•Textbook pg. 160 Q 21-25

•p.167 Q 77, 85

Valence Electrons

• Recall that electron shells are all orbitals of the same principal quantum number.

• Valence electrons, also called outer-shell electrons, are all electrons in the atom’s outermost shell.

• Eg. SulfurSiliconFluorine

Valence Electrons

• Because valence electrons are involved in chemical reactions, electron-dot structures are drawn, which only show the atom’s valence electrons.

• Dots representing valence electrons are drawn around the chemical symbol for the atom.

• Eg. Draw the dot structures for period 2.Textbook p.162 Q 26-28