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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