CHM 25500 Final: Organic Chemistry_Final Exam Study Guide [Helped Me Get An A!]

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CHM 255 - Lecture 1 - Initial Review
Electron Configurations
Atoms consist of a nucleus surrounded by electrons
Electrons exist in shells (can also be thought of as levels)
○ 2n2 electrons in each shell
Electrons move around the nucleus in orbitals within the shells
Shell number: Number of electrons in shell: Orbitals in the shell:
1 2 1 ‘s’ orbital
2 8 4 total; 1 ‘s’ orbital, 3 ‘p’
orbitals
3 18 9 total; 1 ‘s’ orbital, 3 ‘p’
orbitals, 5 ‘d’ orbitals
Atoms can have more shells than you would expect, if the electrons in the atom are in an
excited state, they will occupy higher energy levels (shells) than in the ground state
Energy levels increase with distance from the nucleus
This is the result of the attractive force between the positive nucleus and negative electrons
Valence shell: the highest shell/energy level containing electrons
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Rules For Filling Electron Shells
The Aufbau Principle
Electronic orbitals fill in order of lowest energy to highest energy
Each increasing shell/energy level is of higher energy than the last
p orbitals are of higher energy than s orbitals
The Pauli Exclusion Principle
A maximum of two electrons can occupy an orbital
When two electrons occupy the same orbital, they must have opposite spins (paired spins)
Hund’s Rule
When there are multiple orbitals of equal energy available, and not enough electrons to
completely fill all of them, one electron is added to each before a second electron is added to
any of the orbitals
CHM 255 - Lecture 2 - Initial Review Continued
Valence Electrons
Valence electrons = electrons in the outermost shell
Valence electrons are responsible for bond formation between atoms
Element Electronic Configuration Number of Valence Electrons
H 1s11
He 1s22
Li 1s2 2s11
Be 1s2 2s22
B 1s2 2s2 2p13
O [He] 2s2 2p46
s and p Orbitals
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Orbitals represent the probability of finding an electron in a particular region
Shape of s orbitals:
Shape of p orbitals:
Electronegativity
Electronegativity: the level of attraction an atom has for its electrons
The most electronegative atom is fluorine
4.0 - arbitrary electronegativity value assigned to fluorine
Electronegativity increases as the number of protons on the atom increases (increasing positive
charge)
Electronegativity increases as the distance between valence electrons and the nucleus
decreases (decreasing shielding effect)
Fluorine has the most protons with the least shielding, therefore it is the most electronegative
Chemical Bonds
Octet rule: atoms seek to achieve a set of 8 valence electrons
Atoms can either transfer electrons (ionic bonding) or share electrons (covalent bonding) to
achieve a complete set
Ionic bonding results when an atom such as a group 6 atom (7 valence electrons, highly
electronegative) takes an electron from an atom such as a group 1 atom (1 valence electron,
not as electronegative)
When the group 6 atom gains an electron, it then has 8 valence electrons, achieving the octet
When the group 1 atom loses an electron, it has 8 valence electrons in the next shell down, and
has a full octet in what is now its outer shell
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