describes relative probabilities of finding electrons in an atom

• each electron "fills"/"occupies" an atomic orbital that describes its distribution in space around the nucleus
• an orbital can be occupied by 0, 1, or 2 electrons. If two electrons occupy the same orbital, they must have opposite signs.

1. 1s orbitals (1s, 2s): spherical and most stable / lowest energy
2. 2s orbitals (2px, 2py, 2pz): dumbbell shape of two lobes (+ and -) separated by a region of zero electron density (node), are perpendicular to one another and equal energy (degenerate)

1s < 2s < 2px = 2py = 2pz

lowest energy arrangement

1. Lowest energy orbitals fill first (1s -> 2s -> 2p)
2. Pauli Exclusion Principle: Max of two spin paired (for stability) electrons can occupy an orbitals w/ one electron up and one down
3. If two or more empty orbitals fo equal energy are available, electrons occupy each one with spins parallel until all orbitals have one electron

- 1st shell: max 2 electrons

- 2nd shell: max 8 electrons (octet)

• Ionic bonding: electrostatic attraction between oppositely charge ions where two atoms have a big electronegativity difference
• Covalent bonding: sharing of electrons where each atom contribute 1 electron for sharing (opposite spin); stronger and more common bond

• Sigma bond: Head-on orbital overlap between s and p hybridized orbitals
• pi bond: side-to-side orbital overlap between unhybridized p orbitals (weaker bond)

No! When two bonded atoms are different, one atom may have greater electronegativity* than the other, resulting in a bond dipole (unequal sharing of electrons).

*accounts for the ability of a certain atom to draw electrons towards themselves (increases left to right and decreases up to down on the periodic table)