AP Chem: Unit 1 - Atomic Structures & Properties

What are the 3 main subatomic particles and their charges?

Protons (+1), neutrons (0), electrons (−1).

Where are protons, neutrons, and electrons located?

Protons & neutrons in the nucleus; electrons in orbitals outside nucleus.

Define atomic number (Z).

Number of protons in the nucleus; identifies the element.

Define mass number (A).

Total number of protons + neutrons in an atom.

What is an isotope?

Atoms of the same element (same protons) with different neutrons.

Formula for average atomic mass?

∑(isotope mass × fractional abundance).

Example: An element has 20% isotope X (10 amu) and 80% isotope Y (11 amu). Average atomic mass?

(0.20×10) + (0.80×11) = 10.8 amu.

Why is the periodic table mass rarely a whole number?

It’s a weighted average of isotopes.

Define 1 mole.

6.022×10²³ particles (Avogadro’s number).

What is molar mass?

Mass of 1 mole of a substance, in g/mol (from periodic table).

At STP, 1 mole of any gas occupies what volume?

22.4 L.

Formula for molarity?

M = moles solute ÷ liters solution.

Formula relating energy and frequency?

E = hv

Formula relating wavelength and frequency?

c = λν.

What happens when electrons absorb energy?

They jump to higher energy levels (excited state).

What happens when electrons fall back to lower levels?

They release energy as light (photon).

State the Aufbau principle.

Electrons fill lowest energy orbitals first.

State the Pauli exclusion principle.

Each orbital can hold 2 electrons with opposite spins.

State Hund’s rule.

Place electrons singly in orbitals before pairing.

Electron configuration of Oxygen (8 e^-)?

1s² 2s² 2p⁴.

Noble gas configuration for Oxygen?

[He] 2s² 2p⁴.

Which electrons are valence electrons?

Electrons in the outermost shell (highest n).

Why are valence electrons important?

They determine bonding and reactivity.

What is photoelectron spectroscopy (PES)?

A method to measure electron binding energies by ejecting electrons with light.

What does a peak in a PES spectrum represent?

A group of electrons in a given energy level/orbital.

Define ionization energy.

Energy required to remove an electron from an atom.

Why does ionization energy increase across a period?

More protons (higher nuclear charge) pull electrons closer.

Why does ionization energy decrease down a group?

More shells → increased distance and shielding.

What do “big jumps” in successive ionization energy indicate?

A core electron was removed (new shell).

How does atomic radius change across a period?

Decreases → more protons pull electrons in.

How does atomic radius change down a group?

Increases → more electron shells.

Trend in electronegativity across the table?

Increases up and to the right (F is highest).

Trend in electron affinity across a period?

Generally more negative → atoms want electrons more.

Define shielding.

Inner electrons reduce the pull of the nucleus on outer electrons.

Who discovered the electron with cathode rays?

J.J Thomson

Who discovered the nucleus with the gold foil experiment?

Rutherford

What was Bohr’s atomic model?

Electrons in fixed orbits with quantized energy levels.

Why did Bohr’s model fail for multi electron atoms?

It couldn’t explain electron–electron repulsions.

What is the modern quantum model of the atom?

Electrons exist in orbitals (regions of probability).

State Coulomb’s Law for electrostatic force.

F = (k·q₁·q₂)/r²; attraction ↑ with bigger charges, ↓ with distance.