Chemistry Key Concepts: Atomic Structure, Quantum Numbers, and Periodic Trends

Number of protons in a neutral atom?

Equal to the atomic number (Z).

Number of neutrons in an atom?

Neutrons = Mass number (A) − Protons (Z).

Number of electrons in a neutral atom?

Equal to the number of protons; adjust for ion charge.

What is a paramagnetic atom?

An atom with unpaired electrons.

Hund's Rule

Electrons occupy degenerate orbitals singly first with parallel spins.

Pauli Exclusion Principle

No two electrons in an atom can have the same four quantum numbers.

Aufbau Principle

Electrons fill orbitals from lowest to highest energy.

How do you calculate average atomic mass?

Weighted average: $ ext{Avg. mass} = ext{sum} ( ext{fractional abundance} imes ext{isotopic mass})$

What does "isoelectronic" mean?

Species with the same number of electrons.

Atomic radius trend across a period?

Decreases from left to right.

Atomic radius trend down a group?

Increases from top to bottom.

Ionization energy trend across a period?

Increases from left to right.

Ionization energy trend down a group?

Decreases from top to bottom.

Cation vs. Anion size?

Cations are smaller; anions are larger than neutral atoms.

How many particles are in 1 mole?

$6.022 imes 10^{23}$ (Avogadro's number).

Mole ↔ Mass conversion?

$n = rac{ ext{mass (g)}}{ ext{molar mass (g/mol)}}$

Mole ↔ Particles conversion?

$ ext{particles} = n imes N_A$

How to find formula/molecular mass?

Sum atomic masses of all atoms in the formula/molecule.

Empirical formula vs. Molecular formula

Empirical = simplest whole-number ratio; Molecular = actual number of atoms.

Energy of a photon formula?

$E = h

u = rac{hc}{ ext{λ}}$

Wavelength ↔ Frequency ↔ Energy relationships?

$ ext{λ} = rac{c}{

u}, ext{ } E = h

u$

Energy of 1 mole of photons?

$E_ ext{mol} = E_ ext{photon} imes N_A$

Quantum numbers?

$n$ = principal; $l$ = subshell; $m_l$ = orbital; $m_s$ = spin.

Forbidden quantum number sets?

$m_l$ cannot exceed ±l; any physically impossible combination.

Common electron configuration rules?

Aufbau, Hund, Pauli Exclusion.

How do you write a neutral ionic compound formula?

Combine ions so total charge = 0.

Common polyatomic ions?

Acetate: C₂H₃O₂⁻, Nitrate: NO₃⁻, Chromate: CrO₄²⁻, Permanganate: MnO₄⁻, Phosphite: PO₃³⁻.

Convert grams → moles → particles?

grams → moles using molar mass, then moles → particles using Avogadro's number.

Convert energy of photon → wavelength?

$ ext{λ} = rac{hc}{E}$

How to find number of atoms in a sample?

grams → moles → multiply by Avogadro's number.