Credit Sprint / Study guides / Chemistry Unit 1

Credit Sprint's Guide to Chemistry Unit 1: Atomic Structure and Properties

Chemistry starts with counting: the mole links masses to numbers of particles. Atoms are built from protons, neutrons and electrons, and the arrangement of electrons, revealed by photoelectron spectra, explains the periodic trends in size, ionization energy and electronegativity.

What you need to know

• One mole is 6.022 × 10²³ particles. Moles = mass ÷ molar mass, and molar mass links grams to counted particles.
• Empirical formulas give the simplest whole-number ratio; divide each element's moles by the smallest, then scale to whole numbers.
• Isotopes have the same protons but different neutrons. Average atomic mass is the abundance-weighted mean of isotope masses, read from a mass spectrum.
• Electron configurations fill 1s, 2s, 2p, 3s, 3p, 4s, 3d … (Aufbau), with Hund's rule and the Pauli principle. Cr and Cu are exceptions (half- and full-filled d).
• Transition metal cations lose 4s electrons before 3d electrons.
• A photoelectron spectrum shows one peak per subshell; the peak position is binding energy and the height is the number of electrons.
• Coulomb's law: force ∝ q₁q₂/r². Greater nuclear charge or shorter distance means stronger attraction.
• Across a period, Zeff rises: radius shrinks, ionization energy and electronegativity increase. Down a group, radius grows and ionization energy falls.
• Dips in first ionization energy occur from Be to B (2p above 2s) and N to O (paired electrons repel).
• A big jump in successive ionization energies shows the number of valence electrons.

Equations and rules

Exam traps

Key terms

• Mole: 6.022 × 10²³ particles. The mass of one mole in grams equals the formula mass in u.
• Molar mass: Mass in grams of one mole of a substance (g/mol).
• Empirical formula: The simplest whole-number ratio of atoms in a compound.
• Isotopes: Atoms with the same number of protons but different numbers of neutrons.
• Average atomic mass: Abundance-weighted mean of isotope masses.
• Aufbau principle: Electrons fill the lowest-energy available subshells first.
• Hund's rule: Electrons occupy separate orbitals of a subshell with parallel spins before pairing.
• Pauli exclusion principle: No two electrons in an atom share all four quantum numbers; an orbital holds at most two electrons with opposite spins.
• Valence electrons: Electrons in the outermost energy level; they take part in bonding.
• Photoelectron spectrum: A plot of binding energy versus number of electrons; each peak is a subshell.
• Coulomb's law: Force between charges is proportional to q₁q₂/r².
• Effective nuclear charge (Zeff): The net pull on a valence electron: nuclear charge minus shielding by core electrons.
• First ionization energy: Energy to remove one electron from a gaseous atom; increases across a period and decreases down a group.
• Electronegativity: An atom's tendency to attract bonding electrons; highest for fluorine.
• Isoelectronic: Having the same number of electrons.

Practice questions

Five of the 55 Unit 1 questions in Credit Sprint. Every choice is explained in the app.