Matter and Atomic Structure
Matter and Atomic Structure
This domain tests classification of matter, atomic models, subatomic particles, isotopes, electron configurations, and periodic trends. Expect calculations of average atomic mass and questions that connect position on the periodic table to atomic radius, ionization energy, and electronegativity.
Matter and the atom
- Elements, compounds, and mixtures are distinguished by composition; mixtures are physically separated, compounds chemically decomposed.
- Protons define element identity; neutrons change isotope; electrons change charge. Mass number = protons + neutrons. Atomic mass is a weighted average of isotope masses.
- Models progressed from Dalton to Thomson, Rutherford, Bohr, and quantum mechanical; each explained new evidence and had limits.
Electron configuration and periodicity
- Fill orbitals by Aufbau order, Pauli exclusion, and Hund's rule. Cr and Cu are common exceptions.
- Across a period, atomic radius decreases and ionization energy and electronegativity generally increase; down a group, radius increases and ionization energy decreases.
- Valence electrons determine chemical behavior; ions form to reach noble-gas configurations.
Worked example
Chlorine is 75.78 percent chlorine-35 (34.97 u) and 24.22 percent chlorine-37 (36.97 u). Average atomic mass = 0.7578(34.97) + 0.2422(36.97) = 26.50 + 8.955 = 35.45 u. The answer lies between the isotope masses and closer to the more abundant isotope. Do not simply average 35 and 37 unless abundances are equal.
Traps
Confusing mass number with atomic mass; forgetting that isotopes have the same chemistry but different masses; misapplying Hund's rule; reversing periodic trends; and assigning d electrons before 4s in period 4.