Matter and Its Structure
What to know
This domain tests the particle-level model of matter: classification of matter (elements, compounds, homogeneous and heterogeneous mixtures), atomic structure (protons, neutrons, electrons, isotopes, mass number, average atomic mass), electron configuration and the periodic trends that follow from it, and nuclear behavior (alpha/beta/gamma decay, half-life).
You should be able to move fluidly between macroscopic properties and submicroscopic explanation: why isotopes of an element have identical chemistry, why atomic radius decreases across a period but increases down a group, why ionization energy rises left-to-right. Know the difference between mass number (integer, isotope-specific) and atomic mass (weighted average, non-integer). Know that periods are rows (principal quantum number) and groups are columns (valence count).
Traps
- Confusing atomic number with mass number when counting neutrons: neutrons = mass number − atomic number.
- Forgetting that average atomic mass is a weighted average, so the most abundant isotope dominates.
- Mixing up the trends: electronegativity and ionization energy increase up and to the right; atomic radius increases down and to the left.
- In nuclear equations, balancing mass numbers and charges separately; a beta particle is an electron (mass 0, charge −1), so beta decay raises Z by 1.
- Treating a homogeneous mixture (salt water, air, alloys) as a compound.
Worked rule
Average atomic mass: for isotopes 35 (75%) and 37 (25%), average = 0.75(35) + 0.25(37) = 35.5. Always multiply each mass by its fractional abundance, then sum.
Half-life rule of thumb: after n half-lives, fraction remaining = (1/2)^n. After 3 half-lives, 1/8 remains.