Chemistry: Atoms to AP Chemistry · Module 1 of 15
Atomic Structure & Periodic Trends
What atoms are made of, why isotopes exist, and the patterns the periodic table is built around.
Atomic Structure & Subatomic Particles
- Atoms consist of protons (+, in the nucleus), neutrons (neutral, in the nucleus), and electrons (−, in shells around the nucleus).
- Atomic number = number of protons, which defines the element; a neutral atom has equal protons and electrons.
- Mass number = protons + neutrons; nearly all of an atom's mass comes from the nucleus, since electrons have negligible mass.
An atom has 17 protons, 18 neutrons, and 17 electrons. What is its atomic number and mass number?
Solution. Atomic number = number of protons = 17 (this is chlorine). Mass number = protons + neutrons = 17 + 18 = 35.
Isotopes & Average Atomic Mass
- Isotopes are atoms of the same element (same protons) with different numbers of neutrons, giving different mass numbers.
- The atomic mass shown on the periodic table is a weighted average of all naturally occurring isotopes, based on their relative abundance.
Chlorine has two isotopes: Cl-35 (75% abundance, mass ≈35.0) and Cl-37 (25% abundance, mass ≈37.0). Estimate chlorine's average atomic mass.
Solution. (0.75)(35.0) + (0.25)(37.0) = 26.25 + 9.25 = 35.5, close to the periodic table value of about 35.45.
Electron Configuration & the Periodic Table
- Electrons fill energy levels and orbitals in a predictable order, lowest energy first; the periodic table's rows (periods) correspond to energy levels.
- Valence electrons (outermost shell) determine an element's chemical behavior; elements in the same group (column) have the same number of valence electrons and similar properties.
Sodium (Na) has the electron configuration 1s² 2s² 2p⁶ 3s¹. How many valence electrons does it have, and what does this predict about its reactivity?
Solution. It has 1 valence electron (3s¹), consistent with Group 1 (alkali metals); this makes sodium very reactive, since losing that one electron gives it a stable, full outer shell.
Periodic Trends
- Atomic radius decreases across a period (increasing nuclear charge pulls electrons in) and increases down a group (more electron shells).
- Ionization energy (energy to remove an electron) increases across a period and decreases down a group — the opposite trend from atomic radius.
- Electronegativity (an atom's pull on shared electrons) increases across a period and decreases down a group; fluorine is the most electronegative element.
Which has a larger atomic radius: sodium (Na) or chlorine (Cl), both in period 3?
Solution. Sodium — atomic radius decreases moving left to right across a period, and sodium is to the left of chlorine.
Check yourself
2 of this module's 5 quiz questions. Pick your answers, then check them in the app, with an explanation for every question.
1. An atom has 11 protons and 12 neutrons. What is its mass number?
2. What makes two atoms isotopes of the same element?