Inside the atom
An atom has a tiny, dense nucleus of protons (charge +1) and neutrons (no charge), surrounded by electrons (charge −1) that occupy most of the volume. The atomic number Z is the number of protons and defines the element. The mass number A is protons plus neutrons. A neutral atom has equal protons and electrons; an ion does not. A chloride ion with A = 35 has 17 protons, 18 neutrons and 18 electrons.
Isotopes are atoms of one element with different numbers of neutrons. The atomic mass on the periodic table is a weighted average over the natural isotopes: multiply each isotope mass by its fractional abundance and add.
Electron arrangement
Electrons live in orbitals described by quantum numbers: n (shell, 1, 2, 3 ...), l (subshell shape, 0 to n − 1, labelled s, p, d, f), and m_l (orientation, −l to +l). A shell holds up to 2n^2 electrons. Three rules fill orbitals: the Aufbau principle (lowest energy first), the Pauli exclusion principle (at most two electrons per orbital, with opposite spins), and Hund's rule (spread out in a subshell before pairing up). The filling order runs 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p. Valence electrons are those in the outermost shell and determine bonding; main-group elements in a group share the same number of them.
Periodic trends
- Atomic radius decreases left to right across a period (more nuclear charge pulls the same shell in) and increases down a group (new shells).
- First ionization energy generally increases left to right and decreases down a group. Noble gases are the highest in each period.
- Metals lose electrons to form cations; nonmetals gain electrons to form anions. Main-group ions usually reach a noble gas configuration, so Group 2 gives 2+ and Group 17 gives 1−.
- Species with the same electron count are isoelectronic.
Worked example
A hypothetical element has isotopes of mass 20.00 (60.0 percent), 21.00 (15.0 percent) and 22.00 (25.0 percent). Find its atomic mass.
- Convert percentages to fractions: 0.600, 0.150, 0.250.
- Multiply and add: 0.600 × 20.00 = 12.00; 0.150 × 21.00 = 3.15; 0.250 × 22.00 = 5.50.
- Sum: 12.00 + 3.15 + 5.50 = 20.65.
Notice the answer is not the plain average of 20, 21 and 22 (which is 21.00), because the lightest isotope is the most abundant. Now write the configuration of nitrogen, Z = 7: 1s^2 2s^2 2p^3. The three 2p electrons sit in separate orbitals by Hund's rule, giving five valence electrons.