Atomic Structure & the Periodic Table

Every atom is made of three subatomic particles. Two are found in the nucleus; one orbits it in shells.

Particle	Symbol	Mass (u)	Charge	Location
Proton	p⁺	≈1	+1	Nucleus
Neutron	n⁰	≈1	0	Nucleus
Electron	e⁻	≈0	−1	Orbitals (shells)

Isotopes

Isotopes are atoms of the same element (same Z) but with different mass numbers (different number of neutrons). They have nearly identical chemical properties because chemistry is driven by electrons, not neutrons.

• Chlorine-35: Z=17, A=35, neutrons=18
• Chlorine-37: Z=17, A=37, neutrons=20
• Same element, same electron count, different mass

The atomic mass listed on the periodic table is the weighted average of all naturally occurring isotopes, not the mass number of any single one.

Electrons occupy shells (energy levels, n = 1, 2, 3…), which are subdivided into subshells (s, p, d, f), which contain orbitals. Each orbital holds at most 2 electrons with opposite spins.

Subshell	Orbitals	Max electrons	Shape
s	1	2	Spherical
p	3	6	Dumbbell (3 orientations)
d	5	10	Complex (5 orientations)
f	7	14	Very complex

The Three Rules

Writing Configurations

The valence electrons are those in the outermost shell. They determine chemical bonding behaviour. For main-group elements, the valence electron count equals the group number.

Mendeleev arranged elements by increasing atomic mass (we now use atomic number) and found that properties repeat periodically. The modern table has 7 periods and 18 groups.

Regions of the Periodic Table

Region	Description	Properties
Metals	Left and centre	Lustrous, conduct heat/electricity, malleable, form cations
Metalloids	Diagonal staircase	Intermediate properties, semiconductors (Si, Ge, As…)
Nonmetals	Upper right	Poor conductors, brittle (if solid), form anions or covalent bonds

Key Families

Family	Group	Valence e⁻	Notes
Alkali metals	1	1	Highly reactive; react violently with water; form +1 ions
Alkaline earth metals	2	2	Reactive metals; form +2 ions; Ca and Mg important biologically
Halogens	17	7	Highly reactive nonmetals; need 1 more e⁻; form −1 ions or covalent bonds
Noble gases	18	8 (or 2 for He)	Full outer shells; chemically inert; almost never form compounds

All periodic trends arise from two competing factors: nuclear charge (the pull of protons on electrons) and electron shielding (inner electrons blocking that pull). Going across a period, nuclear charge wins. Going down a group, shielding wins.

Atoms form ions to achieve a filled outer shell (the same electron configuration as the nearest noble gas). This is why ions are stable.

Common Ion Charges by Group

Group	Typical Ion	Example	Configuration after
1	+1	Na⁺	[Ne] — same as neon
2	+2	Ca²⁺	[Ar] — same as argon
13	+3	Al³⁺	[Ne]
15	−3	N³⁻	[Ne]
16	−2	O²⁻	[Ne]
17	−1	Cl⁻	[Ar]
18	0 (no ion)	Ar	Already full — no need

Isoelectronic Series

Atoms or ions with the same number of electrons are isoelectronic. For example, Na⁺, Mg²⁺, Al³⁺, and Ne all have 10 electrons. However, the one with the most protons has the strongest nuclear pull, so it has the smallest radius: Al³⁺ < Mg²⁺ < Na⁺ < Ne (in order of increasing size).

Mistake	What to do instead
Mass number = atomic mass	Mass number (A) is always a whole number (p + n). Atomic mass on the table is a decimal — it is the weighted average of all isotopes.
Confusing periods and groups	Periods are horizontal rows (same number of shells). Groups are vertical columns (same valence electrons).
Atomic radius increases across a period	Atomic radius decreases across a period because more protons pull electrons in. It increases down a group.
IE and EN increase down a group	Both ionization energy and electronegativity decrease down a group (electrons are farther from nucleus, easier to pull away or remove).
Noble gases don’t form ions because they are special	Noble gases have full outer shells (8 electrons, or 2 for He). There is no energetic benefit to gaining or losing electrons — it would require breaking a stable configuration.
Isotopes have different chemical properties	Isotopes have nearly identical chemical properties because chemistry depends on electrons, not neutrons. Cl-35 and Cl-37 behave the same chemically.