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Chemistry Unit 1: Atomic structure and the periodic table

Atoms are made of protons, neutrons and electrons. Their arrangement explains the periodic table and its trends.

Covers: Protons, neutrons, electrons, isotopes, electron configuration, periodic trends, the mole

Study guide

Atoms are made of protons (positive) and neutrons (neutral) in a dense nucleus, surrounded by electrons (negative). The atomic number is the number of protons and identifies the element; the mass number is protons plus neutrons. Isotopes of an element have the same number of protons but different numbers of neutrons, like carbon-12 and carbon-14. The average atomic mass on the periodic table is a weighted average of the isotopes' masses. Ions form when atoms gain electrons (negative anions) or lose them (positive cations).

Electrons occupy energy levels and orbitals. Electron configuration lists how they fill, in order of increasing energy: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ and so on. Valence electrons, in the outermost level, determine chemical behavior, and elements in the same group (column) have similar valence configurations and properties. When excited electrons fall back to lower energy levels, they emit light of specific wavelengths, giving each element a unique emission spectrum.

The periodic table shows trends. Atomic radius decreases across a period (more protons pull electrons closer) and increases down a group (more energy levels). Ionization energy and electronegativity show the opposite trend: they increase across a period and decrease down a group. Fluorine is the most electronegative element. Metals on the left lose electrons easily; nonmetals on the right gain them.

The mole counts particles: 1 mol = 6.022 × 10²³ particles (Avogadro's number). The molar mass of an element in grams equals its atomic mass, linking mass to number of particles.

Periodic trends come from Coulomb's law: the attraction between the nucleus and an electron grows with nuclear charge and shrinks with distance. Inner electrons shield outer ones, so across a period the effective nuclear charge rises and atoms shrink. A cation is smaller than its neutral atom because it has lost its outer shell, and an anion is larger because the extra electrons repel each other. A mass spectrum has one peak for each isotope: its position on the x-axis gives the isotope's mass, and its height gives the relative abundance. The average atomic mass is the abundance-weighted average of the peak masses. Isotopes differ in mass because they have different numbers of neutrons.

In Rutherford's gold-foil experiment, most alpha particles passed straight through the foil, but a few bounced back. This showed that an atom is mostly empty space with a tiny, dense, positive nucleus. Bohr then proposed that electrons occupy fixed energy levels, and the modern model describes orbitals as regions where an electron is likely to be found. The energy of a photon is E = hν, and its speed is c = λν. Higher-frequency (shorter-wavelength) light carries more energy. Each line in an emission spectrum matches the energy gap between two electron energy levels.

Key ideas

• Atomic number = protons. Mass number = protons + neutrons.
• Isotopes have the same protons but different neutrons.
• Electrons fill orbitals in order: 1s 2s 2p 3s 3p 4s 3d…
• Atomic radius decreases across a period and increases down a group.
• Ionization energy and electronegativity increase across a period and decrease down a group.

Formulas and rules

Common mistakes

• Thinking atoms get bigger across a period. They shrink (more protons pull harder).
• Forgetting that losing electrons makes a positive ion.

Words to know

Atomic number

The number of protons in an atom.

Isotopes

Atoms of the same element with different numbers of neutrons.

Atomic radius trend

Decreases across a period, increases down a group.

Electronegativity

An atom's pull on shared electrons; fluorine is highest.

Avogadro's number

6.022 × 10²³

Mass number

Protons + neutrons.

Average atomic mass

The weighted average of an element's isotope masses.

Cation / anion

A positive ion (lost electrons) / a negative ion (gained electrons).

Electron configuration

How electrons fill orbitals, like 1s² 2s² 2p⁶.

Valence electrons

Outermost electrons that determine chemical behavior.

Group (periodic table)

A column; elements with similar properties.

Ionization energy

Energy to remove an electron; increases across a period.

Emission spectrum

Light given off as excited electrons fall to lower levels; unique to each element.

Mole

6.022 × 10²³ particles.

Sample practice questions

15 more questions like these, from level 1 to 5, are in the practice set.