Atomic Structure and Periodic Trends

Vocabulary flashcards summarizing key terms and trends in atomic structure, electron configuration, and the periodic table.

Proton

Positively charged (+1) sub-atomic particle located in the atomic nucleus.

Neutron

Electrically neutral sub-atomic particle found in the nucleus.

Electron

Negatively charged (–1) particle that occupies energy levels around the nucleus.

Atomic Number (Z)

Number of protons in the nucleus; uniquely identifies the element.

Mass Number (A)

Total number of protons plus neutrons in an atom.

Isotope

Atoms of the same element (same Z) that differ in mass number due to different numbers of neutrons.

Ion

Atom or molecule with a net electric charge resulting from the gain or loss of electrons.

Cation

Positively charged ion formed when an atom loses one or more electrons.

Anion

Negatively charged ion formed when an atom gains one or more electrons.

Electron Shell

Discrete energy level around the nucleus where electrons reside (1st, 2nd, 3rd, etc.).

1st Electron Shell

Lowest energy level (1s) that can hold a maximum of 2 electrons.

2nd Electron Shell

Second energy level (2s, 2p) that can hold up to 8 electrons.

3rd Electron Shell

Third energy level (3s, 3p, 3d) that can hold up to 18 electrons.

Aufbau Principle

Electrons fill orbitals starting with the lowest available energy before occupying higher levels.

Pauli Exclusion Principle

No two electrons in the same atom can have identical sets of four quantum numbers; each orbital holds max 2 electrons with opposite spins.

Atomic Radius Trend

Increases down a group (more shells) and decreases across a period (greater nuclear pull).

Transition-Metal Radius Exception

Atomic radii of transition metals remain relatively constant across a period due to similar shielding.

Ionization Energy

Energy required to remove an electron from a gaseous atom; increases across a period, decreases down a group.

Ionization-Energy Exceptions

Group 13 elements have lower IE than Group 2; Group 16 lower than Group 15 owing to p-orbital factors and electron repulsion.

Electron Affinity

Energy change when a neutral atom gains an electron; generally becomes more negative across a period and less negative down a group.

Electron-Affinity Exception

Sulfur has a more negative electron affinity than oxygen despite being lower in the group.

Electronegativity

Measure of an atom’s ability to attract electrons in a bond; increases across a period and decreases down a group.

Most Electronegative Element

Fluorine (F) is the highest on the electronegativity scale.

Electronegativity Order (MCAT Tip)

F > O > N ≈ Cl > Br > I > S > C > H (from most to least electronegative).

Electronegativity Exceptions

Some transition metals deviate from the trend (e.g., Cu > Ni); noble gases lack electronegativity values.