Atomic Structure and the Periodic Table

A comprehensive set of vocabulary flashcards covering key terms and definitions related to atomic structure, isotopes, periodic table organization, periodic trends, electron configuration, and quantum numbers as presented in the lecture notes.

Atom

The smallest unit of an element that retains the element’s chemical properties, composed of protons, neutrons, and electrons.

Dalton’s Atomic Theory

Early 19th-century model stating that elements are made of indivisible atoms that combine in whole-number ratios.

Plum-Pudding Model

J.J. Thomson’s 1897 model portraying the atom as a positive “pudding” studded with negative electrons.

Electron

A subatomic particle with a −1 charge and negligible mass (~1⁄1800 of a proton), found in orbitals around the nucleus.

Proton

A positively charged (+1) subatomic particle in the nucleus that determines an element’s identity.

Neutron

An electrically neutral subatomic particle in the nucleus that contributes to atomic mass and isotope formation.

Nucleus

The dense, positively charged center of an atom containing protons and neutrons.

Atomic Number (Z)

The number of protons in an atom’s nucleus; defines the element and equals the number of electrons in a neutral atom.

Mass Number (A)

The total number of protons and neutrons in a single atom.

Isotope

Atoms of the same element (same Z) that differ in neutron number and thus mass number.

Ion

An atom or group of atoms carrying a net electrical charge due to electron loss or gain.

Cation

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

Anion

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

Relative Atomic Mass

The weighted average mass of an element’s naturally occurring isotopes, expressed in atomic mass units (amu).

Periodic Law

Principle stating that elemental properties recur periodically when elements are arranged by increasing atomic number.

Group (Periodic Table)

A vertical column (1–18) whose elements share the same number of valence electrons and similar chemistry.

Period (Periodic Table)

A horizontal row (1–7) whose elements have the same number of occupied electron shells.

Valence Electrons

Electrons in the outermost shell that participate in chemical bonding.

Metal

Element that tends to lose electrons, is malleable, lustrous, and a good conductor of heat and electricity.

Nonmetal

Element that tends to gain electrons, is brittle or gaseous, and a poor conductor.

Metalloid

Element with properties intermediate between metals and nonmetals, often semiconductive.

Atomic Radius

Half the distance between two nuclei of identical atoms; increases down a group and decreases across a period.

Ionization Energy

Energy required to remove an electron from a neutral atom in the gas phase; increases across a period and decreases down a group.

Electron Affinity

Energy change when an atom gains an electron; generally becomes more negative across a period.

Electronegativity

An atom’s ability to attract shared electrons in a bond; highest for fluorine, increases across a period and decreases down a group.

Metallic Character

Tendency of an element to lose electrons and exhibit metallic properties; increases down a group and decreases across a period.

Non-metallic Character

Tendency to gain electrons and form anions; increases across a period and decreases down a group.

Shielding Effect

Reduction of nuclear attraction on outer electrons caused by inner-shell electron repulsion.

Electron Configuration

Notation showing the distribution of electrons among orbitals (e.g., 1s² 2s² 2p⁶).

Aufbau Principle

Electrons fill orbitals from lowest to highest energy.

Hund’s Rule

Degenerate orbitals are singly occupied before any pairing, and all unpaired electrons have parallel spins.

Pauli Exclusion Principle

No two electrons in the same atom can have the same set of four quantum numbers.

Shell (Principal Energy Level)

Set of orbitals with the same principal quantum number n.

Subshell

Group of orbitals within a shell characterized by the angular momentum quantum number ℓ (s, p, d, f).

Orbital

Region of space where there is a high probability of finding an electron; can hold a maximum of two electrons.

s Orbital

Spherically shaped orbital (ℓ = 0) that can hold 2 electrons.

p Orbital

Dumbbell-shaped orbital (ℓ = 1); three orientations, total of 6 electrons per subshell.

d Orbital

Cloverleaf-shaped orbital (ℓ = 2); five orientations, 10 electrons per subshell.

f Orbital

Complex-shaped orbital (ℓ = 3); seven orientations, 14 electrons per subshell.

Principal Quantum Number (n)

Specifies the energy level and size of an orbital; values are positive integers (1, 2, 3…).

Angular Momentum Quantum Number (ℓ)

Defines the subshell shape; ranges from 0 to n − 1.

Magnetic Quantum Number (mℓ)

Specifies the orientation of an orbital; ranges from −ℓ to +ℓ.

Spin Quantum Number (ms)

Indicates electron spin direction (+½ or −½).

Carbon-14

Radioactive isotope of carbon (t₁⁄₂ = 5730 years) used in radiocarbon dating.

Iodine-131

Radioactive isotope used to diagnose and treat thyroid disorders.

Cobalt-60

Radioisotope employed in radiation therapy for cancer treatment.

Technetium-99m

Metastable isotope widely used in diagnostic nuclear imaging.

Half-life

Time required for half of the nuclei in a radioactive sample to decay.

Energy Level

Discrete allowed energy state of an electron in an atom, associated with a specific n value.

Electron Cloud

Region of space around the nucleus where electrons are likely to be found.

Chemical Bond

Force holding atoms together, arising from electron interactions.

Noble Gas Configuration

Stable electron arrangement ending with a full p subshell (ns² np⁶), characteristic of noble gases.

Radiocarbon Dating

Method of determining the age of organic materials by measuring residual Carbon-14.

Biologically Important Element

Element essential for life processes, such as H, C, N, O, Na, K, Ca, Mg, Cl, P, Fe.

Periodic Trend

Predictable change in a property of elements across periods or down groups due to atomic structure.