Chapter 2 — Elements, Atoms, and the Periodic Table

Vocabulary flashcards covering key terms and definitions from Chapter 2: Elements, Atoms, and the Periodic Table.

Chemical Element

A substance that cannot be broken down into simpler chemical substances.

Chemical Symbol

One- or two-letter abbreviation used to represent a chemical element (e.g., Na for sodium).

Atomic Number (Z)

The number of protons in the nucleus of an atom; defines the element.

Mass Number (A)

The total number of protons plus neutrons in an atomic nucleus.

Isotope

Atoms of the same element that contain different numbers of neutrons and therefore have different mass numbers.

Atomic Mass Unit (u or amu)

A mass equal to 1⁄12 of the mass of a carbon-12 atom (1 u ≈ 1.661 × 10⁻²⁴ g).

Atomic Mass

Weighted average mass of all naturally occurring isotopes of an element, expressed in atomic mass units.

Modern Atomic Theory

The concept that all matter is composed of atoms, each retaining the identity of an element.

Atom

The smallest particle of an element that retains the chemical properties of that element.

Subatomic Particle

Particle smaller than an atom; the three main types are protons, neutrons, and electrons.

Proton

Positively charged subatomic particle located in the nucleus; mass ≈ 1 u.

Neutron

Electrically neutral subatomic particle in the nucleus; mass ≈ 1 u.

Electron

Negatively charged subatomic particle surrounding the nucleus; mass ≈ 0.00055 u.

Nucleus

Dense central region of an atom containing protons and neutrons.

Electron Shell

General energy level around the nucleus where electrons are found.

Subshell

Subdivision of a shell (designated s, p, d, f) that holds a specific maximum number of electrons.

Quantum Mechanics

Modern theory describing electron energies as quantized and organized in shells and subshells.

Electron Configuration

Shorthand notation showing the distribution of electrons among shells and subshells (e.g., 1s²2s²2p⁶).

Valence Electrons

Electrons in the highest-numbered shell of an atom; responsible for chemical behavior.

Core Electrons

Electrons in lower shells that are not involved directly in chemical reactions.

Diatomic Molecule

Molecule composed of two identical atoms (e.g., H₂, O₂, N₂).

Periodic Table

Chart that arranges all known elements by atomic number and recurring chemical properties.

Group (Family)

Vertical column in the periodic table; elements share similar valence-electron configurations.

Period

Horizontal row in the periodic table; elements show a progression of properties across it.

Metal

Element that is shiny, conducts heat and electricity well, and is malleable and ductile.

Nonmetal

Element that is typically dull, brittle, and a poor conductor of heat and electricity.

Semimetal (Metalloid)

Element with properties intermediate between metals and nonmetals.

Alkali Metals

Group 1 elements (Li, Na, K, Rb, Cs, Fr); highly reactive metals with one valence electron.

Alkaline Earth Metals

Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra); reactive metals with two valence electrons.

Halogens

Group 17 elements (F, Cl, Br, I, At); reactive nonmetals that form salts.

Noble Gases

Group 18 elements (He, Ne, Ar, Kr, Xe, Rn); inert gases with filled valence shells.

Transition Metals

Elements in the d-block (groups 3-12) known for variable oxidation states and colored compounds.

Inner Transition Metals

Lanthanide and actinide series; f-block elements located below the main periodic table.

Lanthanide Series

Elements 57-71; first row of inner transition metals.

Actinide Series

Elements 89-103; second row of inner transition metals, many are radioactive.

Atomic Radius

Approximate size of an atom; increases down a group and decreases across a period.

Hydroxyapatite

Calcium-phosphate mineral that makes up tooth enamel and bone.

Fluorapatite

Fluorine-containing mineral formed when enamel reacts with fluoride, more acid-resistant than hydroxyapatite.

Fluoridation

Addition of fluoride (e.g., NaF) to drinking water to reduce tooth decay.

Eutrophication

Over-enrichment of water bodies with nutrients (often phosphorus), leading to algal blooms and oxygen depletion.

Deuterium

Hydrogen isotope with one proton and one neutron (²H).

Tritium

Radioactive hydrogen isotope with one proton and two neutrons (³H).

Alpha Particle

Particle consisting of two protons and two neutrons; emitted during some radioactive decays.

Planetary Model

Rutherford’s atomic model depicting electrons orbiting the nucleus like planets around the sun.

Plum Pudding Model

Early atomic model (Thomson) with electrons embedded in a positively charged “pudding.”

Chemical Symbol Capitalization Rule

First letter uppercase, second letter (if any) lowercase (e.g., Co for cobalt, not CO).

Julius Lothar Meyer

German chemist who organized elements by atomic mass and properties; precursor to the periodic table.

Dmitri Mendeleev

Russian chemist who created a periodic table, predicted properties of undiscovered elements.

Ernest Rutherford

Physicist whose gold-foil experiment led to the nuclear model of the atom.

John Dalton

English scientist who formulated the modern atomic theory in the early 1800s.