Chapter 2: Atoms and Elements

VOCABULARY flashcards covering key terms from the notes on atoms, subatomic particles, isotopes, ions, and the periodic table.

Electron

Negatively charged subatomic particle with a very small mass; located outside the nucleus in the electron cloud; mass ~0.00091 x 10^-27 kg; ~0.00055 amu; charge −1.

Plum-pudding model

Thomson’s model in which electrons are embedded in a sphere of positive charge.

Nucleus

Small, dense core containing most of the atom’s mass and all of its positive charge.

Alpha particle (α)

Positively charged particle (helium nucleus) used in Rutherford’s experiments.

Beta particle (β)

Negatively charged radiation emitted by unstable nuclei.

Gamma ray (γ)

High-energy electromagnetic radiation emitted by unstable nuclei.

Radioactivity

Emission of small energetic particles from unstable atoms; studied by Becquerel and Curie.

J. J. Thomson

Physicist who proposed the plum-pudding model and discovered the electron.

Rutherford’s Gold Foil Experiment

Experimental test where α particles were directed at a thin gold foil; most passthrough, some deflected, some bounced back.

Nuclear Theory of the Atom

Three-part model: (1) nucleus contains most mass and positive charge; (2) atom mostly empty space with dispersed electrons; (3) equal numbers of protons outside the nucleus and electrons to balance charge.

Proton

Positively charged subatomic particle in the nucleus; mass ~1.00727 amu; charge +1.

Neutron

Electrically neutral subatomic particle in the nucleus; mass ~1.00866 amu; contributes to mass but has no charge.

Atomic number (Z)

Number of protons in the nucleus; defines the element; represented by Z.

Mass number (A)

Total number of protons and neutrons in the nucleus; A = p + n.

Isotopes

Atoms of the same element (same Z) with different numbers of neutrons and thus different mass numbers.

Isotope notation (A X)

Notation where A is the mass number and X is the chemical symbol (e.g., Ne-20).

Atomic mass unit (amu)

1/12 the mass of a carbon-12 atom; approximately the mass of a proton or neutron; electron mass is ~0.00055 amu.

Millikan oil-drop experiment

Experiment that measured the charge of the electron: −1.60 x 10^-19 C; in atomic units the electron has a charge of −1.

Charge neutrality

In a neutral atom, the charges of protons and electrons cancel each other out.

Ion

Atom that has gained or lost electrons and carries a net charge.

Cation

Positively charged ion formed when an atom loses electrons.

Anion

Negatively charged ion formed when an atom gains electrons.

Periodic table

Organization of elements by increasing atomic number with groups (columns) and periods (rows).

Mendeleev’s periodic law

When elements are arranged by increasing mass, certain properties recur periodically.

Modern periodic table

Elements are arranged by increasing atomic number (not mass); more complete than Mendeleev’s table.

Noble gases

Group 18 (8A); very unreactive elements; helium is a familiar example used in balloons.

Alkali metals

Group 1A elements; highly reactive metals (e.g., Li, Na, K, Rb, Cs, Fr).

Alkaline earth metals

Group 2A elements; reactive but less than alkali metals (e.g., Mg, Ca, Sr, Ba).

Halogens

Group 7A elements; highly reactive nonmetals (F, Cl, Br, I).

Metalloids

Elements along the zigzag line between metals and nonmetals; have mixed properties and some semiconducting behavior (e.g., B, Si, Ge, As, Sb, Te, At, Ts).

Metals

Elements typically on the left/middle of the periodic table; good conductors, ductile, malleable; tend to lose electrons.

Nonmetals

Elements on the upper-right of the periodic table; poor conductors; tend to gain electrons.

Main-group elements

Elements whose properties follow predictable trends based on position in the table.

Transition metals

Elements in the center of the table; properties are less predictable based solely on position.

Lanthanides

The f-block of elements from La to Lu; part of the periodic table’s rare-earth series.

Actinides

The f-block elements from Ac to Lr; include many radioactive elements.

Group (family)

Vertical column in the periodic table.

Period

Horizontal row in the periodic table.