Atoms, Isotopes, and Nuclear Chemistry

Vocabulary flashcards covering the structure of atoms, isotopes, atomic mass, and nuclear chemistry concepts from the lecture notes.

Atom

The basic unit of matter consisting of a nucleus of protons and neutrons surrounded by an electron cloud; electrically neutral when the number of protons equals the number of electrons.

Nucleus

Dense central region of the atom where protons and neutrons reside; mass is concentrated here.

Proton

Positively charged subatomic particle in the nucleus; mass ≈ 1 atomic mass unit (u).

Neutron

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

Electron

Negatively charged subatomic particle surrounding the nucleus; mass is very small and often neglected in atomic mass calculations.

Atomic number (Z)

Number of protons in the nucleus; defines the element.

Mass number (A)

Total number of protons and neutrons in the nucleus; A = Z + N.

Isotopes

Atoms of the same element with different numbers of neutrons and therefore different mass numbers; chemically similar.

Atomic weight

Weighted average mass of an element’s isotopes, relative to 1/12 the mass of carbon-12; typically the element’s average atomic mass.

Atomic mass unit (amu)

1/12 the mass of a carbon-12 atom; 1 u ≈ 1.661 × 10^-24 g.

Carbon-12 standard

Isotope used as the standard for atomic mass; 1 atom of carbon-12 has exactly 12 u.

Charge neutrality

In a neutral atom, the number of protons equals the number of electrons.

Cathode-ray tube

Device used by J. J. Thomson; revealed that cathode rays are negatively charged electrons.

Radioactivity

Spontaneous decay of unstable nuclei with emission of radiation.

Radioisotope

A radioactive isotope.

Alpha particle

Helium-4 nucleus (2 protons and 2 neutrons); positively charged; emitted in alpha decay.

Beta particle

Electron emitted during beta decay; carries negative charge (β−).

Gamma radiation

High-energy electromagnetic radiation (γ); no electric charge and not deflected by electric fields.

Positron

Positive electron; same mass as an electron but opposite charge; emitted in positron emission.

Electron capture

Process where the nucleus captures an orbital electron, converting a proton to a neutron; decreases Z by 1; mass number unchanged.

Nuclear reaction

A reaction that changes the nucleus of an atom; distinct from chemical reactions.

Isotopic abundance

Relative amounts of each isotope in a natural sample; used to calculate the atomic weight.

Hydrogen isotopes

Protium (1H), Deuterium (2H), Tritium (3H); differ by number of neutrons.

Uranium-235

Isotope with Z = 92 and A = 235; symbol U-235; used as an example of a nuclear isotope.

Atomic weight vs mass number

Mass number is an integer (Z+N); atomic weight is the weighted average mass of the element’s isotopes.

Mass of electron

Mass is about 1/2000 of a proton/neutron and is negligible in most atomic mass calculations.

Relative abundance

Fraction of each isotope present in a natural sample, contributing to the atomic weight.