The Nature of Matter

Front

Atom Noun

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[at-uhm]

Back

Atom

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The basic unit of a chemical element, which is the smallest fragment of a substance that retains its properties.

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Example: This diagram shows an atom's structure: a central nucleus with protons and neutrons, and electrons orbiting the nucleus in distinct energy shells or levels.

Front

Proton Noun

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[proh-ton]

Back

Proton

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A stable subatomic particle with a positive electric charge found in the nucleus of every atom.

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Example: This diagram shows a proton as a particle located inside the nucleus of an atom, along with neutrons, while electrons orbit the nucleus.

Front

Neutron Noun

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[noo-tron]

Back

Neutron

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A subatomic particle of about the same mass as a proton but without an electric charge, present in all atomic nuclei.

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Example: This diagram shows a neutron (blue sphere) as a particle located inside the atom's central nucleus, alongside protons (green spheres).

Front

Electron Noun

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[ih-lek-tron]

Back

Electron

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A stable subatomic particle with a negative electric charge, found in all atoms in the space surrounding the nucleus.

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Example: This diagram shows an electron as a small particle that orbits the central nucleus of an atom, which also contains protons and neutrons.

Front

Nucleus Noun

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[noo-klee-uhs]

Back

Nucleus

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The dense, central region of an atom, consisting of protons and neutrons and containing nearly all its mass.

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Example: This diagram shows a cell's nucleus, which contains the genetic material (chromatin) and is surrounded by a protective nuclear membrane with pores.

Front

Element Noun

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[el-uh-muhnt]

Back

Element

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A pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei.

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Example: This diagram shows a single atom of the element Sodium. All elements are made of only one type of atom, each with a unique structure.

Front

Atomic Number Noun

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[uh-tom-ik nuhm-ber]

Back

Atomic Number

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The number of protons in the nucleus of an atom, which determines the chemical properties and identity of an element.

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Example: This diagram shows the standard notation for an element, using Carbon as an example. The atomic number, 6, is the number of protons.

Front

Isotope Noun

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[ahy-suh-tohp]

Back

Isotope

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Atoms of the same element that have the same number of protons but differ in the number of neutrons.

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Example: Isotopes are atoms of the same element with different numbers of neutrons. The diagram shows Carbon isotopes, each with 6 protons but a different neutron count.

Front

Mass Number Noun

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[mas nuhm-ber]

Back

Mass Number

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The total number of protons and neutrons in the nucleus of a single atom, representing its approximate mass.

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Example: This diagram shows the standard notation for an element (Carbon), where the mass number (12) is the sum of protons and neutrons.

Front

Radioactive Isotope Noun

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[rey-dee-oh-ak-tiv ahy-suh-tohp]

Back

Radioactive Isotope

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An isotope with an unstable nucleus that breaks down at a constant rate over time, emitting radiation.

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Example: This graph shows radioactive decay. As time passes (measured in half-lives), the amount of the radioactive substance decreases by half in each interval.