Atomic Theory

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Atom Noun

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[at-uhm]

Back

Atom

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The smallest particle of an element that retains the chemical properties of that element, consisting of a nucleus and electrons.

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Example: This diagram shows an atom has a central nucleus with protons and neutrons, and electrons that orbit the nucleus in different shells.

Front

Element Noun

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[el-uh-muhnt]

Back

Element

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A pure substance consisting of only one type of atom, which is distinguished by its unique, identifying atomic number.

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Example: This diagram shows a single atom of the element Sodium, with its electrons arranged in shells, representing the basic unit of an element.

Front

Nucleus Noun

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[noo-klee-uhs]

Back

Nucleus

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The small, dense, positively charged center of an atom, containing protons and neutrons and nearly all the atom's mass.

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Example: This diagram shows a cutaway view of the cell's nucleus, labeling its key parts like the chromatin (which holds DNA) and the nucleolus.

Front

Proton Noun

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[proh-ton]

Back

Proton

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A subatomic particle with a positive electric charge found in the nucleus of an atom; its count defines the element.

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Example: This diagram shows a proton (green sphere) is a particle found inside the central nucleus of an atom, along with neutrons (blue spheres).

Front

Neutron Noun

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[noo-tron]

Back

Neutron

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A subatomic particle with no electric charge (neutral) found in the nucleus of an atom, contributing to its mass.

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Example: This diagram shows a neutron as a particle located inside the atom's central nucleus, grouped with protons, while electrons orbit the outside.

Front

Electron Noun

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[ih-lek-tron]

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Electron

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A subatomic particle with a negative electric charge that orbits the nucleus in a region called the electron cloud.

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

Front

Electron Cloud Noun

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[ih-lek-tron kloud]

Back

Electron Cloud

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The region of space surrounding an atom's nucleus where there is a high probability of finding its electrons.

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Example: This image shows an atom's nucleus at the center and the surrounding electron cloud, a region where electrons are most likely to be found.

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 uniquely identifies a chemical element on the periodic table.

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Example: This diagram shows an atom's nucleus, which contains protons (green). The atomic number is the count of protons in the nucleus; this atom's is 6.

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 an atom's nucleus, representing the approximate mass of the atom.

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Example: This diagram shows the mass number is the total number of protons and neutrons, using both a general formula and a Carbon atom as an example.

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 a different number of neutrons.

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Example: This image shows three versions of a hydrogen atom, called isotopes. Each has one proton, but a different number of neutrons (zero, one, or two).