How Atoms Differ

Front

Atomic Number Noun

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[uh-tom-ik num-ber]

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Atomic Number

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The number of protons in an atom's nucleus, a unique value that identifies a specific chemical element.

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Example: This diagram shows an atom's nucleus contains protons (green). The atomic number is the total count of these protons, which uniquely identifies the element.

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Proton Noun

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[pro-ton]

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Proton

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A subatomic particle with a positive charge found in the nucleus of an atom, contributing to its atomic number.

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Example: This diagram shows a proton, a positively charged particle, located with other protons and neutrons inside the central nucleus of an atom.

Front

Neutron Noun

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[noo-tron]

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Neutron

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A subatomic particle with no electrical charge, found in the nucleus of an atom and contributing to its mass.

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Example: This diagram shows a neutron as a neutral particle (no charge) located in the central nucleus of an atom, alongside positively charged protons.

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 charge that exists in the space surrounding the nucleus of an atom.

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

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Isotope Noun

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[ahy-suh-tohp]

Back

Isotope

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Atoms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei.

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Example: This image shows three forms of hydrogen, called isotopes. They all have one proton but different numbers of neutrons, which changes their mass number.

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Mass Number Noun

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[mas num-ber]

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Mass Number

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The total number of protons and neutrons in an atom's nucleus, which is used to identify a specific isotope.

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Example: This diagram shows the standard notation for an element, labeling the mass number (top) and explaining it is the sum of protons and neutrons.

Front

Atomic Mass Noun

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[uh-tom-ik mas]

Back

Atomic Mass

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The weighted average mass of the isotopes of an element, based on the abundance of each isotope found in nature.

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Example: This diagram of a periodic table element tile for Carbon shows that the number at the bottom, 12.01, represents the element's average atomic mass.

Front

Atomic Mass Unit (amu) Noun

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[uh-tom-ik mas yoo-nit]

Back

Atomic Mass Unit (amu)

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A unit of mass used to express atomic weights, defined as one-twelfth of the mass of a carbon-12 atom.

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Example: This diagram shows that an atom's mass comes from its protons and neutrons. The atomic mass unit (amu) is defined using Carbon-12, which has 6 protons and 6 neutrons.

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Periodicity Noun

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[peer-ee-uh-dis-i-tee]

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Periodicity

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The tendency of elements to exhibit recurring trends in properties when arranged by increasing atomic number in the periodic table.

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Example: This diagram shows that atom size changes in a predictable pattern across the periodic table, getting smaller across a row and larger down a column.

Front

Nucleus Noun

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[noo-klee-uhs]

Back

Nucleus

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The dense, positively charged central core of an atom, composed of protons and neutrons and containing nearly all its mass.

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Example: This diagram shows the nucleus, the central part of an atom, which contains positively charged protons and neutral neutrons, all surrounded by electrons.