Atomic structure and isotopes: key terms
6 terms. Say the definition out loud, tap to check, then say whether you knew it. Back to the topic
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Isotopes
Atoms of the same element with different numbers of neutrons and different masses.
Examiners say Same number of protons and electrons, different number of neutrons. "Different mass numbers" alone is not enough when the question asks about atomic structure.
Relative isotopic mass
The mass of an isotope compared with 1/12th of the mass of an atom of carbon-12.
Examiners say The comparison with 1/12th of carbon-12 is the mark; "compared with carbon-12" without the twelfth loses it.
Relative atomic mass
The weighted mean mass of an atom of an element compared with 1/12th of the mass of an atom of carbon-12.
Examiners say Three ideas are credited: weighted mean, of an atom, compared with 1/12th of carbon-12. "Average" without "weighted" is commonly penalised.
Relative molecular mass
The weighted mean mass of a molecule compared with 1/12th of the mass of an atom of carbon-12. Relative formula mass is used for giant structures.
Examiners say Use "relative formula mass" for ionic compounds, which have no molecules.
Mass number
The number of protons plus neutrons in the nucleus of an atom.
Examiners say Atomic number is the number of protons only; it identifies the element.
Ion
An atom or group of atoms with a charge, formed by the loss or gain of electrons.
Examiners say For a positive ion, electrons = protons − charge; for a negative ion, electrons = protons + charge.
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