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Atomic Structure

Mass Number and Isotopes

  • 3 things to know
  • Papers 1 and 3

Overview

Mass number, atomic number, and how isotopes of the same element differ.

What you need to know

Answer each one before you reveal it.

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Key term

How are mass number, atomic number and neutron count related?

Key term

What is the mass number (A) of an atom?

Key term

What is the atomic number (Z) of an atom?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What is the mass number (A) of an atom?

    Model answer

    The total number of protons and neutrons in the nucleus of an atom.

  2. 2

    What is the atomic number (Z) of an atom?

    Model answer

    The number of protons in the nucleus of an atom.

  3. 3

    How do you determine the number of protons in an atom using atomic number?

    Model answer

    The number of protons equals the atomic number.

  4. 4

    How do you determine the number of neutrons in an atom?

    Model answer

    Number of neutrons = mass number − atomic number (A − Z).

  5. 5

    How do you determine the number of electrons in a neutral atom?

    Model answer

    The number of electrons equals the atomic number (same as the number of protons).

  6. 6

    How do you determine the number of electrons in an ion?

    Model answer

    Number of electrons = atomic number − charge (subtract the charge for positive ions, add for negative ions).

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Atomic Structure
Amount of Substance
Bonding
Energetics and Thermodynamics

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AOxidation is the loss of electrons.
BReduction is the gain of electrons.
CAn oxidising agent is an electron acceptor.

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The sum of all mole fractions equals 1.
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