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Atomic Structure

Atomic number, mass number and the composition of ions

  • 3 things to know
  • Unit AS 1

Overview

Atomic number of an element and mass number of an atom.

What you need to know

Answer each one before you reveal it.

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Key term

Define the atomic number of an element.

Key term

Define the mass number of an atom.

Key term

How is the number of neutrons in an atom found from its atomic and mass numbers?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define the atomic number of an element.

    Model answer

    The atomic number is the number of protons in the nucleus of an atom of that element.

  2. 2

    Define the mass number of an atom.

    Model answer

    The mass number is the total number of protons and neutrons in the nucleus of that atom.

  3. 3

    How is the number of neutrons in an atom found from its atomic and mass numbers?

    Model answer

    The number of neutrons is the mass number minus the atomic number.

  4. 4

    In the notation ³⁵₁₇Cl, what does each number represent?

    Model answer

    The upper number 35 is the mass number and the lower number 17 is the atomic number.

  5. 5

    Give the numbers of protons, neutrons and electrons in an atom of ⁵⁶Fe.

    Model answer

    An atom of ⁵⁶Fe has 26 protons, 26 electrons and 30 neutrons.

  6. 6

    How does the electron count of a positive ion compare with that of its parent atom?

    Model answer

    A positive ion has fewer electrons than the neutral atom, one fewer for each unit of positive charge.

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