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Physics · Atomic structure

Mass number, atomic number and isotopes

Use atomic number and mass number, write nuclear symbols, and describe isotopes and ions.

  • 6 key terms
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Warm-up

Answer each one, then check.

  1. 1

    How many protons does carbon have?

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    6

  2. 2

    What is an element?

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    A substance with only one type of atom

  3. 3

    What is an ion?

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    A charged atom

  4. 4

    What does the atomic number tell you?

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    The number of protons

  5. 5

    What is a neutron?

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    A neutral particle in the nucleus

Learning Objectives

  1. 1Define atomic number and mass number.
  2. 2Work out the numbers of protons, neutrons and electrons in an atom.
  3. 3Explain what isotopes are and relate differences in isotope symbols to identities, charges and masses.
  4. 4Explain how atoms become positive ions.

ATOMIC NUMBER AND MASS NUMBER

The atomic number is the number of protons in an atom. The mass number is the total number of protons and neutrons.

Isotopes of an element have the same number of protons but different numbers of neutrons.

Counting Particles

An atom of \(^{14}_{6}\text{C}\) is neutral. How many protons, neutrons and electrons does it have?

Show the solutionHide the solution
  1. 1 Protons Atomic number = 6
  2. 2 Neutrons Mass number − atomic number = \(14 - 6 = 8\)
  3. 3 Electrons Same as protons in a neutral atom = 6

Answer6 protons, 8 neutrons and 6 electrons.

Isotopes

Compare the atoms \(^{35}_{17}\text{Cl}\) and \(^{37}_{17}\text{Cl}\).

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  1. 1 Protons Both have 17
  2. 2 Electrons Both have 17 if neutral
  3. 3 Neutrons \(35 - 17 = 18\) and \(37 - 17 = 20\)

AnswerThey are isotopes: the same number of protons and electrons but different numbers of neutrons (2 more in chlorine-37).

Ions

A sodium atom \(^{23}_{11}\text{Na}\) loses one electron. Describe the ion formed.

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  1. 1 Protons Still 11
  2. 2 Electrons Now 10
  3. 3 Charge One more proton than electrons: +1

AnswerA positive ion, \(\text{Na}^+\), with 11 protons, 12 neutrons and 10 electrons.

Key Ideas

Learn these.

  • Element

    Determined by the number of protons.

  • Isotopes

    Same element, so same chemical properties, but different masses.

  • Charge

    Ions have an unequal number of protons and electrons.

  • Symbols

    The mass number is the top number; the atomic number is the bottom number.

Fill the Table

For \(^{7}_{3}\text{Li}\) and \(^{6}_{3}\text{Li}\), state the number of protons, neutrons and electrons in each neutral atom. Explain why they are isotopes.

1. Use atomic number for protons.

2. Mass number minus atomic number for neutrons.

A good answer shows: Li-7: 3 protons, 4 neutrons, 3 electrons. Li-6: 3 protons, 3 neutrons, 3 electrons. They have the same number of protons but different numbers of neutrons.

Can I...?

  1. 1Define atomic number.
  2. 2Define mass number.
  3. 3Work out neutrons.
  4. 4Read a nuclear symbol.
  5. 5Define isotopes.
  6. 6Compare two isotopes.
  7. 7Explain positive ions.
  8. 8Say why isotopes react the same.

Summary & Exam Focus

  • Atomic number = protons.
  • Mass number = protons + neutrons.
  • Isotopes: same protons, different neutrons.
  • A positive ion has lost electrons.

Exam focus

An atom has 6 protons and 8 neutrons. State its mass number. (1 mark) (1 marks)

Add protons and neutrons.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Atomic number
The number of protons in an atom.
Mass number
The total number of protons and neutrons.
Isotopes
Atoms of the same element with different numbers of neutrons.
Ion
An atom that has lost or gained electrons.
Element
A substance made of atoms with the same number of protons.
Nuclide
A particular type of nucleus, such as carbon-14.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Calculate 2 marks

    An atom of carbon-14 is represented as \(^{14}_{6}\text{C}\). State the number of protons and the number of neutrons in the atom.

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    Model answer

    6 protons; 14 − 6 = 8 neutrons.

    Mark scheme

    • 6 protons — 1 mark
    • 8 neutrons — 1 mark
  2. Question 2 Explain 2 marks

    Explain what is meant by isotopes.

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    Model answer

    Atoms of the same element (same number of protons) that have different numbers of neutrons.

    Mark scheme

    • Same number of protons or same element — 1 mark
    • Different number of neutrons — 1 mark
  3. Question 3 Compare 3 marks

    Chlorine has two isotopes: \(^{35}_{17}\text{Cl}\) and \(^{37}_{17}\text{Cl}\). State one similarity and one difference between atoms of these isotopes.

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    Model answer

    Similarity: same number of protons (17) or electrons. Difference: chlorine-37 has 2 more neutrons (20 rather than 18), so a larger mass number.

    Mark scheme

    • Similarity — 1 mark
    • Difference in neutrons — 1 mark
    • With numbers or different mass — 1 mark
  4. Question 4 Explain 3 marks

    A lithium atom \(^{7}_{3}\text{Li}\) becomes a positive ion. Explain how this happens and give the number of electrons in the ion if the charge is +1.

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    Model answer

    The atom loses an outer electron. It has 3 protons and 2 electrons, so the charge is +1.

    Mark scheme

    • Loses an electron — 1 mark
    • 3 protons and 2 electrons — 1 mark
    • More protons than electrons gives positive charge — 1 mark
  5. Question 5 Calculate 3 marks

    An atom of uranium-235 has 92 protons. Calculate the number of neutrons and the number of electrons in a neutral atom of uranium-235.

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    Model answer

    Neutrons: 235 − 92 = 143. Electrons: 92.

    Mark scheme

    • Neutrons = mass number − atomic number — 1 mark
    • 143 — 1 mark
    • 92 electrons — 1 mark

Quick check

  1. The number of protons is the...

    1. Amass number
    2. Bisotope number
    3. Catomic number
    4. Delectron number
    Show answerHide answer

    C: atomic number

    Atomic number = protons.

  2. Isotopes have the same number of...

    1. Aneutrons
    2. Bnucleons
    3. Cmass
    4. Dprotons
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    D: protons

    Same element.

  3. Carbon-14 has 6 protons and mass number 14. Neutrons =

    1. A8
    2. B6
    3. C14
    4. D20
    Show answerHide answer

    A: 8

    14 − 6 = 8.

  4. A neutral atom with 11 protons has how many electrons?

    1. A12
    2. B11
    3. C10
    4. D22
    Show answerHide answer

    B: 11

    Equal to protons.

  5. An atom that loses an electron is...

    1. Aa negative ion
    2. Ban isotope
    3. Ca positive ion
    4. Dneutral
    Show answerHide answer

    C: a positive ion

    More protons than electrons.

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