National 5ChemistrySQARevision NotesChemical Changes & StructureAtomic Structure & Bonding Related to Properties of MaterialsProtons, Neutrons & Electrons

Protons, Neutrons & Electrons (SQA National 5 Chemistry): Revision Note

Exam code: X813 75

Philippa Platt

Written by: Philippa Platt

Reviewed by: Richard Boole

Updated on

Nuclide notation

  • Nuclide notation is a standard shorthand used in chemistry to provide detailed information about a specific atom or ion.

  • It indicates the number of protons, neutrons, and electrons

The layout

Diagram of a chemical element symbol "X" with labels: "A" for atomic mass above, "Z" for atomic number below, and "C" for charge to the right.
The layout of nuclide notation

  • A - Mass number:

    • The number at the top left

    • It is the total number of protons + neutrons in the nucleus

  • Z - Atomic number:

    • The number at the bottom left

    • It is the number of protons

    • The atomic number is unique to each element

  • X - Element symbol:

    • The symbol for the element

    • For example:

      • Sodium is Na

      • Chlorine is Cl

      • Magnesium is Mg

  • C - Charge:

    • The charge is written at the top right

    • If there is no charge written, the atom is neutral with a charge of 0

    • The charge is used to determine the number of electrons in an ion

  • For example, the nuclide notation for a sodium ion (Na+) is:

Sodium element symbol "Na" with atomic number 11 and atomic mass 23, displayed with a plus sign indicating a positive ion.
The nuclide notation of a sodium ion

  • This shows that a sodium ion has:

    • A mass number of 23

    • An atomic number of 11

    • A chemical symbol of Na

    • A charge of 1+

Determining protons and electrons

  • You can use the nuclide notation to work out the number of each subatomic particle

How to find the number of protons

  • The number of protons is the atomic number, Z

How to find the number of electrons

  • For a neutral atom:

    • There is no charge shown on the nuclide notation

    • So, the number of electrons is the same as the number of protons

  • For an ion

    • There is a charge shown on the nuclide notation

    • A positive charge shows that an atom has lost electrons

      • A 1+ charge means 1 electron has been lost

      • A 2+ charge means 2 electrons have been lost

    • A negative charge shows that an atom has gained electrons

      • A 1- charge means 1 electron has been gained

      • A 2- charge means 2 electrons have been gained

Worked Example

Determine the number of protons and electrons in:

  1. A chlorine-35 atom open parentheses Cl presubscript 17 presuperscript 35 close parentheses

[2]

  1. A sulfide ion open parentheses scriptbase straight S to the power of 2 minus end exponent end scriptbase presubscript 16 presuperscript 32 close parentheses

[2]

Answer:

  1. The chlorine-35 atom:

  • Protons:

    • The atomic number is 17

    • So, there are 17 protons

[1 mark]

  • Electrons:

    • The atom is neutral

    • So, the number of electrons equals the number of protons

    • So, there are 17 electrons

[1 mark]

  1. The sulfide ion:

  • Protons:

    • The atomic number is 16

    • So, there are 16 protons

[1 mark]

  • Electrons:

    • The ion has a 2- charge

    • It has gained 2 electrons

    • So, the number of electrons = number of protons + 2

    • So there are 16 + 2 = 18 electrons

[1 mark]

Determining neutrons

How to find the number of neutrons

  • The mass number is the number of protons plus neutrons

  • The number of neutrons, n, can be calculated by:

number of neutrons = mass number - atomic number

Worked Example

Determine the number of protons, neutrons and electrons in:

  1. A potassium atom open parentheses straight K presubscript 19 presuperscript 39 close parentheses

[3]

  1. A magnesium ion open parentheses scriptbase Mg to the power of 2 plus end exponent end scriptbase presubscript 12 presuperscript 24 close parentheses

[3]

Answer:

  1. The potassium atom:

  • Protons

    • The atomic number is 19

    • So, there are 19 protons

[1 mark]

  • Electrons

    • The atom is neutral

    • So, the number of protons = the number of electrons

    • So, there are 19 electrons

[1 mark]

  • Neutrons

    • The mass number is 39

    • The atomic number is 19

    • The number of neutrons = mass number - atomic number

    • So, there are 39 - 19 = 20 neutrons

[1 mark]

  1. The magnesium ion:

  • Protons

    • The atomic number is 12

    • So, there are 12 protons

[1 mark]

  • Electrons

    • The ion has a 2+ charge

    • So, it has lost 2 electrons

    • So, the number of electrons = number of protons - 2

    • So, there are 12 - 2 = 10 electrons

[1 mark]

  • Neutrons

    • The mass number is 24

    • The atomic number is 12

    • The number of neutrons = mass number - atomic number

    • So, there are 24 - 12 = 12 neutrons

[1 mark]

  • With the right information, the number of neutrons equation can be rearranged to determine the mass number or atomic number:

mass number = number of neutrons + atomic number

atomic number = mass number - number of neutrons

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    Author
    Reviewer
    Philippa Platt

    Author: Philippa Platt

    Expertise: Chemistry Content Creator

    Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener

    Richard Boole

    Reviewer: Richard Boole

    Expertise: Chemistry Content Creator

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.