Subatomic Particles (DP IB Chemistry): Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Philippa Platt

Updated on

Subatomic Particles

  • The atomic number (or proton number) is the number of protons in the nucleus of an atom and has the symbol Z

    • The atomic number is also equal to the number of electrons that are present in a neutral atom of an element

    • E.g. the atomic number of lithium is 3

      • This means that a neutral lithium atom has 3 protons and, therefore, also has 3 electrons

The chemical symbol of a general element

Diagram showing element notation: A as mass number, Z as atomic number, and X as chemical symbol, with labels explaining each component.
The mass (nucleon) and atomic (proton) number are given for each element in the Periodic Table
  • Each element is represented using its chemical symbol, with two key numbers:

    • Mass number (A) — the total number of protons and neutrons (also called nucleons)

    • Atomic number (Z) — the number of protons in the nucleus

  • Protons and neutrons are both nucleons because they are located in the nucleus

  • The number of neutrons can be calculated by:

Number of neutrons = mass number - atomic number

How to work out protons, neutrons and electrons

  • A neutral atom has no overall charge because it has equal numbers of protons and electrons

  • Ions form when electrons are gained (forming a negative ion) or lost (forming a positive ion), creating a charged species

  • You can work out the number of subatomic particles using:

    • Atomic (proton) number = number of protons

    • Mass (nucleon) number = total number of protons + neutrons

    • Charge = difference between number of protons and electrons

Protons

  • The atomic number (Z) tells you the number of protons in the nucleus

  • All atoms and ions of the same element have the same number of protons

    • e.g. Lithium has 3 protons; beryllium has 4 protons

  • If mass number and number of neutrons are known, calculate protons using:

Mass number = number of protons + number of neutrons

Number of protons = mass number - number of neutrons

Worked Example

Determine the number of protons in the following ions and atoms:

  1. Mg2+ ion

  2. Carbon atom

  3. An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1: 

  • The atomic number of a magnesium atom is 12

  • This means that the number of protons in the nucleus of a magnesium atom is 12

  • Therefore, the number of protons in a Mg2+ ion is also 12

    • Remember: The number of protons does not change when an ion is formed

Answer 2: 

  • The atomic number of a carbon atom is 6

  • This means that a carbon atom has 6 protons in its nucleus

Answer 3: 

  • Use the formula to calculate the number of protons

    • Number of protons = mass number - number of neutrons

    • Number of protons = 63 - 34

    • Number of protons = 29

  • Therefore, element X is copper

Electrons

  • A neutral atom has the same number of protons and electrons

  • Ions have a different number of electrons compared to protons:

    • A positive ion (cation) has lost electrons, so it has fewer electrons than protons

    • A negative ion (anion) has gained electrons, so it has more electrons than protons

Worked Example

Determine the number of electrons in the following ions and atoms:

  1. Mg2+ ion

  2. Carbon atom

  3. An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

  • The atomic number of a magnesium atom is 12

  • This means that the number of protons in the nucleus of a magnesium atom is 12

  • However, the 2+ charge in Mg2+ ion indicates that it has lost two electrons

  • Therefore, an Mg2+ ion only has 10 electrons 

Answer 2: 

  • The atomic number of a carbon atom is 6

  • This means that a carbon atom has 6 protons in its nucleus

  • Since there is no overall charge on a neutral carbon atom, there must be 6 negative electrons to balance the charge of the 6 positive protons

Answer 3: 

  • Use the formula to calculate the number of protons

    • Number of protons = mass number - number of neutrons

    • Number of protons = 63 - 34

    • Number of protons = 29

  • Since element X  is neutral, there must be 29 negative electrons to balance the charge of 29 positive protons

Neutrons

  • The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:

Number of neutrons = mass number (A) - number of protons (Z)

Worked Example

Determine the number of neutrons in the following ions and atoms:

  1. Mg2+ ion

  2. Carbon atom

  3. An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

  • The atomic number of a magnesium atom is 12 and its mass number is 24

    • Number of neutrons = mass number (A) - number of protons (Z)

    • Number of neutrons = 24 - 12

    • Number of neutrons = 12

  • The Mg2+ ion has 12 neutrons in its nucleus

Answer 2: 

  • The atomic number of a carbon atom is 6 and its mass number is 12

    • Number of neutrons = mass number (A) - number of protons (Z)

    • Number of neutrons = 12 - 6

    • Number of neutrons = 6

  • The carbon atom has 6 neutrons in its nucleus

Answer 3: 

  • The atomic number of an element atom is 29 and its mass number is 63

    • Number of neutrons = mass number (A) - number of protons (Z)

    • Number of neutrons = 63 - 29

    • Number of neutrons = 34

  • The neutral atom of element X has 34 neutrons in its nucleus

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Richard Boole

Author: 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.

Philippa Platt

Reviewer: 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

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