Sub-Atomic Particles (Edexcel International A Level Chemistry)

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Sub-Atomic Particles

  • All matter is composed of atoms, which are the smallest parts of an element that can take place in chemical reactions
  • Atoms are mostly made up of empty space around a very small, dense nucleus that contains protons and neutrons
  • The nucleus has an overall positive charge
    • The protons have a positive charge and the neutrons have a neutral charge

  • Negatively charged electrons are found in orbitals in the empty space around the nucleus

Carbon atom structure, IGCSE & GCSE, AS & A Level Chemistry revision notes

The basic structure of an atom (not to scale)

  • Subatomic particles are the particles an element is made up of and include protons, neutrons and electrons
  • These subatomic particles are so small that it is not possible to measure their masses and charges using conventional units (such as grams and coulombs)
  • Instead, their masses and charges are compared to each other using ‘relative atomic masses’ and ‘relative atomic charges
  • These are not actual charges and masses but they are charges and masses of particles relative to each other
    • Protons and neutrons have a very similar mass so each is assigned a relative mass of 1 whereas electrons are 1836 times smaller than a proton and neutron
    • Protons are positively charged, electrons negatively charged and neutrons are neutral

  • The relative mass and charge of the subatomic particles are:

Relative Mass & Charge of Subatomic Particles Table

Atomic Structure Table_Subatomic Particles, downloadable AS & A Level Chemistry revision notes

Examiner Tip

The relative mass of an electron is almost negligible.

The charge of a single electron is -1.602 x 10-19  coulombs whereas the charge of a proton is +1.602 x 10-19  coulombs, however, relative to each other, their charges are -1 and +1 respectively.

Chemical Elements

  • The atomic number (or proton number) is the number of protons in the nucleus of an atom and has symbol Z
    • The atomic number is equal to the number of electrons present in a neutral atom of an element
    • E.g. the atomic number of lithium is 3 which indicates that the neutral lithium atom has 3 protons and 3 electrons

  • The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom and has symbol A
  • The number of neutrons can be calculated by:

Number of neutrons = mass number - atomic number

    • Protons and neutrons are also called nucleons

Examiner Tip

Atomic structure notation, IGCSE & GCSE AS & A Level Chemistry revision notes

The mass (nucleon) and atomic (proton) number are given for each element in the Periodic Table

  • An atom is neutral and has no overall charge
  • Ions on the other hand are formed when atoms either gain or lose electrons, causing them to become charged
  • The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge

Protons

  • The atomic number of an atom and ion determines which element it is
  • Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
    • E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)

  • The number of protons equals the atomic (proton) number
  • The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:

Mass number = number of protons + number of neutrons

Number of protons = mass number - number of neutrons

Electrons

  • An atom is neutral and therefore has the same number of protons and electrons
  • Ions have a different number of electrons to their atomic number depending on their charge
    • A positively charged ion has lost electrons and therefore has fewer electrons than protons
    • A negatively charged ion has gained electrons and therefore has more electrons than 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 protons, electrons and neutrons for 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

    • Therefore, the number of protons in a Mg2+ ion is also 12 
    • However, the 2+ charge in Mg2+ ion suggests it has lost two electrons
      • Therefore, the Mg2+ ion only has 10 electrons left now

   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 = 12
      • The Mg2+ ion has 12 neutrons in its nucleus

   Answer 2:

   The atomic number of a carbon atom is 6

    • Therefore, the number of protons in a carbon atom is also 6 

   The atom has no overall charge so the number of protons = the number of electrons

    • Therefore, the carbon atom has 6 electrons

   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 = 6
      • The carbon atom has 6 neutrons 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
    • (Element X is therefore copper)

   The atom is not charged so the number of protons = the number of electrons

    • Therefore, the atom of element X has 29 electrons

   The number of neutrons is 34 (given in the question)

Isotopes

  • The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
    • E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively

  • Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons

Atomic Structure Hydrogen Isotopes, downloadable AS & A Level Chemistry revision notes

The atomic structure and symbols of the three isotopes of hydrogen

Properties of Isotopes

  • Isotopes have similar chemical properties but different physical properties

Chemical properties

  • Isotopes of the same element display the same chemical characteristics
  • This is because they have the same number of electrons in their outer shells
  • Electrons take part in chemical reactions and therefore determine the chemistry of an atom

Physical properties

  • The only difference between isotopes is the number of neutrons
  • Since these are neutral subatomic particles, they only add mass to the atom
  • As a result of this, isotopes have different physical properties such as small differences in their massdensity, melting point and boiling point 

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Richard

Author: Richard

Expertise: Chemistry

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.