Edexcel International A Level Chemistry

Revision Notes

1.3.1 Sub-Atomic Particles

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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

Exam 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

Exam 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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