ASChemistryCIERevision Notes1. Atomic Structure1.1 Particles in the Atom & Atomic RadiusDetermining Subatomic Structure

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Determining Subatomic Structure (CIE AS Chemistry)

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Key Terms of Atomic Structure

  • 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 present in a neutral atom of an element
    • E.g. the atomic number of lithium is 3, meaning that a neutral lithium atom has 3 protons and therefore, also has 3 electrons

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

Number of neutrons = mass number - atomic number

    • Protons and neutrons are also called nucleons, because they are found in the nucleus

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

Determining the Subatomic Structure of Atoms & Ions

  • 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

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 suggesting that the number of protons in the magnesium element is 12
  • Therefore, the number of protons in a Mg2+ ion is also 12

Answer 2:

  • The atomic number of a carbon atom is 6 suggesting 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
  • Element X is therefore copper

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

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 suggesting that the number of protons in the neutral magnesium atom is 12
  • However, the 2+ charge in the Mg2+ ion suggests it has lost two electrons
  • It only has 10 electrons left now

Answer 2:

  • The atomic number of a carbon atom is 6 suggesting that the neutral carbon atom has 6 electrons orbiting around the nucleus

Answer 3:

  • The number of protons of element X can be calculated by:
    • Number of protons = mass number - number of neutrons
    • Number of protons = 63 - 34
    • Number of protons = 29
  • The neutral atom of element X  therefore also has 29 electrons

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 an atomic number of 29 and a mass number of 63

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 X 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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