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Subatomic Particles (HL IB Chemistry)

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Chemistry

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

Labelled chemical symbol for a general element

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

  • The mass number (or nucleon number) is the total number of protons + neutrons in the nucleus of an atom, and has the symbol A
    • Protons and neutrons are also called nucleons, because they are found 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

  • An atom is neutral and has no overall charge
  • Ions on the other hand have either gained or lost 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
    • 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 of 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

  • An atom is neutral and therefore has the same number of protons and electrons
  • Ions have a different number of electrons to the number of protons, 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 protons of 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 protons of 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 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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