Isotopes (Edexcel AS Chemistry)

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Atomic Structure Calculations

  • 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

Properties

  • 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 
    • For example samples of uranium hexafluoride gas, UF6, can undergo uranium enrichment by using a gas centrifuge cylinder
    • This is because nuclear reactors require higher amounts of the lighter U235 isotope 
    • UF6 gas is placed in a gas centrifuge cylinder and rotated at a high speed
    • This rotation creates a strong centrifugal force so that the heavier gas molecules (UF6 containing the heavier U238 isotope) move towards the outside of the cylinder
    • The lighter gas molecules (containing the U235 isotope) collect closer to the centre

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

Answers

Answer 1: The atomic number of a magnesium atom is 12 indicating 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 indicating 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 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

Answers

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 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 of the following ions and atoms:

  1. Mg2+ ion
  2. Carbon atom
  3. An unknown atom of element X with mass number 63 and 29 protons

Answers

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

Defining & Calculating 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
    • For example, three isotopes of hydrogen are protium, deuterium and tritium

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

The atomic structure and symbols of the three isotopes of hydrogen

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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.