Calculating the Particles (OCR Gateway GCSE Chemistry)

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Defining Terminology

Atomic Number

  • The atomic number (or proton number) is the number of protons in the nucleus of an atom
  • The symbol for this number is Z
  • It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table
  • The proton number is unique to each element, so no two elements have the same number of protons
  • Electrons can be lost, gained, or shared during chemical processes but the proton number of an atom does not change in a chemical reaction

Mass Number

  • The mass number (or nucleon number) is the total number of protons and neutrons in the nucleus of an atom
  • The symbol for this number is A
  • The mass number minus the proton number gives you the number of neutrons of an atom
  • Note that protons and neutrons can collectively be called nucleons
  • The atomic number and mass number for every element is on the Periodic Table

Periodic Table Symbols - AQA

Diagram showing the notation used on the Periodic Table

Isotopes

  • Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons
  • The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number
  • So, C-14 is the isotope of carbon which contains 6 protons and 6 electrons, but the 14 signifies that it has 8 neutrons (14 - 6 = 8)
    • It can also be written as 14C

  • Isotopes display the same chemical characteristics
  • This is because they have the same number of electrons in their outer shells, and this is what determines their chemistry
  • The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only

The Atomic Structure and Symbols of the Three Isotopes of Hydrogen

Defining Isotopes table, IGCSE & GCSE Chemistry revision notes

Ions

  • An ion is an atom or group of atoms that has an electrical charge, either positive and negative
  • Atoms have an equal number of protons and electrons and so do not have an overall charge
  • Atoms with incomplete outer electron shells are unstable
  • By either gaining or losing electrons, atoms can obtain full outer electron shells and become stable
  • When this happens, atoms have an unequal number of protons and electrons and so have an overall charge.
  • This is how atoms become ions
  • An atom that loses electrons has more protons than electrons and so has a positive overall charge
    • This is called a positive ion or cation
  • An atom that gains electrons has more electrons than protons and so has a negative overall charge
    • This is called a negative ion or anion

Calculating the Numbers

Protons

  • The atomic number is equal to the number of protons (p) in an atom
  • Since atoms are neutral, then it is also the same as the number of electrons (e)
  • The mass number is the number of protons plus neutrons
  • The number of neutrons (n) can thus be calculated by subtracting the atomic number from the mass number
    • For example, beryllium has an atomic number of 4, therefore it has 4 protons and 4 electrons
    • The mass number of beryllium is 9, so it has 9 - 4 = 5 neutrons
    • The PEN numbers for beryllium are thus:
      p = 4
      e = 4
      n = (9 - 4 =) 5

Periodic Table Symbol for Beryllium - AQA, downloadable IGCSE & GCSE Chemistry revision notes

The symbol key for beryllium as represented on the periodic table

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

Answer 1: The atomic number of a magnesium atom is 12, therefore the number of protons in the magnesium atom is 12

Therefore, the number of protons in a Mg2+ ion is also 12 - the number of protons does not change when an ion is formed


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

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) - atomic number (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 mass number 63 and 34 neutrons

Answer

Answer 1: The atomic number of a magnesium atom is 12 and its mass number is 24

Number of neutrons = mass number (A) - atomic number (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) - atomic number (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) - atomic number (Z)

Number of neutrons = 63 - 29

Number of neutrons = 34

The neutral atom of element X has 34 neutrons in its nucleus

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 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

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

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.