1.3.1 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

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

Exam Tip

• 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

• 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

   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

The atomic structure and symbols of the three isotopes of hydrogen