Sub-Atomic Particles (Edexcel International AS Chemistry): Revision Note

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

Chemical Elements

• 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

• 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)

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

Properties of Isotopes

• 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 mass, density, melting point and boiling point