Atomic Structure
- 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
Subatomic Particles
- 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
Mass Number & Isotopes
- The mass of an atom is concentrated in the nucleus, because the nucleus contains the heaviest subatomic particles (the neutrons and protons)
- The mass of the electron is negligible
- The nucleus is also positively charged due to the protons
- Electrons orbit the nucleus of the atom, contributing very little to its overall mass, but creating a ‘cloud’ of negative charge
- The electrostatic attraction between the positive nucleus and negatively charged electrons orbiting around it is what holds an atom together
- 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
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
- E.g. carbon-12 and carbon-14 are isotopes of carbon containing 6 and 8 neutrons respectively
- Isotopes have the same chemical properties but different physical 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
- 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 and density
- Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
- These are atoms of the same elements but with different mass numbers
- Because of this, the mass of an element is given as relative atomic mass (Ar) by using the average mass of the isotopes
- The relative atomic mass of an element can be calculated by using the relative abundance values
- The relative abundance of an isotope is either given or can be read off the mass spectrum
- Average atomic mass=