Atomic Structure
- Atoms are the components that make up all elements.
- Atoms are made up of three types of sub-atomic particles – protons, neutrons, and electrons
- Protons and neutrons make up the nucleus, where most of the mass is concentrated. Electrons orbit the nucleus in shells
Particle | Relative Mass | Relative Charge |
Proton | 1 | +1 |
Neutron | 1 | 0 |
Electron | 1/2000 | -1 |
Mass Number & Isotopes
- Element, X
- Mass number, A, is the total number of protons and neutrons in the nucleus
- Atomic number, Z, is the number of protons. The number of positively charged protons is equal to the number of negatively charged electrons in an atom, making the atom neutrally charged
- Mass number = number of protons + number of neutrons
- Atomic number = number of protons = number of electrons
- Ions are formed by atoms losing or gaining electrons.
- A charge of x- means that the number of electrons in the ion is the atomic number + x
- A charge of x+ means that the number of electrons in the ion is the atomic number - x
- Isotopes are atoms with the same number of protons and different numbers of neutrons. Therefore, they have different mass numbers but the same atomic number.
- Isotopes of the same element have the same electronic configuration so react in the same way in chemical reactions but have slightly different physical properties.
Relative Masses
- Relative isotopic mass is the mass of an atom of an isotope compared with 1/12th of the mass of an atom of carbon-12. For an isotope, the relative isotopic mass = its mass number.
- Relative atomic mass is the ratio of the average mass of an atom of an element to 1/12th of the mass of an atom of carbon-12.
- Relative molecular mass is the ratio of the average mass of a molecule of an element or compound to 1/12th of the mass of an atom of carbon-12.
- Relative formula mass is similar to relative molecular mass but applies to ionic compounds.
Mass Spectrometry
- Mass spectrometry is a form of molecular chemical analysis that allows the masses of individual molecules or isotopes to be determined.
- Mass spectrometry can be used to provide structural information, identify an unknown compound, or determine the relative abundance of each isotope of an element.
- The mass spectrum gives information about the relative abundance of isotopes on the y axis and about the relative isotopic mass on the x axis.
- The mass spectrum can be used to determine the relative atomic mass (Ar)
- A mass spectrum for a molecular sample shows the relative molecular mass on the x axis.
