Atomic structure

• The plum pudding model of an atom consists of a sphere of positive charge with small negative charges distributed evenly within it.
• The Rutherford scattering experiment in 1911 showed that most of an atom was empty space and that most of the mass was concentrated in a central positive nucleus.
• The maximum number of orbiting electrons in any shell can be calculated as 2 n^2 where n is the number of the shell.
• The mass number (A) of an element is the sum of protons and neutrons in an atom.
• The atomic number (Z) of an element is equal to the number of protons in an atom.
• Relative atomic mass (Ar) is the mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope. This takes into account relative abundances of the different isotopes of an element.
• Isotopes are atoms of the same element with the same atomic number but a different mass number, ie the same number of protons but a different number of neutrons.
• Isotopes of an element will react chemically in the same way as their proton number and electron configuration is unchanged, but will have different physical properties due to a different mass.
• Ions are formed when an atom loses or gains electrons meaning it has an overall charge.
• An s -orbital has a spherical shape, a p -orbital has an infinity shape and a d -orbital has a flower shape.
• An s-orbital can hold two electrons, a p-orbital can hold six electrons and a d-orbital can hold ten electrons.
• The s -block is the first two groups on the periodic table, the d -block is the transition metals, the p -block is the right hand side of the periodic table and the f -block is the lanthanides and actinides.
• The energy of an orbital increases from s to p to d, and then to the next numbered shell up, and so the orbitals are filled in this order, with the exception of 4s having less energy than 3d and so fills first (but also empties first).
• Within an orbital, electrons pair up with opposite spin so that the atom is as stable as possible.
• Electrons with the same spin fill up an orbital first before pairing.
• Paired electrons in shells experience electron pair repulsion and so in some cases electrons may take on a different arrangement to improve stability.
• Ionisation energy is the minimum energy required to remove one mole of electrons from one mole of atoms in the gaseous state and is measured in kJ/mol.
• Successive ionisation energies occur when further electrons are removed. This requires more energy as the electrostatic force of attraction between the positive nucleus and the negative outer electron increases.