Models of the atom An early model of the atom was the Bohr model (GCSE model) (2 electrons in first shell, 8 in second etc.) with electrons in spherical orbits. Early models of atomic structure predicted that atoms and ions with noble gas electron arrangements should be stable. Electrons are arranged on: The A-level model Principle energy levels numbered 1,2,3,4.. 1 is closest to nucleus Sub energy levels labelled s , p, d and f s holds up to 2 electrons p holds up to 6 electrons d holds up to 10 electrons f holds up to 14 electrons Split into Split into Orbitals which hold up to 2 electrons of opposite spin Shapes of orbitals Orbitals represent the mathematical probabilities of finding an electron at any point within certain spatial distributions around the nucleus. Each orbital has its own approximate, three dimensional shape. It is not possible to draw the shape of orbitals precisely. Principle level 1 2 3 4 Sub-level 1s 2s, 2p 3s, 3p, 3d 4s, 4p, 4d, 4f An atom fills up the sub shells in order of increasing energy (note 3d is higher in energy than 4s and so gets filled after the 4s 1s2s2p3s3p 4s3d4p5s4d5p Writing electronic structure using letters and numbers For oxygen 1s2 2s2 2p4 Number of main energy level Name of type of sub-level Number of electrons in sub-level 2s 2p 1s Using spin diagrams For fluorine An arrow is one electron The arrows going in the opposite direction represents the different spins of the electrons in the orbital Box represents one orbital •s sublevels are spherical • p sublevels are shaped like dumbbells 2p When filling up sub levels with several orbitals, fill each orbital singly before starting to pair up the electrons The periodic table is split into blocks. A s block element is one whose outer electron is filling a s-sub shell Electronic structure for ions When a positive ion is formed electrons are lost Mg is 1s2 2s2 2p6 3s2 but Mg2+ is 1s2 2s2 2p6 When a negative ion is formed electrons are gained O is 1s2 2s2 2p4 but O2- is 1s2 2s2 2p6
2.2.1 Electron structure
Energy levels, shells, sub-shells, atomic orbitals, electron configuration (a) the number of electrons that can fill the first four shells (b) atomic orbitals, including: (i) as a region around the nucleus that can hold up to two electrons, with opposite spins (ii) the shapes of s- and p-orbitals (iii) the number of orbitals making up s-, p- and d-sub-shells, and the number of electrons that can fill s-, p- and d-sub-shells HSW1,7 Development of models to explain electron structure. (c) filling of orbitals: (i) for the first three shells and the 4s and 4p orbitals in order of increasing energy (ii) for orbitals with the same energy, occupation singly before pairing Learners are expected to be familiar with the ‘electrons in box’ representations. HSW1 Development of refined models for electron structure. (d) deduction of the electron configurations of: (i) atoms, given the atomic number, up to Z = 36 (ii) ions, given the atomic number and ionic charge, limited to s- and p-blocks up to Z = 36. Learners should use sub-shell notation, i.e. for oxygen: 1s22s22p4.
Credits: Neil Goalby