



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 Calcium 1s2 2s2 2p6 3s2 3p6 4s2 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 e.g. sodium 1s2 2s2 2p6 3s 1 A p block element is one whose outer electron is filling a p-sub shell e.g. chlorine 1s2 2s2 2p6 3s2 3p5 A d block element is one whose outer electron is filling a d-sub shell e.g. vanadium 1s22s22p63s23p6 4s23d3 Electronic structure of d-block elements The electronic structure of the d-block has some complications. As mentioned earlier, conventionally we say that 4s fills before 3d and so we write them in that order. There is, however, disagreement in the scientific community about whether this is true. If you look at the electronic structures below you will see both Chromium and copper have an unusual arrangement in having a half filled 4s sub shell. You will also see that when d block elements form ions they lose the 4s electrons first. You may find if you research different reasons for these observations. It may well be many of the reasons are false and we have to accept that some things in chemistry don’t neatly follow patterns we can exp. When a positive ion is formed electrons are lost from the outermost shell Mg is 1s2 2s2 2p6 3s2 becomes Mg2+ is 1s2 2s2 2p6 When a negative ion is formed electrons are gained O is 1s2 2s2 2p4 becomes O2- is 1s2 2s2
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3.1.1.3 Electron configuration
Electron configurations of atoms and ions up to Z = 36 in terms of shells and sub-shells (orbitals) s, p and d.