![]() ![]() Another example is calcium, which has 20 electrons. Using the aufbau diagram we obtain an electronic configuration of 1s 2, 2s 2, 2p 6, 3s 2 in beautiful agreement with experiments that can examine the configuration directly by looking at the spectra of atoms. The atom of magnesium has a total of 12 electrons. But let’s not get sidetracked by these anomalies and instead concentrate on a far deeper problem with this approach. This diagram, when combined with a knowledge of how many electrons can be accommodated in each kind of orbital and the number of such available orbitals in each shell, is now supposed to give us a prediction of the complete electronic configuration of all but about 20 atoms in which further irregularities occur, such as the cases of chromium and copper. This process gives the order of filling of orbitals with electrons according to this sequence:ġs > 2s > 2p > 3s > 3p > 4s > 3d > 4p > 5s > 4d The order of filling is said to be obtained by starting at the top of the diagram and following the arrows. This is demonstrated using the aufbau diagram, which lies at the heart of the trouble. But we are also reassured that there is a nice simple pattern that governs the order of shell and consequently of orbital filling. ![]() ![]() Now comes the magic ingredient in the sloppy version of this principle that claims to predict the order in which these orbitals fill (and here is where the fallacy lurks): rather than filling the shells around the nucleus in a simple sequence, where each shell must fill completely before moving onto the next shell, we are told that the correct procedure is more complicated. The aufbau diagram lies at the heart of the trouble As a result there is potentially one s orbital, three p orbitals, five d orbitals, seven f orbitals and so on for each shell. For s orbitals l = 0, for p orbitals l = 1, for d orbitals l = 2 and so on. The answer is provided by the formula 2 l + 1, where l takes different values depending on whether we are speaking of s, p, d or f orbitals. Next, we need to know how many of these orbitals occur in each shell. Each shell of electrons can be broken down into various orbitals and as we move away from the nucleus each shell contains a progressively larger number of types of orbital: the first shell only contains a 1s orbital, the second shell 2s and 2p orbitals, the third shell 3s, 3p and 3d orbitals, the fourth shell 4s, 4p, 4d and 4f orbitals and so on. The different atomic orbitals come in various kinds that are distinguished by labels such as s, p, d and f. These orbitals, at least in their simplest form, nowadays come from solving the Schrödinger equation for the hydrogen atom. The next ingredient is a knowledge of the atomic orbitals into which the electrons are progressively placed. Bohr proposed that the atoms of the periodic table can be thought of as being progressively built up one electron at a time: starting from the simplest atom of all, hydrogen with just one electron, moving onto helium with two electrons, lithium with three, all the way to uranium – which at that time (1913) was the heaviest known atom – with 92 electrons. He was also one of the first to fundamentally explain the periodic table in terms of arrangements of electrons (electronic configurations). The aufbau method was initially proposed by the Danish physicist Niels Bohr, who was the first person to use quantum mechanics to study atomic structure. © Science Photo Library Starting at the beginning ![]()
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