Electronic structure of the first twenty elements

The old orange street lights were sodium. The red glow in a shop sign is neon. A sodium atom has one more electron than a neon atom, just one. Neon forms no compounds at all. Sodium, though, reacts with water so fiercely that it can catch fire. One electron is the difference between the two - so where that electron sits must matter. This video places every electron in the first twenty elements.

This is video six of seven in Atomic structure, and it needs only Protons, neutrons and electrons behind it. If your course left out Relative atomic mass from isotopic abundances and you skipped it, nothing here depends on it.

Electrons sit in shells around the nucleus. The shells fill from the inside out. The lowest energy level, the shell nearest the nucleus, always fills first. For the first twenty elements, the first shell holds two electrons. The second shell holds eight. The third holds eight, and then a fourth shell starts. The electronic structure is how the electrons are arranged in those shells. You write it two ways: as a diagram of rings with the electrons on them, and as numbers, one per shell, with commas between. Hydrogen has one electron, so its structure is one. Helium has two, and that fills the first shell: two. Lithium has three electrons. The first shell takes two - so the third electron has to start the second shell. Lithium is two, one. Neon has ten: two, eight, and its second shell is full. Sodium has eleven. Two in the first shell, eight in the second, and the eleventh starts the third shell. Sodium is two, eight, one. The outer shell is the single outermost shell with electrons in it. An atom has exactly one outer shell. Neon's is full, and sodium's holds one electron on its own. Now, one for you to write out yourself. Calcium has twenty electrons. What is its electronic structure? Two, eight, eight, two. The first shell takes two, the second eight, the third eight, and the remaining two go into a fourth shell.

The structure also tells you where an element sits in the periodic table, from two counts. A period is a row of the table, and a group is a column. Count the shells, and you have the period. Sodium, two, eight, one, has three shells, so sodium is in period three. Count the electrons in the outer shell, and you have the group. Sodium has one outer electron, so it is in group one. A full outer shell, like neon's, puts an element in group zero. Now calcium, two, eight, eight, two: which period and which group? Period four, because there are four shells. Group two, because there are two electrons in the outer shell. The number of electrons, though, is not the structure. Twenty on its own tells you nothing about calcium's period. Two, eight, eight, two tells you both counts. A student writes chlorine as two, eight, seven, and then says chlorine is in period seven and group three. The structure is right - the slip happens when it is read. What has the student swapped, and what should the answer be? The two counts. Chlorine has three shells, so it is in period three. It has seven outer electrons, so it is in group seven. One examiner's report describes students who read the period off a single digit. Some "said that magnesium was in the second period as shown by the 'two' in the electronic configuration." The same report adds that "some had mentioned three shells, as taken from the electronic configuration, but had not linked that to the period number." The fix is to count, never to read a digit. Count the shells in the drawing for the period, then count the electrons in the outer shell for the group. Here is a comparison question, answered in terms of electronic structure only. Lithium two, one; sodium two, eight, one: one similarity, one difference? Similarity: both have one electron in the outer shell. Difference: sodium has one more shell than lithium, three shells against two. Say outer shell as a singular, always. Another examiner's report, on a sodium and potassium question, says "the phrase 'outer shell' is always taken to mean the single outermost shell." Here is your handle for this video, the two counts in their fixed order: shells for the period, outer electrons for the group.

To finish, some elements this video has not drawn. Aluminium has thirteen electrons. How are they arranged in shells? Two, eight, three. Two in the first shell, eight in the second, and the last three in the third. Next, from two, eight, three alone: which period and which group? Period three, from three shells. Group three, from three electrons in the outer shell. Now the street light: sodium has one more electron than neon. Where does it go? Into a new shell, on its own. Neon's two, eight has a full outer shell, and sodium adds a lone electron in a third shell. Why that lone electron makes sodium so reactive is a story for the chapter on the periodic table.

Give this one the thumb if you could draw any of the first twenty from its electron count and read off the period and group; then this one's yours. If the filling order still trips you, save it, because shell diagrams often land on a second viewing.

Next in the chapter: How the model of the atom changed.

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