Period 1

The first row of the periodic table contains two elements and a very large gap. On a standard eighteen-column layout, hydrogen sits at the far left, helium at the far right, and the sixteen positions between them are empty — not because nothing has been discovered there, but because nothing can exist there.

The reason is the quantum number that labels the shell. For n = 1 the angular momentum quantum number l can only take the value zero, which means a single s orbital and no p, d or f orbitals at all. One orbital holds two electrons. Two electrons means two elements, and then the shell is full and the row ends.

This is the cleanest available demonstration that the shape of the periodic table is dictated by orbital structure rather than by chemistry. Counting the members of each successive row gives two, then a pair of eights, then a pair of eighteens, then a pair of thirty-twos; each new value of l becomes available one shell later than its label suggests, which is why 3d electrons fill during period 4 and 4f electrons during period 6, and why the rows lengthen in pairs. Period 1 is the base case with the arithmetic stripped bare.

Neither element will stay where it is put

Both members of this row are placed by convention rather than by consensus, and both arguments are still live.

Hydrogen has the configuration 1s¹, which is the configuration of an alkali metal, and that is why it is drawn above lithium. Almost nothing else about it agrees. It is a nonmetal that exists as a diatomic gas; it holds its single electron far more tightly than any group 1 metal does; and while it does form the H⁺ ion, a bare proton is so small and so charge-dense that it never travels alone in solution — what exists in water is the hydronium ion, bound to something.

The counter-argument places it above fluorine, on the grounds that a single vacancy in its shell is exactly what a halogen has, and that it does form a genuine H⁻ anion in the hydrides of reactive metals. Its Pauling electronegativity of 2.20 sits close to carbon's and refuses to support either case. A third proposal notes that a half-filled shell is a half-filled shell and puts it above carbon in group 14. Many modern tables duck the question by floating hydrogen above the body of the table, unattached to any column.

Helium's placement is disputed for the opposite reason: convention and configuration disagree, and convention wins. By behaviour it plainly belongs with the noble gases, and that is where every teaching table puts it. Its configuration argues otherwise: helium's two electrons close the 1s subshell and nothing further, which makes beryllium rather than neon its nearest structural relative and group 2 the column it ought to be printed in. Charles Janet's left-step table of 1928, which orders elements strictly by the filling sequence, does exactly that, and it has never quite gone away — periodically a physicist argues that the standard table is privileging chemical similarity over electronic structure and should be redrawn. The standard table's answer is that helium's chemistry is what a reader needs first.

The largest isotope effect in the table

Hydrogen holds a privilege granted to no other element: IUPAC accepts distinct names and symbols for its isotopes. Deuterium is D as well as ²H, tritium is T as well as ³H, and the ordinary isotope is sometimes called protium when a distinction is needed.

The exception exists because the mass ratio is extreme. A deuterium nucleus is twice the mass of a protium nucleus, whereas carbon-13 is only about eight per cent heavier than carbon-12. Mass affects the vibrational frequency of a bond and therefore its zero-point energy and the height of the barrier to breaking it, so reactions involving a broken hydrogen bond can run several times more slowly with deuterium in place. Heavy water freezes and boils a few degrees higher than ordinary water, is slightly denser and more viscous, and interferes with cell division in organisms fed on it exclusively — the only case in the periodic table where swapping one isotope for another changes biology.

Deuterium is about one part in 6,400 of terrestrial hydrogen, and it is a Big Bang leftover: the ratio of deuterium to hydrogen in unprocessed gas is one of the tighter constraints cosmologists have on the density of ordinary matter in the universe. Tritium is not primordial at all — its half-life is 12.3 years, so any that formed early is long gone, and what exists is made by cosmic rays in the upper atmosphere or bred deliberately in reactors.

The row that is most of the universe

Counting atoms rather than elements, this two-element row is roughly 98 per cent of the observable universe. Hydrogen accounts for something like 92 atoms in every hundred and helium for nearly all of the rest; by mass the split is closer to three-quarters and one-quarter. Every other element in the table together makes up about two per cent of the atoms and rather less of that by count in the very oldest stars, where the heavier elements had not yet been made.

Almost all of that hydrogen and most of that helium was produced in the first few minutes after the Big Bang, and the reason none of it went further is a peculiarity of the nuclei at the end of this row. Two rungs of the mass ladder are simply missing — nothing at mass 5 holds together, and nothing at mass 8 does either. Helium-4 is exceptionally tightly bound — that binding is why alpha particles exist as a decay product at all — and adding a proton or a neutron to it produces something that falls apart in about 10⁻²¹ seconds. Two helium-4 nuclei fusing give beryllium-8, which comes apart again almost as fast.

Primordial nucleosynthesis therefore ran into a wall at helium and stopped, and the universe waited hundreds of millions of years for stars to find a way around it. The route stars found is the triple-alpha process, in which three helium-4 nuclei combine effectively simultaneously — possible only because carbon-12 happens to have an excited state at just the right energy. Fred Hoyle predicted that resonance in 1953 on the grounds that carbon exists and therefore something must make it, and it was found experimentally soon afterwards. Every carbon atom in every reader of this page got past the end of period 1 through that gap.

What is missing from this row, and why it matters

Because there is no p subshell, period 1 has no group 13 through 17 and therefore no metalloid, no halogen, no walk from metal to nonmetal across the row. Every other period tells a story about character changing as a shell fills. This one cannot, because the shell it is filling has no internal structure to change.

What the row does contain is a pair of extremes. Hydrogen is the smallest and simplest atom there is — one proton, one electron, and the only atom whose Schrödinger equation has an exact analytical solution, which is why every model of atomic structure is calibrated against it. Helium is the least polarisable atom, the hardest to disturb, and the point at which the first shell closes. Between them they define the ends of the scale that the remaining six rows work within.

Hydrogen has one further trick that belongs to no other element in this row's story. Under pressures of several million atmospheres it is predicted to become metallic, and the interiors of Jupiter and Saturn are thought to consist largely of liquid metallic hydrogen — which would be the origin of their magnetic fields. Laboratory attempts to make it have been contested; a 2017 claim from Harvard was disputed on its interpretation of the reflectivity data, and the sample was lost before it could be re-examined. The lightest element in the table is still the subject of an unresolved argument about what it does under conditions that exist in abundance one planet away.