The periodic table, Year 10: how the table is arranged

Atomic (proton) number

HintIt is a count of one kind of particle in the nucleus.

The whyEach element has one more proton than the element before it. Ordering by this count puts every element in a group with others like it, which ordering by mass did not always do.

Their atoms have the same number of outer-shell electrons

HintCompare the last figure in 2,1 and 2,8,1 and 2,8,8,1.

The whyChemical reactions involve the outer electrons, so atoms with the same number of them react in similar ways. Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) each have one, and all three are reactive metals of Group 1.

Group 2, Period 3

HintOne answer comes from the last number in the structure, the other from how many numbers there are.

The whyThe number of outer electrons gives the group, and the number of occupied shells gives the period. Two outer electrons means Group 2, and three shells means Period 3. The element is magnesium.

Similar properties recur at regular intervals

HintThink of something that comes round again and again with an even gap, like a weekly lesson.

The whyGoing through the elements in order of atomic number, a reactive metal turns up, then several elements later another one, and so on. Stacking these repeats under one another makes the columns called groups.

Rubidium hydroxide and hydrogen

HintElements in the same column react in the same way, so follow the pattern and swap the metal's name.

The whyRubidium has one outer electron, like every Group 1 metal, so it reacts in the same way and gives the same kinds of product. Because reactivity increases down the group, its reaction is more violent than potassium's.

Some elements ended up in groups with elements quite unlike them

HintThink what happens to a column of look-alikes if one member is slotted in purely by its weight.

The whyThe early tables were also incomplete, because many elements had not yet been discovered. Following weight order with no gaps pushed some elements into the wrong columns. Mendeleev dealt with both problems: he left gaps and, in places, changed the order.

Isotopes

HintThey differ from one another only in their number of neutrons.

The whyAn element's atomic weight is an average over its different atoms, so it depends on how many neutrons the common ones have, not just on the protons. An element can therefore weigh more than the next one while having fewer protons.

It is ordered by atomic number: argon has 18 protons, potassium 19

HintMass is not what the modern table counts; think of the particle that fixes which element an atom is.

The whyArgon has 18 protons and potassium has 19. Placed by atomic number, argon falls with the unreactive noble gases and potassium with the reactive alkali metals, where their properties say they belong. Ordered by mass, the two would be swapped.

Protons, neutrons and electrons

HintThey are the three subatomic particles you use to describe any atom today.

The whyWith no knowledge of protons, chemists could not order the elements by atomic number, so they used atomic weight, which they could measure. That makes it all the more striking that Mendeleev's arrangement, guided by properties, turned out to match the order of proton numbers.

They contain more neutrons

HintMass comes from two kinds of particle in the nucleus, and one kind is already accounted for.

The whyRelative atomic mass depends on protons plus neutrons, averaged over the isotopes. The abundant isotopes of tellurium are rich enough in neutrons to outweigh iodine's extra proton. This is why Mendeleev was right to put tellurium before iodine, going by properties, even though the weights said otherwise.

Positive ions

HintMetal atoms give electrons away, and electrons carry a negative charge.

The whyAn element whose atoms form positive ions when it reacts is a metal; one whose atoms do not is a non-metal. The definition works even for mercury, which is a liquid and so fails the everyday test of being hard and solid.

They lose their outer electrons

HintA metal atom has only one, two or three of something in its outside shell, and gives them up.

The whyLosing negative electrons leaves more protons than electrons, so the ion is positive. Sodium, 2,8,1, loses one electron and becomes an ion with a 1+ charge.

2,8,5 — a non-metal

HintCount how many electrons end up in the third shell, and think whether it is easier to lose that many or to gain a few.

The whyFifteen electrons fill the shells as 2,8,5. With five outer electrons the atom does not lose them to form a positive ion, so the element is a non-metal. It is phosphorus, on the right-hand side of Period 3.

To the left and towards the bottom

HintThink where sodium and caesium sit, compared with fluorine.

The whyThe non-metals sit at the right-hand side and near the top. Metals are on the left because their atoms have few outer electrons, which they lose when they react. Going down a group the elements become more metallic too, which is why tin and lead are metals although they share a group with carbon.

It gains one electron and becomes full

HintCount how far 7 is from the stable arrangement of the nearest noble gas.

The whyA non-metal atom with a nearly full outer shell takes electrons rather than losing them, so it does not form a positive ion. The electron comes from the metal atom, which becomes a positive ion while the chlorine atom becomes a negative one.