How the Periodic Table is arranged

1. The Periodic Table places every known element in order of increasing proton number, which is also called the atomic number.
2. Hydrogen, with one proton in each nucleus, comes first, helium with two protons comes second, and each element after that has one more proton than the element before it.
3. Every element has its own proton number, so every element has exactly one position in the table.
4. The horizontal rows are called periods, and they are numbered from Period 1 at the top to Period 7 at the bottom.
5. The vertical columns are called groups, and the main groups are numbered with Roman numerals from Group I on the far left to Group VIII on the far right.
6. The block of metals between Group II and Group III contains the transition elements, which are not given a group number in this numbering.
7. Each box in the table shows the element's symbol, its name, its proton number and its relative atomic mass.
8. Elements in the same period have the same number of occupied electron shells.
9. Elements in the same group have the same number of outer-shell electrons.

Metals to non-metals across a period

1. Moving from left to right across a period, the elements change from metals to non-metals.
2. In Period 3, sodium, magnesium and aluminium are metals, silicon has properties between those of metals and non-metals, and phosphorus, sulfur, chlorine and argon are non-metals.
3. Atoms on the left of a period have only one, two or three outer-shell electrons, and they lose these electrons in reactions to form positive ions.
4. Losing electrons to form positive ions is typical metallic behaviour.
5. Atoms on the right of a period have nearly full outer shells, so they tend to gain or share electrons instead.
6. Gaining or sharing electrons is typical non-metallic behaviour.
7. Metallic character therefore decreases across a period, while non-metallic character increases.
8. The physical properties follow the same pattern, so the shiny solids that conduct electricity on the left give way to dull solids and gases that do not conduct on the right.
9. The metals of a period sit to the left of a stepped line that runs down the right-hand side of the table, and the non-metals sit to the right of this line.

Group number and the charge on ions

1. Atoms form ions by losing or gaining electrons until their outer shell is full, giving them the same electronic configuration as a noble gas.
2. For Groups I to VII, the group number equals the number of outer-shell electrons, so it tells you how many electrons each atom must lose or gain.
3. Groups I, II and III: atoms lose their outer-shell electrons and form positive ions.
   1. Group I atoms lose one electron and form ions with a charge of $+1$, such as ${Na}^{+}$.
   2. Group II atoms lose two electrons and form ions with a charge of $+2$, such as ${Mg}^{2+}$.
   3. Group III atoms lose three electrons and form ions with a charge of $+3$, such as ${Al}^{3+}$.
4. Groups V, VI and VII: atoms gain electrons to fill their outer shell and form negative ions.
   1. Group V atoms gain three electrons and form ions with a charge of $-3$, such as $N^{3-}$.
   2. Group VI atoms gain two electrons and form ions with a charge of $-2$, such as $O^{2-}$.
   3. Group VII atoms gain one electron and form ions with a charge of $-1$, such as ${Cl}^{-}$.
5. For Groups I to III, the size of the positive charge equals the group number.
6. For Groups V to VII, the size of the negative charge equals eight minus the group number.
7. Group IV atoms such as carbon and silicon would have to lose or gain four electrons, so they usually share electrons in covalent bonds and do not form simple ions.
8. Group VIII atoms already have a full outer shell, so they do not form ions at all.

Similar properties within a group

1. Elements in the same group have the same number of electrons in their outer shell.
2. Lithium ($2,1$), sodium ($2,8,1$) and potassium ($2,8,8,1$) each have one outer-shell electron.
3. Fluorine ($2,7$) and chlorine ($2,8,7$) each have seven outer-shell electrons.
4. In a chemical reaction, it is the outer-shell electrons that are lost, gained or shared.
5. Atoms with the same number of outer-shell electrons therefore react in the same way, so elements in the same group have similar chemical properties.
6. They form ions with the same charge, so sodium and potassium both form $+1$ ions.
7. They also form compounds with the same type of formula, so sodium chloride is $NaCl$ and potassium chloride is $KCl$.
8. The properties are similar rather than identical, because each element further down a group has one more occupied shell than the element above it.
9. This extra shell produces gradual changes in properties down a group, such as a steady change in reactivity.

Predicting properties from position

1. Period number: this tells you the number of occupied electron shells in an atom of the element.
2. Group number: this tells you the number of outer-shell electrons for Groups I to VII.
   1. From the outer-shell electrons you can predict the charge on the ion the element forms.
   2. You can also predict that the element reacts in a similar way to the other members of its group.
3. Position across the period: this tells you whether the element is likely to be a metal or a non-metal.
   1. An element towards the left of a period is likely to be a metal that forms positive ions.
   2. An element towards the right of a period is likely to be a non-metal that forms negative ions or shares electrons.
4. Known members of the same group: an unfamiliar element is expected to behave like the elements above and below it, with its properties continuing the pattern down the group.