Electron Structure and Periodic-table Organisation

Atomic number connects the atom to the table

The periodic table arranges elements in order of increasing atomic number. Atomic number is the number of protons in an atom's nucleus, so each element has its own atomic number. Lithium has atomic number 3; sodium has atomic number 11.

A neutral atom has equal numbers of positively charged protons and negatively charged electrons. Its atomic number therefore also tells you how many electrons it has. A neutral sodium atom has 11 protons and 11 electrons.

Those electrons are not all at the same energy level. Their arrangement explains why an element belongs in a particular row and column of the periodic table—and why it reacts in a particular way.

Arranging electrons in shells

Electrons occupy particular energy levels, also called shells, around the nucleus. They occupy the lowest available energy levels first: the innermost available shells fill before electrons occupy higher energy levels.

For the first 20 elements, use this filling pattern:

• Place up to 2 electrons in the first shell.
• Place up to 8 electrons in the second shell.
• Place up to 8 electrons in the third shell before starting the fourth.
• The remaining electron or electrons in potassium and calcium occupy the fourth shell.

This is the shell model needed here; it is not a rule that every shell can hold only eight electrons.

Electronic structure means the arrangement of electrons in these shells. In numerical notation, write the number in each occupied shell, starting with the innermost, and separate the numbers with commas.

For example, sodium's 11 electrons are arranged as two in the first shell, eight in the second and one in the third. Its electronic structure is 2,8,1. The numbers add up to 11, and each number represents a shell—not an individual electron.

Calcium has atomic number 20. After placing two electrons in the first shell, eight in the second and eight in the third, two remain for the fourth shell. Its electronic structure is 2,8,8,2.

Electronic structures of the first 20 elements

The same method gives the electronic structure of every neutral atom in this table.

Reading rows and columns from electronic structure

The horizontal rows of the periodic table are called periods. The period number tells you the number of occupied shells. Sodium, 2,8,1, has three occupied shells, so it is in Period 3. Potassium, 2,8,8,1, has four, so it is in Period 4.

The vertical columns are called groups. Using the GCSE group numbering 1–7 and 0, the group number for Groups 1–7 matches the number of outer-shell electrons. Chlorine, 2,8,7, has seven outer electrons, placing it in Group 7. Its three occupied shells also place it in Period 3.

Group 0 contains the noble gases, whose outer shells are full. Neon, 2,8, and argon, 2,8,8, each have eight outer electrons. Helium has only two, but its first shell is full with two electrons, so it also belongs in Group 0.

Hydrogen has one outer electron and is often shown above Group 1, but it is a non-metal, not an alkali metal. Its properties differ from those of lithium, sodium and potassium.

Lithium, sodium and potassium have different total numbers of electrons and occupied shells, yet each has one outer electron. That shared outer-shell arrangement is the connection between their positions in the same column.

Why chemical properties repeat

An atom's outer electrons determine how it takes part in chemical reactions. Elements with the same number of outer electrons therefore have similar chemical properties.

Moving across Period 2, the outer shell builds up from one electron in lithium to eight in neon. Sodium then starts a new occupied shell with one electron. Its outer-shell arrangement resembles lithium's, so its chemical behaviour also resembles lithium's. Potassium repeats this pattern in the next period.

This recurrence of similar properties is why the table is called periodic. Arranging elements by atomic number reveals a repeating pattern of outer-electron arrangements, and elements with similar properties line up in groups.

Using position to predict reactions

The periodic table lets you make reasoned predictions, rather than treating every element as an unrelated substance. Its broad pattern also separates metals, mainly on the left and towards the bottom, from non-metals, mainly towards the upper right.

Group 1: the alkali metals. Lithium, sodium and potassium each have one outer electron, which they lose in reactions. All react with water to form hydrogen and a metal hydroxide. Sodium, for example, forms hydrogen and sodium hydroxide. Their shared group position helps predict the same type of reaction.

They are not equally reactive: reactivity increases down Group 1, so potassium reacts more vigorously with water than sodium, and sodium more vigorously than lithium. Down the group there are more occupied shells. The outer electron is farther from the nucleus and more shielded by inner electrons, so it is less strongly attracted and easier to remove.

Group 7: the halogens. These non-metals have seven outer electrons and can gain one electron to complete their outer shell. They react with metals to form metal halides; sodium and chlorine, for example, form sodium chloride.

Here, reactivity decreases down the group. An incoming electron is attracted less strongly as the outer shell becomes farther from the nucleus and shielding increases. Chlorine is therefore more reactive than bromine or iodine.

This allows a specific prediction: chlorine will displace bromine from potassium bromide solution, forming potassium chloride and bromine. Bromine will not displace chlorine from potassium chloride solution because bromine is less reactive.

Group 0: the noble gases. Their full outer shells make them stable and very unreactive. Neon, 2,8, and argon, 2,8,8, are therefore unlikely to react under ordinary conditions.

Group position predicts similarities in reactions; position within a group helps predict differences in reactivity. The link running through both is the arrangement of electrons.

From atomic number to electronic structure

• Atomic number = number of protons = number of electrons in a neutral atom.
• Electrons occupy the lowest available energy levels first.
• For the first 20 elements, fill shells using 2,8,8, then start the fourth shell.
• Numerical notation lists electrons from the innermost occupied shell outwards: sodium 2,8,1, potassium 2,8,8,1, calcium 2,8,8,2.

From electronic structure to position

• Elements are arranged by increasing atomic number.
• Period: number of occupied shells.