Arranging the Elements

Section 1 of 6How the table is ordered

Atomic number

• Atomic number Elements are arranged in order of increasing atomic number, so each element has one more proton than the one before it.
• Atomic number is unique to each element, like a fingerprint, and does not change in a chemical reaction.

Groups

• Groups Groups are the vertical columns, numbered 1 to 7 and then 0.
• Elements in a group have the same number of electrons in their outer shell, which equals the group number.

Periods

• Periods Periods are the horizontal rows, numbered 1 to 7.
• The period number equals the number of electron shells in the atoms of the elements in that row.

Why 'periodic'

• Why 'periodic' Similar properties recur at regular intervals, so elements with similar behaviour fall in the same column.

Section 2 of 6Using group number

1Group number

• Group number The group tells you how many electrons are in the outer shell.

2Outer electrons decide reactions

• Outer electrons decide reactions The number of outer electrons controls how an atom gains, loses or shares electrons.

3Similar reactions

• Similar reactions Elements in the same group react in similar ways, so one member can be used to predict the others.

4Trends

• Trends Comparing several elements in a group lets you spot trends, such as increasing reactivity down Group 1.

Section 3 of 6Reading the Periodic Table

Feature	What it tells you	Example
Group number	Outer electrons	Group 2 atoms have 2 outer electrons
Period number	Number of shells	Period 3 atoms have 3 shells
Atomic number	Number of protons (and electrons in an atom)	Atomic number 11 is sodium
Group 0	Full outer shell, unreactive	Helium, neon, argon

Section 4 of 6Predicting a reaction

1

Find the element on the Periodic Table and note its group.

2

Identify the number of outer electrons from the group number.

3

Look at what is known about another element in the same group, such as lithium, sodium and potassium in Group 1, which all form ionic compounds with Group 7 elements.

4

Predict that your element will react in a similar way, with the speed of reaction changing in line with the trend down the group.

5

Remember the exception: helium is in Group 0 but has only 2 outer electrons, because it has just one shell.

Section 5 of 6Key terms

Term	Meaning
Atomic number	The number of protons in an atom; the order of the elements in the Periodic Table.
Group	A vertical column of elements with the same number of outer electrons.
Period	A horizontal row of elements with the same number of occupied shells.
Outer shell	The highest-energy occupied shell of an atom, containing the electrons involved in reactions.
Periodic	Occurring at regular intervals, used because properties repeat down the table.

Section 6 of 6Exam technique

1

Confusing groups and periods. Groups go down in columns; periods go across in rows.

2

Saying the table is ordered by relative atomic mass. The modern table is ordered by atomic number.

3

Stating that Group 0 elements have 0 outer electrons. They have full outer shells, with 8 (or 2 for helium).

4

Assuming elements in the same period behave alike. It is elements in the same group that react similarly.

5

Thinking atomic number changes when an atom forms an ion. Only the number of electrons changes.