The periodic table is arranged by atomic number
Definition
Periodic table
A table in which elements are arranged by increasing atomic number and placed in groups with other elements that have similar chemical properties.
- The elements are placed in order of increasing atomic number, which is the number of protons.
- They are arranged so that elements with similar chemical properties line up in the same column.
- Metals are on the left and centre; non-metals are on the right.
- The pattern that repeats across the table is why it is called periodic.
Key Idea
- Order is set by atomic number, not by mass.
- Elements in the same column behave in similar ways.
Groups: columns with the same number of outer electrons
Definition
Group
A vertical column of the periodic table containing elements with the same number of outer electrons and similar chemical properties.
- A group is a vertical column of the periodic table.
- The group number tells you the number of electrons in the outer shell of each atom.
- Elements in the same group react in similar ways, because chemical reactions depend on the outer electrons.
- For example, every Group 1 element has 1 outer electron, and every Group 7 element has 7 outer electrons.
Note
Because they share the same outer electron count, group members show trends you can predict.
Periods: rows showing the number of shells
Definition
Period
A horizontal row of the periodic table, whose number shows the number of occupied electron shells in an atom.
- A period is a horizontal row of the periodic table.
- The period number tells you the number of occupied electron shells in each atom.
- Moving one step across a period, the atomic number increases by one, adding one proton and one electron.
- The added electrons fill the same outer shell until it is full, which is why properties change gradually across a period.
Self review
- What decides the order of the elements in the periodic table?
- Answer: increasing atomic number (number of protons).
- What does the group number tell you?
- Answer: the number of electrons in the outer shell.
- What does the period number tell you?
- Answer: the number of occupied electron shells.
- Why do elements in the same group react similarly?
- Answer: they have the same number of outer electrons.
