The periodic table arranges the elements in order of atomic number. Rows are called periods and columns are called groups. Elements in the same group have similar chemical properties because they have the same number of electrons in their outer shell. Metals are on the left and in the middle; non-metals are on the right.
Group 1, the alkali metals, are soft and very reactive, and they get more reactive down the group: lithium fizzes in water, but potassium bursts into flame. Group 7, the halogens, are reactive non-metals that get less reactive down the group. Group 0, the noble gases, are very unreactive because their outer shells are full. Mendeleev left gaps for undiscovered elements and predicted their properties.
Worked example
Sodium is in Group 1 and Period 3. How many outer electrons does it have?
- The group number for Groups 1–7 gives the number of outer electrons.
- Sodium is in Group 1.
Answer:So it has 1 outer electron.
Key words
- group
- a column; similar properties
- period
- a row
- alkali metals
- Group 1: soft, reactive metals
- halogens
- Group 7: reactive non-metals
- noble gases
- Group 0: unreactive gases
New questions every time.
NGSS-referenced: MS-PS1-1