Metals occupy the left and centre of the periodic table; non-metals are mainly on the right. Metals generally conduct electricity and heat, are malleable, and tend to lose electrons in reactions. Many non-metals conduct poorly and form negative ions or covalent bonds. These are broad patterns: graphite conducts despite being a non-metal. A table position helps make predictions but does not replace a stated property.
Group 1 alkali metals are reactive metals with one outer electron. Group 17 halogens are reactive non-metals with seven outer electrons. Group 18 noble gases have full outer shells and usually low reactivity. Similarities within a group arise partly from outer-electron structure. Reactivity is not identical for every group member, so distinguish a shared pattern from a claim that every property is exactly the same.
Example 1
An unknown element is malleable and conducts electricity. Which broad classification is most consistent?
These are typical metallic properties, so a metal is the best supported classification. One property alone may have exceptions.
Example 2
Why do chlorine and bromine show related chemistry?
They are both group−17 halogens with seven outer electrons. This shared outer-electron pattern helps explain related reactions.
Watch out: They share chemical patterns, but properties such as state, density and reactivity can differ.