Periodic Table: Test Your Understanding
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Which property of an element on the periodic table determines its identity?
The atomic number, or number of protons in the nucleus, determines the identity of an element.
Across periods, do elements become steadily more reactive, less reactive, or is there no continuous pattern
to the reactivity? Explain.
Sample answer: Reactivity is not continuous across a period. The group 18 noble gases are unreactive,
compared to the highly reactive group 17 halogens. The number of valence electrons increases across a
period; a full valence shell (group 18) confers different chemical properties from one that is almost full
(group 17).
Critical Thinking: The atoms of chlorine easily gain one electron and react with metals to form ionic
compounds. Where on the periodic table would a similar element be located?
Because similarly reacting elements occur in the same family or group, a similar element will be located
within the same column as the element described.
