(i) State the number of elements in period 1, period 2, and period 3 of the periodic table.
(ii) Name the elements in period 1.
(iii) What happens to the atomic size of elements on moving from left to right in a period?
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Number of elements
(b) The elements in period 1 are hydrogen and helium.
(c) The atomic size of elements decreases on moving from left to right in a period. As the atomic number increases, effective nuclear charge also increases; this overcomes the electron–electron repulsion in the atom.