Final answer:
Group 1 elements in the periodic table all have one valence electron. This is a periodic trend and can be determined by looking at the number of the group they belong to.
Step-by-step explanation:
Group 1 elements all have the same number of valence electrons, specifically one valence electron. An easy way to find this is to look at the number of the group in the periodic table. For example, hydrogen and the alkali metals, which make up Group 1, all have one valence electron.
This is consistent across the table, with alkali metals in Group 2 having two valence electrons, and this trend continues with other groups accordingly. Therefore, for Group 1 elements, you would complete the statement with the understanding that Group 1 elements have one valence electron.