To determine in which group of the periodic table an element that forms a 1- ion would be found, consider the common properties of the elements within each group.
1. Group 1 (Alkali Metals): Elements in this group, such as sodium (Na) and potassium (K), typically lose one electron to form a 1+ ion. They do not usually form 1- ions.
2. Group 2 (Alkaline Earth Metals): Elements here, such as magnesium (Mg) and calcium (Ca), typically lose two electrons to form 2+ ions. Again, these elements do not form 1- ions.
3. Group 17 (Halogens): Elements in Group 17 include fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). These elements have seven valence electrons and quite easily gain one additional electron to complete their outer shell, forming a 1- ion.
4. Group 18 (Noble Gases): Elements in Group 18, such as helium (He), neon (Ne), and argon (Ar), have a complete valence shell and are generally inert. They do not typically form ions, let alone 1- ions.
Given this information, the elements that form a 1- ion are found in:
Group 17.
Therefore, the correct answer is:
A. 17
