Step-by-step explanation:
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To form ionic compounds we need that one of the elements is a metal and the other a nonmetal. In this way the polarities will be distributed.
Let's analyze each of the options:
1- iron and cholrine: forms ionic bonds because one of the elements is metal and the other is not metal.
2- oxygen and magnesium: forms ionic bonds because one of the elements is metal and the other is not metal.
3- fluorine and sodium: forms ionic bonds because one of the elements is metal and the other is not metal.
4- potassium and sulfur: forms ionic bonds because one of the elements is metal and the other is not metal.
5- nitrogen and iodine: both elements are non-metals, they do not form an ionic bond.
6- lithium and calcuim: the two elements are metals, they do not form an ionic bond.