Decide which element probably forms a compound with oxygen that has a chemical formula most and least similar to the chemical formula of the compound formed by oxygen and barium.

The chemical formula for the compound formed by oxygen and barium is BaO. This is because barium is in Group 2 of the periodic table and therefore has a valency of +2, while oxygen is in Group 16 and has a valency of -2. When these two elements combine, they form a compound with a 1:1 ratio, hence the formula BaO.

To find an element that forms a compound with oxygen with a similar chemical formula, we should look for another element with a valency of +2. Elements in Group 2 of the periodic table, such as magnesium (Mg), calcium (Ca), or strontium (Sr), would form similar compounds with oxygen (MgO, CaO, SrO).

To find an element that forms a compound with oxygen with a least similar chemical formula, we should look for elements with different valencies. For example, elements in Group 1 of the periodic table, such as lithium (Li), sodium (Na), or potassium (K), have a valency of +1 and would form compounds with oxygen with a 2:1 ratio (Li2O, Na2O, K2O). Similarly, elements in Group 15, such as nitrogen (N) or phosphorus (P), have a valency of -3 and would form compounds with oxygen with a 2:3 ratio (N2O3, P2O3).