To determine the formula of the compound formed when element X (atomic number 12) combines with element Y (atomic number 17), we need to consider their valence electrons.
Element X has an atomic number of 12, which means it has 12 protons and 12 electrons. The electronic configuration of element X is 2, 8, 2. Therefore, it has two valence electrons.
Element Y has an atomic number of 17, which means it has 17 protons and 17 electrons. The electronic configuration of element Y is 2, 8, 7. Therefore, it has seven valence electrons.
In chemical reactions, elements tend to gain, lose, or share electrons to achieve a stable electron configuration. Since element X has two valence electrons and element Y has seven valence electrons, it is likely that element X will lose its two valence electrons, and element Y will gain one electron to achieve a stable configuration.
When element X loses two electrons, it forms a cation with a charge of +2, represented as X^2+. When element Y gains one electron, it forms an anion with a charge of -1, represented as Y^-.
Now, to balance the charges, we need to combine two cations (X^2+) with three anions (Y^-) to obtain an electrically neutral compound.
Thus, the formula of the compound formed between element X and element Y is X2Y3.