Two elements, A and B, have the following configurations—A: 2, 8, 5 B: 2, 8, 7 (i) Which element is more reactive? (ii) Will A and B form ionic or covalent bonds when they combine? Explain using electron transfer or sharing. (iii) Predict the formula of the compound they would form.
Element B is more reactive because it has 7 valence electrons and needs only one more electron to complete its octet, whereas element A has 5 valence electrons and needs three more electrons to achieve stability. The closer an element is to achieving a stable electronic configuration, the more reactive it is.
A and B will form covalent bonds when they combine. Both elements have more than 4 valence electrons, so they tend to gain or share electrons rather than donate them. Since A needs 3 electrons and B needs 1 electron, neither can easily transfer electrons to the other. Instead, they share electrons to complete their octets, forming a covalent bond.
The formula of the compound formed is AB₃. Since element A needs 3 electrons to complete its octet and element B needs only 1 electron, one atom of A shares its 3 electrons with 3 atoms of B. Each B atom shares 1 electron with A, satisfying the octet of all atoms.
Explanation
This question tests understanding of electronic configuration, reactivity trends, and chemical bonding. Element B (2,8,7) is more reactive as it is just one electron away from a stable noble gas configuration. Both A and B are non-metals with more than 4 valence electrons, so they share electrons rather than transfer them, forming covalent bonds. The formula AB₃ is derived from the fact that A needs 3 electrons (shares with 3 B atoms) and each B needs 1 electron (shares with 1 A atom).