Problem 48 - Chemistry: An Atoms-Focused Approach

Question

Does each atom in a pair that is covalently bonded always contribute the same number of valence electrons to form the bonds between them?

Step-by-Step Solution

Answer

Explain your answer with examples. Answer: No, two atoms that form a covalent bond do not always contribute the same number of valence electrons. While equal sharing of electrons is common in covalent bonds, such as in hydrogen gas (H_2) or oxygen gas (O_2), there are cases where atoms contribute an unequal number of valence electrons. This happens in coordinate covalent bonds (also known as dative bonds), like the one formed between boron trifluoride (BF_3) and ammonia (NH_3), where nitrogen contributes both electrons while boron contributes none.

Step 1: Understanding Covalent Bonding

Covalent bonding is the sharing of electron pairs between atoms. In a covalent bond, two atoms share one or more pairs of electrons to complete their valence electron shell, which results in a more stable configuration. In most cases, both atoms involved in a covalent bond contribute an equal number of electrons to form the electron pairs being shared.

Step 2: Considering Equal Contribution of Valence Electrons

In many cases, atoms contribute the same number of valence electrons to form covalent bonds. For example, in hydrogen gas (H_2), each hydrogen atom contributes one electron to create a single covalent bond between them. Similarly, in an oxygen molecule (O_2), each oxygen atom contributes two electrons to form a double covalent bond. In both cases, the electrons being shared are equal between the two atoms.

Step 3: Examining Cases with Unequal Contribution of Valence Electrons

However, not all covalent bonds involve an equal sharing of electrons. In some cases, one atom may contribute more valence electrons than the other. This occurs in a coordinate covalent bond (also known as a dative bond). An example of this type of bond is the bond formed between boron trifluoride (BF_3) and ammonia (NH_3). In this case, boron has only 6 valence electrons, and it can readily accept a pair of electrons to complete its octet. On the other hand, nitrogen in ammonia has a lone pair of electrons it can contribute to form a bond with boron. Here, nitrogen contributes both electrons to the dative covalent bond while boron contributes none.

Step 4: Conclusion

In summary, while it is common for atoms in a covalently bonded pair to contribute an equal number of valence electrons to form the bonds between them, unequal electron contributions can also occur in some cases, such as coordinate covalent bonds.