Covalent bonds hold atoms together in a molecule primarily because they do what at the level of shared electrons and atomic shells?

Introduction / Context:
Covalent bonding is one of the two main types of chemical bonding studied in basic chemistry, the other being ionic bonding. Understanding why covalent bonds hold atoms together is essential for explaining the structures and properties of molecules such as water, methane and oxygen gas. This question asks you to look a little deeper than the simple phrase share electrons and to think about how shared electrons help fill valence shells and how electrostatic attractions between electrons and nuclei stabilise the bond.

Given Data / Assumptions:

• The question is about covalent bonds, where atoms share pairs of electrons.
• The options mention filling shells, bringing electrons closer to protons in nuclei and the forces involved.
• We are considering ordinary covalent bonds at typical chemical energy scales.
• The focus is on why the bond is stable and why the atoms remain attached.

Concept / Approach:
A covalent bond forms when two atoms share one or more pairs of electrons. This sharing allows each atom to achieve a more stable electron configuration, often resembling the filled valence shell of a noble gas. The shared electrons are attracted simultaneously by the positive charges of both nuclei, which lowers the overall potential energy of the system. Thus, covalent bonds hold atoms together because they both fill valence shells without creating highly charged ions and because the shared electrons are pulled toward both nuclei, creating a strong electrostatic attraction that keeps the atoms close together.

Step-by-Step Solution:
Step 1: Consider statement A. Sharing electrons lets atoms complete their valence shells without fully losing or gaining electrons, so they avoid large ionic charges. This is true. Step 2: Consider statement B. In a covalent bond, the shared electrons are located in the region between the nuclei and are attracted to the positively charged protons in both nuclei. This also is true. Step 3: Because both A and B describe real aspects of covalent bonding, they combine to explain why the bond is stable. Step 4: Statement D is too vague and does not clearly mention the role of shared electrons in lowering energy, so it is less precise than the combination of A and B. Step 5: Therefore the best answer is that both A and B together explain why covalent bonds hold atoms together.

Verification / Alternative check:
You can verify the reasoning by looking at simple molecules. In hydrogen gas, two hydrogen atoms share a pair of electrons so that each effectively has two electrons in its first shell, similar to helium. The shared pair lies between the nuclei, where electrostatic attraction to both protons lowers the energy. In water, each O H bond involves a shared pair that helps give oxygen a stable octet and hydrogen a stable duet. Quantum mechanical calculations show that the energy of the bonded system is lower than that of separate atoms, confirming that electron sharing and electron nucleus attraction work together to stabilise the covalent bond.

Why Other Options Are Wrong:
- Fill valence shells without giving atoms large charge alone: This is part of the explanation but ignores the essential role of electrostatic attraction between electrons and nuclei.
- Bring electrons closer to protons alone: This is also part of the picture, but on its own it does not emphasise the stability gained by filled or nearly filled valence shells.
- Use forces between nuclei as well as forces between electrons: While interactions exist between all charged particles, the most important stabilising factor is the attraction between shared electrons and both nuclei, together with shell filling.
- Only attract electrons to one of the bonded atoms: This describes a highly polar or ionic situation, not a typical covalent bond in which electrons are shared between both atoms.

Common Pitfalls:
A common mistake is to think of covalent bonds as purely mechanical sticks between atoms, without linking them to electron configurations and electrostatic energy. Another pitfall is to confuse covalent and ionic bonds and assume that full electron transfer always occurs. Remember that covalent bonding is about sharing electrons so that atoms obtain stable valence shell configurations, while the attraction between shared electrons and both nuclei provides the binding force that holds the atoms together in a low energy arrangement.

Final Answer:
Covalent bonds hold atoms together because they both fill shells without giving atoms much charge and bring electrons closer to the nuclei, so the correct choice is Both A and B.