According to basic bonding theory, what types of atoms usually combine to form covalent bonds by sharing pairs of electrons?

Introduction / Context:
Chemical bonding is central to understanding how substances form and behave. This question focuses on covalent bonding, one of the two major bonding types in introductory chemistry. It asks which type of atoms normally form covalent bonds by sharing electrons rather than transferring them.

Given Data / Assumptions:
- We are looking for the usual, most common pattern in simple covalent bonding.
- Covalent bonds involve sharing pairs of electrons between atoms.
- Ionic bonds involve transfer of electrons, typically between metals and non metals.

Concept / Approach:
Non metals tend to have relatively high electronegativity and hold their valence electrons strongly. When two non metal atoms interact, neither readily loses or gains electrons to form ions. Instead they share electrons to achieve more stable electron configurations. This shared pair of electrons constitutes a covalent bond. In contrast, metals and non metals usually form ionic bonds, and metals with metals form metallic bonds rather than simple covalent bonds.

Step-by-Step Solution:
Step 1: Recall that covalent bonding is based on sharing electron pairs between atoms. Step 2: Remember that non metals such as hydrogen, oxygen, nitrogen, carbon, and halogens often form molecules like H2, O2, N2, CH4, HCl, and Cl2 through sharing electrons. Step 3: Recognise that when two non metals interact, both tend to gain electrons, so they compromise by sharing. Step 4: Metals and non metals usually form ionic compounds like NaCl or MgO, characterised by electron transfer rather than sharing. Step 5: Conclude that the typical covalent bond is formed between non metal and non metal atoms.

Verification / Alternative check:
Look at common covalent substances: water H2O, methane CH4, carbon dioxide CO2, ammonia NH3, and chlorine gas Cl2. In each case, only non metals are involved. Metals such as sodium or magnesium rarely appear in such simple covalent molecules; instead they form ionic solids. This pattern confirms that non metal and non metal is the correct combination for covalent bond formation in basic chemistry.

Why Other Options Are Wrong:
- Metal and metal: This combination leads to metallic bonding with a sea of delocalised electrons, not typical covalent bonds.
- Metal and non metal: This is the classic situation for ionic bonding, where electrons are transferred from metal to non metal, producing ions.
- Metal and noble gas: Noble gases are generally inert and rarely form bonds; this is not a typical covalent pairing.
- Any combination of atoms: This is too broad and incorrect, since not every combination leads to covalent sharing; different combinations give ionic or metallic bonds instead.

Common Pitfalls:
Learners sometimes generalise that all bonds are similar or that any atoms can share electrons equally. Another mistake is to assume that metals often make covalent bonds in simple binary compounds, which is not usually the case at this level. Remembering that covalent usually means non metal with non metal is a useful rule of thumb for introductory problems.

Final Answer:
Covalent bonds are usually formed when Non metal and non metal atoms share pairs of electrons.