Water, ammonia, methane and larger covalent structures
| English | 中文 | Pinyin |
|---|---|---|
| polymer/ˈpɒlɪmə/ | 聚合物 | jù hé wù |
| repeat unit/rɪˈpiːt ˈjuːnɪt/ | 重复单元 | chóng fù dān yuán |
What would explain this observation?
- A water molecule has two bonds but also two lone pairs on oxygen. A displayed line formula makes connections easy to see while leaving some electron information out.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- In H₂O, oxygen shares one pair with each of two hydrogen atoms and retains two lone pairs. In NH₃, nitrogen shares one pair with each of three hydrogens and retains one lone pair. In CH₄, carbon shares four pairs, one with each hydrogen, and has no lone pairs in the outer-electron diagram. Each hydrogen attains two shell electrons; carbon, nitrogen and oxygen count eight around themselves.
- polymer 聚合物: A substance containing very large molecules built from repeated units; repeat unit 重复单元: The group of atoms repeated along a polymer molecule.
Which description of ammonia’s outer-electron diagram is correct?
Covalent substances can consist of small molecules, very large molecules such as polymers, or giant networks such as diamond and silicon dioxide. A polymer has many repeated units linked along each molecule: [–CH₂–CH₂–]ₙ represents poly(ethene), with bonds passing through brackets and n large. A giant network diagram shows only a small part of continuing covalent connections, not a separate molecule with that exact pictured atom count.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Covalent substances can consist of small molecules, very large molecules such as polymers, or giant networks such as diamond and silicon dioxide. A polymer has many repeated units linked along each molecule: [–CH₂–CH₂–]ₙ represents poly(ethene), with bonds passing through brackets and n large. A giant network diagram shows only a small part of continuing covalent connections, not a separate molecule with that exact pictured atom count.
- For electron diagrams, use one mark type for the central atom’s electrons and another for hydrogen’s. Count bonds, lone pairs and all outer electrons. For a molecular formula, count each element in the complete stated molecule. For a polymer, identify the bracketed repeat rather than calling the drawing a small molecule. Ball-and-stick drawings enlarge gaps and do not fix scale; a 2D line formula omits the real 3D arrangement.
Which two habits make the investigation or model in this case more defensible?
For electron diagrams, use one mark type for the central atom’s electrons and another for hydrogen’s. Count bonds, lone pairs and all outer electrons. For a molecular formula, count each element in the complete stated molecule. For a polymer, identify the bracketed repeat rather than calling the drawing a small molecule. Ball-and-stick drawings enlarge gaps and do not fix scale; a 2D line formula omits the real 3D arrangement.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: methane has 4+4×1=8 outer electrons in total, all in four shared pairs. Ammonia also has 5+3×1=8: six shared electrons and a two-electron lone pair. A poly(ethene) model containing fifty C₂H₄ repeat units contains 100 carbon and 200 hydrogen atoms in those repeat units; chain-end details are excluded from that simplified count.
A simplified polymer contains 30 C₂H₄ repeat units. How many carbon atoms are in those units? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
A simplified polymer contains 30 C₂H₄ repeat units. How many carbon atoms are in those units?
The result is 60 atoms. Known: methane has 4+4×1=8 outer electrons in total, all in four shared pairs. Ammonia also has 5+3×1=8: six shared electrons and a two-electron lone pair. A poly(ethene) model containing fifty C₂H₄ repeat units contains 100 carbon and 200 hydrogen atoms in those repeat units; chain-end details are excluded from that simplified count.
Check the conclusion and its limits
- A lone pair is not a spare hydrogen or an additional bond. Do not apply molecular formula counting to a clipped giant network without understanding its continuation. The number n is a count of repeat units, not a charge or an electron shell number. Polymer formation is developed further in Organic chemistry.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Every covalently bonded substance is made from small separate molecules. This claim is false: A lone pair is not a spare hydrogen or an additional bond. Do not apply molecular formula counting to a clipped giant network without understanding its continuation. The number n is a count of repeat units, not a charge or an electron shell number. Polymer formation is developed further in Organic chemistry.
Water, ammonia, methane and larger covalent structures: Covalent substances can consist of small molecules, very large molecules such as polymers, or giant networks such as diamond and silicon dioxide. A polymer has many repeated units linked along each molecule: [–CH₂–CH₂–]ₙ represents poly(ethene), with bonds passing through brackets and n large. A giant network diagram shows only a small part of continuing covalent connections, not a separate molecule with that exact pictured atom count.
Every covalently bonded substance is made from small separate molecules.
A lone pair is not a spare hydrogen or an additional bond. Do not apply molecular formula counting to a clipped giant network without understanding its continuation. The number n is a count of repeat units, not a charge or an electron shell number. Polymer formation is developed further in Organic chemistry.
A substance containing very large molecules built from repeated units: write the technical term.
polymer means A substance containing very large molecules built from repeated units.