Water, ammonia, methane and larger covalent structures
Diperkenalkan| English | Makna |
|---|---|
| polymer/ˈpɒlɪmə/ | polimer |
| repeat unit/rɪˈpiːt ˈjuːnɪt/ | unit berulang |
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.
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?
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Nitrogen has five outer electrons; three go into shared pairs with the three hydrogens, leaving one lone pair. Four shared pairs would give NH4+, and ammonia forms by sharing, never by ionic transfer.
Match each technical term to its precise meaning.
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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.
In the lab: 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?
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Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- 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.
Your turn: A simplified polymer contains 30 C₂H₄ repeat units. How many carbon atoms are in those units?
A simplified polymer contains 30 C₂H₄ repeat units. How many carbon atoms are in those units?
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The result is 60 atoms. Known: 30 C2H4 repeat units, each containing 2 carbon atoms. Carbon atoms = 30 x 2 = 60 atoms. The repeat-unit count multiplies the unit's contents; chain ends are excluded in this model.
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.
Not true: Every covalently bonded substance is made from small separate molecules. Diamond, graphite and silica are covalently bonded yet form giant networks extending through the whole crystal. Covalent bonding describes the bond type, not the size of the structure.
- In H₂O, oxygen shares one pair with each of two hydrogen atoms and retains two lone pairs.
- Covalent substances can consist of small molecules, very large molecules such as polymers, or giant networks such as diamond and silicon dioxide.
- Watch for: A lone pair is not a spare hydrogen or an additional bond.
Diamond, graphite and silica are covalently bonded yet form giant networks extending through the whole crystal.
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True. Diamond, graphite and silica are covalently bonded yet form giant networks extending through the whole crystal. Covalent bonding describes the bond type, not the size of the structure.
A substance containing very large molecules built from repeated units: write the technical term.
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polymer means A substance containing very large molecules built from repeated units.