Group 7: molecules, compounds and opposite trends
| English | 中文 | Pinyin |
|---|---|---|
| halogen/ˈhælədʒn/ | 卤素 | lǔ sù |
| diatomic molecule/ˌdaɪəˈtɒmɪk ˈmɒlɪkjuːl/ | 双原子分子 | shuāng yuán zi fèn zǐ |
What would explain this observation?
- Chlorine is more reactive than iodine, but iodine has higher melting and boiling points. Chemical reactivity and physical state follow different explanations.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Group 7 halogens 卤素 are non-metals with seven outer electrons. Their elements consist of diatomic molecules 双原子分子 such as Cl₂, Br₂ and I₂. Near ordinary room conditions chlorine is a pale green gas, bromine a red-brown liquid and iodine a grey solid forming purple vapour when heated. Down the group, relative molecular mass, melting point and boiling point increase. Reactivity decreases chlorine→bromine→iodine.
- halogen: A Group 7 non-metal element with seven outer electrons; diatomic molecule: A molecule containing two atoms.
Which comparison is correct?
A halogen gains an electron to form a −1 halide ion when reacting with a metal, giving an ionic salt such as NaCl. With non-metals it generally forms covalent compounds through shared electron pairs, such as HCl. Down the group the outer shell is farther from the nucleus and more shielded, reducing attraction for an incoming electron despite greater nuclear charge. This is the opposite reactivity trend to Group 1, which loses an electron.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- A halogen gains an electron to form a −1 halide ion when reacting with a metal, giving an ionic salt such as NaCl. With non-metals it generally forms covalent compounds through shared electron pairs, such as HCl. Down the group the outer shell is farther from the nucleus and more shielded, reducing attraction for an incoming electron despite greater nuclear charge. This is the opposite reactivity trend to Group 1, which loses an electron.
- Compare supplied molecular drawings, states and reaction data without handling halogens independently. Chlorine and bromine require teacher-approved containment and ventilation; appearance cards can provide the required evidence safely. Use the same conditions when comparing reactivity. From a trend predict whether an unfamiliar halogen is more or less reactive, and avoid fabricating an exact boiling point.
Which two habits make the investigation or model in this case more defensible?
Compare supplied molecular drawings, states and reaction data without handling halogens independently. Chlorine and bromine require teacher-approved containment and ventilation; appearance cards can provide the required evidence safely. Use the same conditions when comparing reactivity. From a trend predict whether an unfamiliar halogen is more or less reactive, and avoid fabricating an exact boiling point.
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: using supplied relative atomic masses Cl=35.5 and Br=80, relative molecular masses are Cl₂=2×35.5=71 and Br₂=2×80=160. Bromine has the larger molecular mass, while chlorine is the more reactive element. Neither value has units of grams, and each refers to the diatomic molecule.
Using iodine Ar=127, calculate the relative molecular mass of I₂. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Using iodine Ar=127, calculate the relative molecular mass of I₂.
The result is 254 . Known: using supplied relative atomic masses Cl=35.5 and Br=80, relative molecular masses are Cl₂=2×35.5=71 and Br₂=2×80=160. Bromine has the larger molecular mass, while chlorine is the more reactive element. Neither value has units of grams, and each refers to the diatomic molecule.
Check the conclusion and its limits
- A halogen element and its halide ion have different electron counts and properties. Chlorine, Cl₂, is not written as Cl⁻. A rising boiling point does not establish a rising rate of electron gain. Solid sodium chloride is white even though chlorine gas is green.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
A halogen with a higher boiling point must be more chemically reactive. This claim is false: A halogen element and its halide ion have different electron counts and properties. Chlorine, Cl₂, is not written as Cl⁻. A rising boiling point does not establish a rising rate of electron gain. Solid sodium chloride is white even though chlorine gas is green.
Group 7: molecules, compounds and opposite trends: A halogen gains an electron to form a −1 halide ion when reacting with a metal, giving an ionic salt such as NaCl. With non-metals it generally forms covalent compounds through shared electron pairs, such as HCl. Down the group the outer shell is farther from the nucleus and more shielded, reducing attraction for an incoming electron despite greater nuclear charge. This is the opposite reactivity trend to Group 1, which loses an electron.
A halogen with a higher boiling point must be more chemically reactive.
A halogen element and its halide ion have different electron counts and properties. Chlorine, Cl₂, is not written as Cl⁻. A rising boiling point does not establish a rising rate of electron gain. Solid sodium chloride is white even though chlorine gas is green.
A Group 7 non-metal element with seven outer electrons: write the technical term.
halogen means A Group 7 non-metal element with seven outer electrons.