Group 1: reactions of lithium, sodium and potassium
| English | Français |
|---|---|
| alkali metal/ˈælkəlaɪ ˈmetl/ | alkali metal |
| shielding/ˈʃiːldɪŋ/ | shielding |
What would explain this observation?
- Lithium, sodium and potassium all have one outer electron. Comparable reactions become more vigorous down the group even though proton number also increases.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Group 1 alkali metals 碱金属 have one outer electron and form +1 ions by losing it. With water they produce a metal hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂. Lithium fizzes and moves on water; sodium also melts into a moving ball; potassium can ignite with a lilac flame in a controlled demonstration. Solutions become alkaline. With chlorine they form white metal chlorides, for example 2Na + Cl₂ → 2NaCl.
- alkali metal: A Group 1 metal with one electron in its outer shell; shielding 屏蔽作用: Reduction in the attraction experienced by outer electrons due to inner electrons.
Why does Group 1 reactivity increase down the group?
With oxygen, these metals tarnish and burn to form oxygen-containing solids; lithium burns crimson, sodium yellow and potassium lilac. The simple oxide model uses 4Li + O₂ → 2Li₂O; sodium and potassium burning products can include peroxide or superoxide, so do not invent a single oxide formula for every demonstration. Reactivity increases Li→Na→K because the outer electron is farther from the nucleus and more shielded by inner shells, so it is lost more easily despite increased nuclear charge.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- With oxygen, these metals tarnish and burn to form oxygen-containing solids; lithium burns crimson, sodium yellow and potassium lilac. The simple oxide model uses 4Li + O₂ → 2Li₂O; sodium and potassium burning products can include peroxide or superoxide, so do not invent a single oxide formula for every demonstration. Reactivity increases Li→Na→K because the outer electron is farther from the nucleus and more shielded by inner shells, so it is lost more easily despite increased nuclear charge.
- Interpret teacher demonstrations or supplied observations under school risk assessment. Alkali-metal water reactions need approved very small quantities, protective screens and trained handling; students should not independently scale them up. Compare the same reaction conditions and distinguish qualitative vigour from a measured rate. Oxygen and chlorine demonstrations require their own approved controls.
Which two habits make the investigation or model in this case more defensible?
Interpret teacher demonstrations or supplied observations under school risk assessment. Alkali-metal water reactions need approved very small quantities, protective screens and trained handling; students should not independently scale them up. Compare the same reaction conditions and distinguish qualitative vigour from a measured rate. Oxygen and chlorine demonstrations require their own approved controls.
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: 2K + 2H₂O → 2KOH + H₂ shows two potassium atoms reacting per hydrogen molecule in the particle model. Six potassium atoms correspond to three H₂ molecules and six KOH formula units. This is a stoichiometric model; it is not instructions for preparing hydrogen or a quantitative mole calculation.
In 2K + 2H₂O → 2KOH + H₂, how many H₂ molecules correspond to ten K atoms? Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
In 2K + 2H₂O → 2KOH + H₂, how many H₂ molecules correspond to ten K atoms?
The result is 5 molecules. Known: 2K + 2H₂O → 2KOH + H₂ shows two potassium atoms reacting per hydrogen molecule in the particle model. Six potassium atoms correspond to three H₂ molecules and six KOH formula units. This is a stoichiometric model; it is not instructions for preparing hydrogen or a quantitative mole calculation.
Check the conclusion and its limits
- The larger nuclear charge alone does not explain the down-group trend: distance and shielding matter. Group 1 reactions are not all identical in appearance. A purple flame is not evidence that potassium chloride itself is a purple solid, and a water-reaction equation must include hydrogen.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
The outer electron becomes harder to remove down Group 1. This claim is false: The larger nuclear charge alone does not explain the down-group trend: distance and shielding matter. Group 1 reactions are not all identical in appearance. A purple flame is not evidence that potassium chloride itself is a purple solid, and a water-reaction equation must include hydrogen.
Group 1: reactions of lithium, sodium and potassium: With oxygen, these metals tarnish and burn to form oxygen-containing solids; lithium burns crimson, sodium yellow and potassium lilac. The simple oxide model uses 4Li + O₂ → 2Li₂O; sodium and potassium burning products can include peroxide or superoxide, so do not invent a single oxide formula for every demonstration. Reactivity increases Li→Na→K because the outer electron is farther from the nucleus and more shielded by inner shells, so it is lost more easily despite increased nuclear charge.
The outer electron becomes harder to remove down Group 1.
The larger nuclear charge alone does not explain the down-group trend: distance and shielding matter. Group 1 reactions are not all identical in appearance. A purple flame is not evidence that potassium chloride itself is a purple solid, and a water-reaction equation must include hydrogen.
A Group 1 metal with one electron in its outer shell: write the technical term.
alkali metal means A Group 1 metal with one electron in its outer shell.