Group Properties
Resumen
In a school preparation room there is a jar of soft grey metal sitting under a layer of oil, and the oil is not there to keep it clean. It is there because the metal cannot be left alone with the air, let alone with water. A few columns away, in a fume cupboard, sits a bottle of dense red-brown liquid that stains everything it touches and attacks the lungs of anyone careless with the stopper. These two substances behave as unlike each other as any pair of elements can, yet the reason for both is the same, and it is written into their position on the chart.
This lesson is about what changes as you travel down a column. Group 1 gets more violent the further you go; Group 7 gets calmer. That looks contradictory until you see what the two trends have in common, at which point one sentence about outer electrons explains both at once. Along the way you will learn to predict the behaviour of elements too reactive to be handed to you in a bottle, and to read the outcome of a reaction from nothing more than a colour change in a test tube. It is also one of the most heavily examined topics on the chemistry paper, so the effort pays back quickly.
Objetivos
- The elements in Group 1 of the periodic table (known as the alkali metals): are metals with low density (the first three elements in the group are less dense than water); react with non-metals to form ionic compounds in which the metal ion carries a charge of +1. The compounds are white solids that dissolve in water to form colourless solutions; react with water, releasing hydrogen; form hydroxides that dissolve in water to give alkaline solutions.
- In Group 1, the further down the group an element is, the more reactive the element.
- The elements in Group 7 of the periodic table (known as the halogens) react with metals to form ionic compounds in which the halide ion carries a charge of -1.
- In Group 7, the further down the group an element is: the less reactive the element; the higher its melting point and boiling point.
- A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt.
- The trends in reactivity within groups in the periodic table can be explained because the higher the energy level of the outer electrons: the more easily electrons are lost; the less easily electrons are gained. Students should be able to explain the relative reactivities of the elements in Group 1 and 7.
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Nota de la lección
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- Which statement about the elements of Group 1 is correct? A. They form ions with a charge of -1. B. They become less reactive going down the group. C. They react with water to release hydrogen. D. Their compounds are coloured solids. Answer: C
- Lithium, sodium and potassium all float when added to water. This is because they: A. have a density less than that of water B. have low melting points C. are held up by the hydrogen produced D. do not dissolve in water Answer: A
- Going down Group 7, the melting point of the elements: A. increases and the reactivity increases B. increases and the reactivity decreases C. decreases and the reactivity increases D. decreases and the reactivity decreases Answer: B
- Chlorine water is added to potassium bromide solution. Which observation is expected? A. No change, because bromine is more reactive than chlorine. B. The solution turns orange-brown as bromine is displaced. C. A white precipitate of potassium chloride forms. D. Hydrogen gas is given off. Answer: B
- Why is potassium more reactive than sodium? A. A potassium atom has more outer electrons than a sodium atom. B. A potassium atom has a lower density than a sodium atom. C. The outer electron of potassium is in a higher energy level and is lost more easily. D. A potassium atom gains an electron more easily than a sodium atom. Answer: C
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