Groups 1, 7 and 0

Three groups of the periodic table show clear patterns. The alkali metals (group 1) are soft, reactive metals that get more reactive going down. The halogens (group 7, or group 17 in the 1–18 numbering) are non-metals that exist as two-atom molecules and get less reactive going down. The noble gases (group 0, or group 18) have full outer shells and hardly react at all.

Key facts

  • Group 1 metals have 1 outer electron, which they lose to form 1+ ions. Reactivity increases down the group because the outer electron is further from the nucleus and more easily lost.
  • Alkali metals react with water to make a metal hydroxide and hydrogen; the solution is alkaline. Lithium fizzes; sodium melts into a ball; potassium burns with a lilac flame.
  • Group 7 elements have 7 outer electrons and gain one to form 1− ions. They exist as diatomic molecules: F2, Cl2, Br2, I2.
  • Down group 7, reactivity decreases, and melting and boiling points increase: chlorine is a green gas, bromine a red-brown liquid, iodine a grey-black solid.
  • A more reactive halogen displaces a less reactive one from a solution of its salt: chlorine + potassium bromide → potassium chloride + bromine.
  • Noble gases have full outer shells, so they are very unreactive and exist as single atoms. Their boiling points increase down the group.
  • Uses: helium in balloons (light and does not burn), argon in light bulbs and welding (unreactive), neon in signs.

Worked examples

Example 1

Write the word and balanced symbol equations for sodium reacting with water, and say what you would see.

  1. An alkali metal and water make the metal hydroxide and hydrogen: sodium + water → sodium hydroxide + hydrogen
  2. Balance the symbol equation: two Na give two NaOH, which need two H2O; the four H atoms leave one H2. 2Na + 2H2O → 2NaOH + H2
  3. Sodium is less dense than water, so it floats; it fizzes (hydrogen), melts into a ball and whizzes about. Universal indicator turns purple: the solution is alkaline.

Answer: sodium + water → sodium hydroxide + hydrogen; 2Na + 2H2O → 2NaOH + H2; it fizzes, melts into a ball and moves about

Example 2

Chlorine water is added to potassium iodide solution. What happens and why?

  1. Reactivity in group 7: chlorine > bromine > iodine.
  2. Chlorine is more reactive, so it takes iodine's place: chlorine + potassium iodide → potassium chloride + iodine
  3. Iodine dissolved in water is brown, so the colourless solution turns brown. Cl2 + 2KI → 2KCl + I2

Answer: the solution turns brown as iodine forms; chlorine is more reactive than iodine and displaces it

Example 3

Use the trends to predict the state of astatine (below iodine) and how caesium (below rubidium) reacts with water.

  1. Down group 7 the halogens go from gas (F, Cl) to liquid (Br) to solid (I), and get darker. Astatine comes next: a black solid.
  2. Down group 1 the metals get more reactive. Caesium is below rubidium, so it reacts even more violently.
  3. The trends work because each group has the same number of outer electrons, with one more shell each step down.

Answer: astatine: a black solid; caesium: reacts even more violently than rubidium (explosively)