The periodic table: groups 1, 7 and 0
The periodic table is arranged so that elements with similar properties sit in the same column. If you know why, you can predict how an element behaves without memorising every reaction.
Part 1 of 3: Learn it
In short
- Elements are in order of atomic number; groups are columns and periods are rows.
- Elements in a group have the same number of outer electrons, so they react in similar ways.
- Group 1 gets more reactive down the group; group 7 gets less reactive down the group.
Where this is in your specification
Spec points: AQA 4.1.2.1 to 4.1.3, Edexcel Topics 1 and 6, OCR C2.1
| Board | Topic: Atomic structure and the periodic table |
|---|---|
| AQA 8462 | 4.1 (8462); 5.1 (8464) |
| Edexcel 1CH0 | Topics 1, 2 and 6 |
| OCR J248 | C1 and C2.1 |
How the table is arranged
- Elements are arranged in order of atomic number (protons).
- The group number is the number of electrons in the outer shell (for groups 1 to 7); group 0 has a full outer shell.
- The period number is the number of electron shells.
- Metals are on the left and in the middle; non-metals are on the right. Metals form positive ions, non-metals tend to form negative ions.
Mendeleev left gaps for elements not yet discovered and predicted their properties. When those elements were found and matched his predictions, his table was accepted. Ordering by atomic number came later, once protons were discovered.
Group 1: the alkali metals
Lithium, sodium and potassium are soft metals with one outer electron. They react with water to form a metal hydroxide (an alkaline solution) and hydrogen:
Reactivity increases down the group: the outer electron is further from the nucleus and more shielded, so it is lost more easily. Potassium reacts more violently than sodium, which reacts more than lithium.
Group 7: the halogens
Fluorine, chlorine, bromine and iodine exist as diatomic molecules (Cl₂). Down the group, melting and boiling points increase and reactivity decreases: the atoms are bigger, so it is harder to gain the extra electron.
A halogen higher up group 7 pushes a lower one out of its compound in solution: this is a displacement reaction.
Group 0: the noble gases
Helium, neon, argon and the rest have full outer shells, so they are very unreactive and exist as single atoms. Their boiling points increase down the group.
Why do elements in the same group react in similar ways?
Show the answer
They have the same number of electrons in their outer shell.
Part 2 of 3: See it worked
Worked examples
Example 1
Bromine water is added to potassium iodide solution. Predict what happens and write the word equation.
- Bromine is above iodine in group 7, so it is more reactive
- Bromine displaces iodine from the salt
- The solution turns brown as iodine forms
Answer: bromine + potassium iodide → iodine + potassium bromide. The iodine set free turns the solution brown.
Example 2
Rubidium is below potassium in group 1. Predict how it reacts with water.
- Same group, so the same products: a hydroxide and hydrogen
- Further down group 1, so more reactive than potassium
Answer: It reacts even more violently than potassium, forming rubidium hydroxide and hydrogen.
Common mistakes
- Saying group 7 gets more reactive down the group, like group 1. It is the other way round.
- Writing chlorine as Cl instead of Cl₂ in an equation.
- Saying noble gases are unreactive because they have 8 outer electrons. Helium has 2: the point is that the shell is full.
- Expecting bromine to displace chlorine. A less reactive halogen cannot displace a more reactive one.
What gas is made when lithium reacts with water?
Show the answer
Hydrogen.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Why do elements in the same group react in similar ways?
They have the same number of electrons in their outer shell.
What gas is made when lithium reacts with water?
Hydrogen.
Will iodine displace bromine from potassium bromide?
No: iodine is less reactive than bromine.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Estos apuntes están en inglés porque siguen los programas de examen del Reino Unido.
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