The Periodic Table
How the Periodic Table is arranged and the trends in Group I, Group VII, the transition elements and the noble gases.
📌 Key points
- Elements are arranged in order of increasing proton number. Groups (columns) have the same number of outer-shell electrons and similar properties; periods (rows) have the same number of occupied shells.
- Metals are on the left and non-metals on the right; across a period the elements change from metallic to non-metallic.
- Group I (alkali metals) are soft, have low densities and react with water to form an alkaline hydroxide and hydrogen. Down the group reactivity and density increase, and melting point decreases.
- Alkali metals become more reactive down the group because the outer electron is further from the nucleus, so it is lost more easily.
- Group VII (halogens) are diatomic non-metals. At r.t.p. chlorine is a pale yellow-green gas, bromine a red-brown liquid and iodine a grey-black solid. Down the group reactivity decreases, and density and melting point increase.
- A more reactive halogen displaces a less reactive halide from its solution, e.g. Cl2 + 2KBr → 2KCl + Br2.
- Transition elements have high densities and high melting points, form coloured compounds, act as catalysts (e.g. iron in the Haber process) and can have ions with variable oxidation numbers.
- Noble gases (Group VIII) are monatomic and unreactive because they have a full outer shell. Helium is used in balloons; argon is used in lamps.
- Trends can be used to predict the properties of other elements in a group, e.g. rubidium reacts more vigorously with water than potassium.
📖 Key terms
- Group
- A vertical column of elements with the same number of outer-shell electrons.
- Period
- A horizontal row of elements with the same number of occupied electron shells.
- Alkali metal
- A Group I metal that reacts with water to form an alkaline solution.
- Halogen
- A Group VII element; halogens exist as diatomic molecules.
- Transition element
- A metal from the central block: dense, high melting point, coloured compounds, catalyst.
- Noble gas
- A Group VIII element: monatomic and unreactive due to a full outer shell.
- Displacement reaction
- A more reactive element takes the place of a less reactive one in a compound.
✏️ Worked example
Rubidium is below potassium in Group I. Predict how rubidium reacts with water and write the equation.
- Reactivity increases down Group I, so rubidium is more reactive than potassium.
- It reacts very vigorously (possibly explosively), fizzing as hydrogen is given off.
- An alkaline solution of rubidium hydroxide forms.
- Equation: 2Rb + 2H2O → 2RbOH + H2.
Describe what you would see: "fizzing", "floats", "moves around", "melts into a ball". Writing "hydrogen is produced" is a conclusion, not an observation.
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