What you'll learn
Group 1, the alkali metals, are among the most reactive metals and show clear trends down the group. In this guide you will learn the properties of the alkali metals, how they react with water, oxygen and chlorine, how and why their reactivity increases down the group, and how electronic structure explains their behaviour. These ideas illustrate how the periodic table groups elements with similar properties and predictable trends.
Key terms and definitions
Group 1 / alkali metals — the metals in the first column of the periodic table (lithium, sodium, potassium, etc.).
Reactivity — how readily an element takes part in chemical reactions.
Outer shell electron — the electron in the highest occupied energy level, which determines chemical behaviour.
Shielding — the way inner shells of electrons reduce the attraction between the nucleus and the outer electron.
Ion — a charged particle formed by losing or gaining electrons; Group 1 metals form 1+ ions.
Hydroxide — a compound containing OH⁻; alkali metal hydroxides are alkaline.
Core concepts
Properties of the alkali metals
The alkali metals (lithium, sodium, potassium, and below) are soft (can be cut with a knife), have low densities (lithium, sodium and potassium float on water), and have relatively low melting points for metals. They are stored under oil because they react quickly with air and water. Each has one electron in its outer shell, so they all form 1+ ions and have similar chemical properties.
Reaction with water
Alkali metals react vigorously with water to produce a metal hydroxide and hydrogen gas:
alkali metal + water → metal hydroxide + hydrogen
For example, sodium + water → sodium hydroxide + hydrogen. The metal floats and fizzes as hydrogen is released, and the solution becomes alkaline (the hydroxide). The reactions become more vigorous down the group — lithium fizzes steadily, sodium melts into a ball and moves rapidly, and potassium reacts so fast it ignites with a lilac flame.
Reaction with oxygen and chlorine
Alkali metals react with oxygen to form metal oxides (e.g. sodium → sodium oxide), tarnishing quickly in air. They react with chlorine to form white metal chloride salts (e.g. sodium + chlorine → sodium chloride). These reactions also get more vigorous down the group.
Trend in reactivity down the group
Reactivity increases down Group 1. This is because each element down the group has its outer electron in a shell further from the nucleus, with more inner shells shielding it. The attraction between the nucleus and the outer electron is therefore weaker, so the outer electron is lost more easily — making the metal more reactive. (This is the opposite of the trend in Group 7.)
Why they form 1+ ions
Each alkali metal has one outer electron, which it loses to gain a stable, full outer shell, forming a 1+ ion. This is why they all behave similarly and form compounds with the same formulae types (e.g. Na⁺, K⁺).
Worked examples
Example 1: Reaction with water
Write the word equation for potassium reacting with water.
Potassium + water → potassium hydroxide + hydrogen. The solution is alkaline and hydrogen gas is released.
Example 2: Explaining the trend
Why is potassium more reactive than sodium?
Potassium's outer electron is in a shell further from the nucleus with more shielding, so it is held less strongly and lost more easily, making potassium more reactive.
Example 3: Predicting a product
What forms when lithium reacts with chlorine?
Lithium chloride (LiCl), a white solid, since Group 1 metals react with chlorine to form metal chlorides.
Common mistakes and how to avoid them
Getting the trend backwards. Reactivity increases down Group 1 (unlike Group 7, where it decreases).
Forgetting hydrogen in the water reaction. Products are metal hydroxide + hydrogen.
Saying the solution is acidic. Alkali metal hydroxides make the solution alkaline.
Not explaining the trend properly. Mention distance from nucleus, shielding and weaker attraction on the outer electron.
Wrong ion charge. Group 1 metals form 1+ ions by losing one electron.
Exam technique for Group 1
State key properties — soft, low density, one outer electron, 1+ ions.
Write reactions with water, oxygen and chlorine, naming the products.
Explain the reactivity trend using distance, shielding and attraction.
Describe observations (fizzing, floating, flame colours) for the water reactions.
Link one outer electron to similar properties across the group.
Quick revision summary
Group 1, the alkali metals (lithium, sodium, potassium…), are soft, have low densities and low melting points, and each has one outer-shell electron, so they form 1+ ions and have similar properties. They react with water to give a metal hydroxide + hydrogen (an alkaline solution), with oxygen to give metal oxides, and with chlorine to give white metal chlorides. Reactivity increases down the group: lithium reacts steadily, sodium melts and skims across the water, and potassium ignites with a lilac flame. This is because, further down, the outer electron is in a shell further from the nucleus with more shielding, so it is less strongly attracted and lost more easily. They form 1+ ions by losing their single outer electron to gain a full outer shell. Remember: the trend is the opposite of Group 7. State the properties, write the reactions with their products, describe the observations, and explain the increasing reactivity using distance, shielding and weaker attraction.