Group 1: The Alkali Metals

Introduction

The first column of the periodic table holds some of the most striking elements in chemistry: metals so soft you can cut them with a knife, light enough to float on water, and so reactive that they are stored under oil. These are the alkali metals of Group 1. Their shared behaviour is a textbook example of how a single valence electron shapes an entire family of elements.

Core explanation

Members of the group. Group 1 contains lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr). Francium is extremely rare and radioactive, so little is known about it directly. Hydrogen is often placed at the top of the column but is a non-metal and is not an alkali metal.

One valence electron. Every alkali metal atom has one electron in its outer shell :

- Li: 2,1 - Na: 2,8,1 - K: 2,8,8,1

This lone electron is easily lost, so each alkali metal forms an ion with a 1+ charge (Li⁺, Na⁺, K⁺). Because they lose this electron so readily, the alkali metals are the most reactive metals of all.

Physical properties. The alkali metals are unusual metals:

Element Density (g/cm³) Melting point (°C) --- --- --- Lithium 0.53 181 Sodium 0.97 98 Potassium 0.86 63 Rubidium 1.53 39 Caesium 1.93 28

- They are soft and can be cut with a knife, becoming softer down the group. - They have low densities ; lithium, sodium and potassium are less dense than water. - They have low melting points , which decrease down the group. - A freshly cut surface is shiny, but it quickly tarnishes as the metal reacts with oxygen and moisture in the air. - Like all metals, they conduct heat and electricity well.

The general trend in density is an increase down the group, although potassium is slightly less dense than sodium.

Why so soft and low-melting? Each atom contributes only one electron to the sea of delocalised electrons, and the atoms are large. The metallic bonding is therefore weak compared with metals such as iron, so less energy is needed to melt them. Down the group the atoms get even larger, the attraction between the ions and the electron sea weakens further, and the melting point falls.

Characteristic reactions. The alkali metals react with oxygen to form oxides (for example Na₂O), with chlorine to form white ionic chlorides such as NaCl, and with water to form alkaline hydroxide solutions and hydrogen gas — the reaction that gives the group its name. All of these reactions become more vigorous down the group.

Safe storage. Because they react with air and water, alkali metals are stored under oil and handled only in small amounts with protective equipment. Rubidium and caesium react so violently that they are kept sealed in glass ampoules and are not used in school laboratories.

Step-by-step reasoning

To predict the formula of potassium bromide:

1. Potassium is in Group 1, so it has one valence electron and forms K⁺. 2. Bromine is in Group 17 and forms Br⁻. 3. One K⁺ balances one Br⁻. 4. The formula is KBr, a white ionic solid like other alkali metal halides.

Visual explanation

Draw the atoms of lithium, sodium and potassium one above another. Each has a single dot in its outermost ring, but the rings get larger down the column, so the lone electron sits further and further from the nucleus.

Real-world analogy

The alkali metals are like people carrying one balloon on a long string. The longer the string (the bigger the atom), the more easily a gust of wind takes the balloon away. Lithium's string is short; caesium's is very long.

Real-world example

Lithium's low density makes it ideal for rechargeable batteries in phones and electric cars: lithium-ion cells store a lot of energy for their mass. Sodium compounds are everywhere, from table salt (NaCl) to baking soda (NaHCO₃), and yellow sodium-vapour street lamps rely on the characteristic glow of excited sodium atoms.

Why?

Why are they called alkali metals? When they react with water, they form hydroxides such as sodium hydroxide, NaOH, which dissolve to give strongly alkaline solutions. Their oxides are also strongly basic, reflecting their highly metallic character.

Common misconception

"Sodium is found as a shiny metal in nature." Alkali metals are far too reactive to exist uncombined. They occur only in compounds, such as sodium chloride in seawater and rock salt, and the pure metals are produced industrially by electrolysis.

Worked example

Question: Rubidium lies below potassium in Group 1. Predict its melting point relative to potassium's, its charge in compounds and the formula of rubidium chloride.

Reasoning: Melting points decrease down Group 1, so rubidium melts below 63 °C (the actual value is about 39 °C). Like all alkali metals, it has one valence electron and forms Rb⁺. Chlorine forms Cl⁻, so one of each is needed.

Answer: Lower melting point than potassium; Rb⁺; RbCl.

Quick check

1. How many electrons are in the outer shell of a sodium atom? Answer: One.

Exam focus

Learn the trends: down Group 1, melting point decreases, softness and reactivity increase, and density generally increases. Be ready to explain properties using the single valence electron and weak metallic bonding, and to predict properties of rubidium and caesium.

Advanced insight

Alkali metal ions give characteristic flame colours: lithium crimson, sodium yellow-orange and potassium lilac. Energy from the flame promotes an electron to a higher energy level, and light of a specific colour is emitted when it falls back. Caesium was discovered in 1860 through a pair of blue lines in its spectrum, and its name comes from the Latin for "sky blue".

Summary

The alkali metals in Group 1 each have one valence electron, which they lose easily to form 1+ ions. They are soft, low-density metals with low melting points that fall down the group, and they tarnish quickly in air. They react with oxygen, chlorine and water, becoming more reactive down the group, and are stored under oil for safety.

Practice questions

1. Why are alkali metals stored under oil? Answer: To stop them reacting with oxygen and water vapour in the air. 2. Describe the trend in melting point down Group 1 and explain it. Answer: Melting point decreases, because the atoms get larger and the metallic bonding between the ions and the delocalised electrons becomes weaker. 3. What is the charge on a lithium ion? Explain. Answer: 1+, because a lithium atom loses its single valence electron. 4. Give two ways in which alkali metals are unlike typical metals. Answer: They are soft enough to cut with a knife, and some have densities lower than water (they also have low melting points).