Chemistry 4, 5, 6:
Chemists have arranged the chemical elements into a table called the periodic table. This helps us to make sense of the different properties of the elements and their compounds. It also helps us to predict how they will behave in different situations.
In the periodic table, the elements are arranged in order of proton number, also called atomic number. This is the number of positive protons in each atom. It’s written underneath each element. Dmitri Mendeleev put the known elements in order of relative atomic mass but left gaps for undiscovered elements. He also predicted what the properties of these undiscovered elements might be.
Another number is shown above it. This is the relative atomic mass of the element. It is a comparative measurement of the mass of one atom of the element. You can use it to see how much heavier an atom of one element is compared with an atom of another element.
The periodic table is divided into horizontal rows and vertical columns.
Across each row, the elements on the left are metals, while those on the right are non-metals.
Each column in the table contains elements with similar properties, called a group. Each has a group number, shown across the top of the table. So group 1 contains the elements lithium (Li) to francium (Fr), and group 7 contains the elements fluorine (F) to astatine (At).
Elements in the same group of the periodic table have similar properties. Group 1 elements are called the alkali metals. Group 7 elements are known as the halogens.
Group 1 appearance
Lithium, sodium and potassium are all soft metals that are easily cut with a scalpel or knife. The freshly cut surface is a shiny, silver colour, but this tarnishes quickly to a dull grey as the metal reacts with oxygen and water in the air. Pieces of such metals are stored in oil to prevent these reactions.
Handling alkali metals
Because the alkali metals are so reactive, care has to be taken when using them. They must not be touched because they will react with the water in sweat on the skin. Gloves may be used, and goggles should be worn.
Physical properties of the alkali metals:
Melting point
The alkali metals have low melting and boiling points compared to most other metals. Lithium has the highest melting point in the group (group 1). The melting points then decrease as you go down the group.
Boiling point
The boiling points of these alkali metals show a similar pattern to the melting points.
Density
The alkali metals have low densities compared with most other metals. Lithium has the lowest density in the group. The densities then generally increase as you go down the group.
Hardness
The alkali metals are very soft. Lithium is the hardest alkali metal and they become softer as you go down the group.
Reaction with cold water
All the alkali metals react vigorously with cold water. In each reaction, hydrogen gas is given off and the metal hydroxide is produced. The speed and violence of the reaction increases as you go down the group. This shows that the reactivity of the alkali metals increases as you go down group 1.
For example: Sodium
When sodium is added to water, it melts to form a ball that moves around on the surface. It fizzes rapidly, and the hydrogen produced may burn with an orange flame before the sodium disappears.
Sodium + water → sodium hydroxide + hydrogen
2Na(s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
Reaction with chlorine
All of the alkali metals react vigorously with chlorine gas. Each reaction produces a white crystalline salt. The reaction gets more violent as you move down group 1, showing how reactivity increases down the group.
Lithium- white powder is produced and this is the salt lithium chloride. Lithium + chlorine → lithium chloride. 2Li(s) + Cl2 (g) → 2LiCl(s)
Sodium- the sodium burns with a bright yellow flame. Clouds of white powder are produced. This is the salt sodium chloride.
The reaction of sodium with chlorine is similar to the reaction with lithium, but more vigorous.