C4.1 What are the patterns in the properties of elements
Elements are the ‘building blocks’ of all materials – the atoms of each element have a different proton number. The elements are arranged in order of ascending atomic (or proton) number, this is the number of positive protons in each atom.
Putting elements in this order gives a repeating pattern of their properties.
Knowledge of chemical facts began to grow in the 18th and 19th centuries – as a result scientists tried to find patterns in order to stop themselves being overwhelmed by the mass of information and to provide a basis for understanding the facts. Many of the early attempts of classification were dismissed by the scientific community as more information emerged.
Three of the most significant developments were made by Döbereiner, Newlands and Mendeleev.
During, the period of the 18th century actual masses could not be measured so they were compared against the mass of hydrogen (the lightest) – this is called relative atomic mass. It is now measured relative to 1/12 the mass of a carbon atom.
The periodic table gives us a great deal of information about each of the elements. Firstly the name and symbol of each element are shown
The mass number is above the element –the total number of protons and neutrons in the atom
The atomic number (proton number) is shown below each element – proton = electrons
A vertical column of elements is called a group.
Elements in the same group have the same number of electrons on their outer shell (except helium). This number also matches with the group number. For example group 1 elements have one electron in their outer shell and group 7 elements have seven electrons in their outer shell.
Elements in the same group have similar properties
A horizontal row of elements is called a period.
The periods to which an element belongs corresponds to the number of shells of electrons it has. For example sodium (Na), aluminium (Al) and chlorine (Cl) all have three shells of electrons, so they are found in the third period.
Across each row, the elements on the left are metals, while those on the right are non-metals – the diving line is shown in red:
Because there are patterns in the way the elements are arranged in the periodic table, it can be used to predict their properties and interpret data
Group 1 – Alkali metals
Least reactive
Most reactive
There are six metals in Group 1
As we go down the group, the alkali metals become more reactive.
The alkali metals have low melting and boiling points compared to most other metals. The melting and boiling points decrease as we go down the group
The density of a substance is a measure of how much mass it has for its size. The alkali metals have low densities compared to most other metals (they feel lighter)
The alkali metals are very soft – lithium is the hardest alkali metal and they became softer as you go down the group.
The alkali metals are stored under oil because they react vigorously with water and oxygen. Their surfaces are shiny when freshly cut but tarnishes rapidly moist air due to reaction with oxygen – it will go dull and become covered in a layer of metal oxide. For example:
Lithium + Oxygen → Lithium Oxide
4Li(s) + O2(g) → 2Li2O(s)
Lithium, sodium and potassium float on top of cold water (due to their low density). The heat from the reaction is great enough to melt sodium and potassium into liquids. Lithium reacts gently, sodium more aggressively and potassium so aggressively it catches fire.
Alkali metals react with water to form hydrogen and an alkaline solution of a hydroxide with the formula MOH. For example:
Potassium + Water → Potassium Hydroxide + Hydrogen
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
All of the alkali metals react vigorously with chlorine gas – each reaction forms a colourless crystalline salt called metal chlorides. The reaction gets more violent as you move down group 1, showing how reactivity increases down the group. The formula is MCl for example:
Lithium + Chlorine → Lithium Chloride
2Li(s) + Cl2(g) → 2LiCl(s)
Some chemicals can harm us if they are not handled carefully – hazard symbols are used to identify the danger posed by each chemical and what care should be taken
The following hazard symbols are found on alkali metals and their compounds:
Therefore when working with Group 1 metals, the following precautions should be taken:
- Use small amounts of very dilute concentrations
- Wear safety glasses and use safety screens
- Avoid working near naked flames
Group 7 – The halogens
There are five non-metals in group 7
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The halogens have low melting points and boiling points – this is a typical property of non-metals. Fluorine has the lowest melting point and boiling point – the melting points and boiling points then increase as you go down the group.
The halogens become less reactive as you go down the group – Fluorine at the top of the group is the most reactive halogen.
The densities increase as we go down the group
Room temperature is usually taken as being 25˚C – at this temperature fluorine and chlorine are gases, bromine is a liquid and iodine and astatine are solids. There is a trend in state from gas to liquid to solid down the group.
At room temperature the halogens become darker as you go down the group. Fluorine is very pale yellow, Chlorine is yellow-green (as a gas chlorine is green), Bromine is a red-brown (as a gas bromine is brown). Iodine crystals are shiny purple – but easily turn into a dark purple vapour when they are warmed up.
All halogens consist of diatomic molecules (i.e. they only exist in pairs of atoms) e.g. Cl2, Br2 and I2
The reactivity of the halogens decreases as we move down the group – this can be seen by looking at displacement reactions. A more reactive halogen will displace a less reactive halogen from an aqueous solution of its salt.
E.g. When chlorine (as a gas or dissolved in water) is added to sodium bromide solution the chlorine takes the place of the bromine. Because chlorine is more reactive than bromine, it displaces bromine from sodium bromide:
Chlorine + sodium bromide → sodium chloride + bromine
Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)
When halogens react, they form compounds that are similar – the reactivity decreases as we go down the group. For example, this is seen clearly when halogens react with the alkali metals or iron – the reaction between chlorine and iron is more vigorous than that between iodine and iron.
The halogens carry the following hazard symbols
Therefore working with halogens, the following precautions should be taken:
- Wear safety goggles
- Work in a fume cupboard
- Make sure the room is well ventilated
- Use small amounts of very dilute concentrations
- Avoid working near naked flames
Need to know
- Hydrogen, H2
- Water, H2O
- Chlorine, Cl2
- Bromine, Br2
- Iodine, I2
Halide – a binary (dual) compound of a halogen with another element or group
Each of the group 1 halides has a formula with one symbol for the metal and one for the halogen. So for sodium chloride the formula in NaCl
