Resource 4: Resource for classifying elements
Background information / subject knowledge for teacher
Elements
The first elements to be discovered were gold, silver, carbon and sulfur. This is because they occur naturally and are relatively unreactive. Gradually, as people became more interested in science, more and more elements were discovered. Chemists were keen to classify the elements and to understand the similarities and differences between them. They wanted to understand why some are reactive and some are unreactive. All sorts of suggestions were made, but there were always exceptions to the rule and none of the systems suggested were very helpful.
In 1869 a Russian scientist called Dimitrich Mendeleev came up with yet another suggestion. He placed the elements in order of increasing mass. Other people had also done this but it hadn’t worked very well. However, Mendeleev had what proved to be a brilliant idea. First of all, he realised that some of the elements were actually quite similar to each other: lithium and sodium for example, and bromine and iodine. He changed the order slightly so the elements which were similar formed a column. This meant that some of the elements were not all in order of increasing mass. He also realised that some of the elements might not yet have been discovered. So he left some gaps in his table. Furthermore, he made some predictions about some of the elements that had not been discovered. He predicted that there would be an element between silicon and tin, and was able to give quite a bit of detail about what he thought this element would be like. A few years later, germanium was discovered and it turned out that Mendeleev had been right! His predictions about its properties were very accurate. Much later, when scientists discovered the proton, they found that Mendeleev had put the elements in order of increasing atomic number.
Mendeleev was obviously a clever scientist but it was his creativity that led to this significant discovery.
Information for Activity 2
Make as many sets of 20 cards as you can, with information about the first 20 elements. (You could print off and cut out the ones below). Each card should contain:
symbol
atomic number
electron arrangement
mass number
appearance
state at room temperature
reactivity.
Give each group a set of cards and ask them to devise a way of classifying them. It does not matter if they don’t come up with the ‘right’ answer – the important thing is that they think about how you might classify elements. Some will sort them into solids, liquids and gases; some will sort them into metals and non-metals. Some might even group them according to reactivity. It is important that you let them devise their own method.
Hydrogen H Atomic No: 1 Mass No: 1 Electron arrangement: 1 Appearance: colourless, odourless State at room temperature: gas Reactivity: reactive; reacts explosively with oxygen | Helium He Atomic No: 2 Mass No: 4 Electron arrangement: 2 Appearance: colourless, odourless State at room temperature: gas Reactivity: completely unreactive | Lithium Li Atomic No: 3 Mass No: 7 Electron arrangement: 2,1 Appearance: soft, silvery metal State at room temperature: solid Reactivity: reactive; discolours in air, reacts with cold water, stored in oil | Beryllium Be Atomic No: 4 Mass No: 9 Electron arrangement: 2,2 Appearance: white, grey metal State at room temperature: solid Reactivity: does not appear reactive owing to a protective, layer of oxide |
Boron B Atomic No: 5 Mass No: 11 Electron arrangement: 2,3 Appearance: brown, black State at room temperature: solid Reactivity: chemically inert; only reacts with hot, concentrated acids | Carbon C Atomic No: 6 Mass No: 12 Electron arrangement: 2,4 Appearance: dark grey slippery solid, black powder or glass-like gem stone (diamond) State at room temperature: solid Reactivity: reacts with air if heated | Nitrogen N Atomic No: 7 Mass No: 14 Electron arrangement: 2,5 Appearance: colourless, odourless State at room temperature: gas Reactivity: unreactive; reacts with oxygen if heated with a platinum catalyst | Oxygen O Atomic No: 8 Mass No: 16 Electron arrangement: 2,6 Appearance: colourless, odourless State at room temperature: Gas Reactivity: reactive; reacts with metals and non-metals – sometimes requires heat |
Fluorine F Atomic No: 9 Mass No: 19 Electron arrangement: 2,7 Appearance: pale yellow, pungent smell State at room temperature: gas Reactivity: very reactive; can etch glass | Neon Ne Atomic No: 10 Mass No: 20 Electron arrangement: 2,8 Appearance: colourless, odourless State at room temperature: colourless, odourless Reactivity: completely unreactive | Sodium Na Atomic No:11 Mass No: 23 Electron arrangement: 2,8,1 Appearance: verysoft, silvery metal State at room temperature: solid Reactivity: very reactive; stored in oil, tarnishes rapidly in air, reacts with water (melts) | Magnesium Mg Atomic No: 12 Mass No: 24 Electron arrangement: 2,8,2 Appearance: silvery grey metal State at room temperature: solid (often kept as ribbon) Reactivity: reacts vigorously with air when heated, slowly with cold water, vigorously with steam |
Aluminium Al Atomic No: 13 Mass No: 27 Electron arrangement: 2,8,3 Appearance: shiny silver metal State at room temperature: solid Reactivity: tarnishes in air, forms a protective layer | Silicon Si Atomic No: 14 Mass No: 28 Electron arrangement: 2,8,4 Appearance: grey, shiny, solid State at room temperature: solid Reactivity: unreactive | Phosphorous P Atomic No: 15 Mass No: 31 Electron arrangement: 2,8,5 Appearance: Two forms: red phosphorous (powder) and white Phosphorous (pale grey solid – can be cut with a knife) State at room temperature: solid Reactivity: white phosphorous ignites in air and has to be stored in water; red phosphorous is unreactive | Sulphur S Atomic No: 16 Mass No: 32 Electron arrangement: 2,8,6 Appearance: yellow State at room temperature: solid Reactivity: burns when heated in air; reacts with metals when heated |
Chlorine Cl Atomic No: 17 Mass No: 35 or 37 Electron arrangement: 2,8,7 Appearance: green, yellowy, pungent smell. State at room temperature: gas Reactivity: reactive; reacts with metals, especially if heated | Argon Ar Atomic No: 18 Mass No: 40 Electron arrangement: 2,8,8 Appearance: colourless, odourless State at room temperature: gas Reactivity: completely unreactive | Potassium K Atomic No: 19 Mass No: 39 Electron arrangement: 2,8,8,1 Appearance: extremely soft, silvery metal State at room temperature: solid Reactivity: stored in oil, tarnishes in air, catches fire when it reacts with water | Calcium Ca Atomic No: 20 Mass No: 40 Electron arrangement: 2,8,8,2 Appearance: light grey metal State at room temperature: solid Reactivity: tarnishes in air, reacts with air on heating |
Resource 3: Differentiating Work
