# 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 HAtomic No: 1Mass No: 1Electron arrangement: 1Appearance: colourless, odourlessState at room temperature: gasReactivity: reactive; reacts explosively with oxygen Helium HeAtomic No: 2Mass No: 4Electron arrangement: 2Appearance: colourless, odourlessState at room temperature: gasReactivity: completely unreactive Lithium LiAtomic No: 3Mass No: 7Electron arrangement: 2,1Appearance: soft, silvery metalState at room temperature: solidReactivity: reactive; discolours in air, reacts with cold water, stored in oil Beryllium BeAtomic No: 4Mass No: 9Electron arrangement: 2,2Appearance: white, grey metalState at room temperature: solidReactivity: does not appear reactive owing to a protective, layer of oxide Boron BAtomic No: 5Mass No: 11Electron arrangement: 2,3Appearance: brown, blackState at room temperature: solidReactivity: chemically inert; only reacts with hot, concentrated acids Carbon CAtomic No: 6Mass No: 12Electron arrangement: 2,4Appearance: dark grey slippery solid, black powder or glass-like gem stone (diamond)State at room temperature: solidReactivity: reacts with air if heated Nitrogen NAtomic No: 7 Mass No: 14Electron arrangement: 2,5Appearance: colourless, odourlessState at room temperature: gasReactivity: unreactive; reacts with oxygen if heated with a platinum catalyst Oxygen OAtomic No: 8Mass No: 16Electron arrangement: 2,6Appearance: colourless, odourlessState at room temperature: GasReactivity: reactive; reacts with metals and non-metals – sometimes requires heat Fluorine FAtomic No: 9Mass No: 19Electron arrangement: 2,7Appearance: pale yellow, pungent smellState at room temperature: gasReactivity: very reactive; can etch glass Neon NeAtomic No: 10Mass No: 20Electron arrangement: 2,8Appearance: colourless, odourlessState at room temperature: colourless, odourlessReactivity: completely unreactive Sodium NaAtomic No:11Mass No: 23Electron arrangement: 2,8,1Appearance: verysoft, silvery metalState at room temperature: solidReactivity: very reactive; stored in oil, tarnishes rapidly in air, reacts with water (melts) Magnesium MgAtomic No: 12Mass No: 24Electron arrangement: 2,8,2Appearance: silvery grey metalState at room temperature: solid (often kept as ribbon)Reactivity: reacts vigorously with air when heated, slowly with cold water, vigorously with steam Aluminium AlAtomic No: 13Mass No: 27Electron arrangement: 2,8,3Appearance: shiny silver metalState at room temperature: solidReactivity: tarnishes in air, forms a protective layer Silicon SiAtomic No: 14Mass No: 28Electron arrangement: 2,8,4Appearance: grey, shiny, solidState at room temperature: solidReactivity: unreactive Phosphorous PAtomic No: 15Mass No: 31Electron arrangement: 2,8,5Appearance: Two forms: red phosphorous (powder) and white Phosphorous (pale grey solid – can be cut with a knife)State at room temperature: solidReactivity: white phosphorous ignites in air and has to be stored in water; red phosphorous is unreactive Sulphur SAtomic No: 16Mass No: 32Electron arrangement: 2,8,6Appearance: yellowState at room temperature: solidReactivity: burns when heated in air; reacts with metals when heated Chlorine ClAtomic No: 17Mass No: 35 or 37Electron arrangement: 2,8,7Appearance: green, yellowy, pungent smell.State at room temperature: gasReactivity: reactive; reacts with metals, especially if heated Argon ArAtomic No: 18Mass No: 40Electron arrangement: 2,8,8Appearance: colourless, odourlessState at room temperature: gasReactivity: completely unreactive Potassium KAtomic No: 19Mass No: 39Electron arrangement: 2,8,8,1Appearance: extremely soft, silvery metal State at room temperature: solidReactivity: stored in oil, tarnishes in air, catches fire when it reacts with water Calcium CaAtomic No: 20Mass No: 40Electron arrangement: 2,8,8,2Appearance: light grey metalState at room temperature: solidReactivity: tarnishes in air, reacts with air on heating

Resource 3: Differentiating Work

Resource 5: Mining Tantalum – a controversial issue