9. Atoms, Elements and Compounds

9. ATOMS, ELEMENTS AND COMPOUNDS

CONCEPT MAP

can be properties accounted for by

matter is made of

is mixture of can be can explain

two or more

two or more combined make

can be represented by can be

may have

has a particle a particle

INORGANIC

can be

9.1 MATTER AND MATERIALS

Aims:

To remind pupils that matter can exist in three states.
To make pupils aware that the materials or substances of which things are made may be natural or man-made.
To make pupils aware of the role of the chemical industry in the production of useful man-made materials and substances.
To remind pupils that the uses of materials depend on their properties.
To provide advance information to help pupils organise their ideas about the topics covered in this chapter.

Activities:

Discussion of the nature of matter, and of the distinction between natural and man-made materials or substances, based on pupils’ previous experience.
Demonstration/practical activity looking at and handling an exhibition, prepared by the teacher, of as wide a range as possible of different materials and substances.
Discussion of the relationship of the properties of materials to their uses.
Presentation of a brief general overview of the main topics to be studied in the remainder of the chapter.

Answers:

Q1. The following are man-made materials: plastic, glass, aluminium, paper, charcoal (usually man-made). Water, cotton, salt and haematite are all found in nature. (Cotton cloth is made by twisting natural fibres from the cotton plant into threads and then weaving the threads into a cloth. The cloth is man-made but the material, cotton, is still basically the same as the cotton that grew).
Q2. Substances found in many kitchens include: water, soap, washing powder, bleach, disinfectant, cooking oil, salt, vinegar, flour, milk, sugar, charcoal, kerosene (paraffin). (Foods and spices, and materials like wood, are not usually called substances).
Q3. (i) The property of metals that makes them suitable for electric wires is that they are good conductors of electricity. (ii) The property of plastics that makes them suitable for covering electric wires is that they are good insulators of electricity.
Q4. (i) Bio-chemistry is the chemistry of living organisms. A bio-chemist studies the chemistry of life processes such as respiration and photosynthesis. (ii) Organic chemistry is the study of substances found in, or made by, living organisms. An organic chemist might study starch as a substance (e.g. how to make it and how to make different things from it).

9.2 THE KINETIC THEORY

Aims:

To inform pupils that matter is made of tiny particles called atoms or molecules.
To inform pupils that molecules are small groups of atoms joined together.
To inform pupils of the kinetic theory model of matter as regards the arrangement and behaviour of particles in solids, liquids and gases.

Activities:

Discussion of the nature of matter leading to the idea of particles that are called atoms or molecules (small groups of atom joined together).
Presentation and discussion of the kinetic theory model for solids, liquids and gases.
Demonstration/practical activity with small spheres such as polystyrene balls, marbles or small fruits. These can be arranged in rows or stacks to represent solids, and shaken in the corner of a tilted tray to represent a liquid. A few shaken quite vigorously on a horizontal tray can represent a gas.

Answers:

Q1. The Ancient Greeks called atoms "things that can not be divided" because an atom is the smallest particle of matter.
Q2. A molecule is a small group of atoms joined together.
Q3. (i) The particles in a gas have the most energy. (ii) When water is heated until it boils the particles move faster and faster until they shoot out of the crowd and fly around on their own as a gas (see Module 9.3).

9.3 USING THE KINETIC MODEL TO EXPLAIN THINGS

Aims:

To remind pupils of the nature of scientific theories using the kinetic theory as an example.
To show pupils how the kinetic theory model can be used to explain familiar phenomena including: the characteristics of solids, liquids and gases, changes of state, diffusion in gases, dissolving and diffusion in liquids, the regular shape of crystals and thermal expansion.
To give pupils the opportunity to use the kinetic theory to explain the above phenomena, and also the compressibility of gases, in their own words.

Activities:

Discussion of the nature of scientific theories using the kinetic theory as an example.
Demonstration and discussion of each of the examples given in the text to show how the kinetic theory model can explain what happens. Demonstrations could include the following. Use pupils to represent particles (lined up in rows for a solid, jostling in a crowd to represent a liquid, and running around outside to represent a gas). Remove the top from a bottle of strong scent to demonstrate diffusion. Drop a crystal of potassium permanganate of other strongly coloured solid into a gas of water to demonstrate dissolving and diffusion in liquids. Stack marbles or small fruits to demonstrate the regular shape of crystals. Repeat demonstration from Module 7.5 to show thermal expansion and from Module 6.1 to show compressibility of gases.
Pupils try end-of-page questions and afterwards discuss answers with their peers and their teacher.

Answers:

Q1. The particle near the top of the box is evaporating because it has enough energy to leave the crowd.
Q2. Particles of the liquid solvent are moving. They bump into the solid solute. Some of the solute particles are knocked off the solid and by the solvent particles. The solute particles diffuse through the crowd of solvent particles.
Q3. Air and other gases are compressible (Module 6.1) because, in a gas, there are large spaces between the particles. Liquids and solids are almost incompressible because there is very little space between the particles.
Q4. I am a water molecule. I move around in a crowd of water molecules. I am always bumping intoand pushing past my neighbours who are moving around too. (i) When I am put in a freezer I lose almost all my energy. I cannot move about and have to stay in one place. I am lined up in rows with my neighbours. (ii) When I am heated in the kettle I get more and more energy. I move faster and faster and fly off on my own away from the crowd. All my neighbours are flying away on their own too.

9.4 ELEMENTS AND SYMBOLS

Aims:

To interest pupils in elements and their symbols.
To define and explain the terms atom and element.
To inform pupils of the names and symbols of some common elements.
To inform pupils that, in some common gaseous elements, pairs of atoms join together to form molecules, and to show them how to write the formulae for such molecules.

Activities:

Introduction and discussion of the idea of element and of atom as the smallest particle of an element.
Identifying elements already known to pupils. Looking at and discussing lists of elements, and actual examples of any elements available.
Learning the symbols for about 20 common elements (refer to requirements of local syllabus).
Discussing the nature of, and formulae for, simple molecules of the elements hydrogen, oxygen and nitrogen.

Answers:

Q1. The 2 liquid elements are mercury Hg, and bromine Br. The 11 gaseous elements are hydrogen H, nitrogen N, oxygen O, fluorine F, chlorine Cl, and the inert gases helium He, neon Ne, argon Ar, xenon Xe, krypton Kr, and radon Rn.
Q2. Lead is Pb, silver is Ag, gold is Au, and zinc is Zn.
Q3. The formula H2 stands for a molecule of hydrogen - it shows that the molecule contains 2 atoms of hydrogen joined together.
Q4. (i) In Module 6.12 iodine was used to test for starch; in Module 3.13 you read about the use of chlorine to kill micro-organisms in drinking water; in Module 3.5, blue copper sulphate was used to make solutions and crystals - copper sulphate contains copper, sulphur and oxygen all combined together. (ii) Carbohydrates such as starch and sugar are substances that contain carbon, hydrogen and oxygen combined together (Module 8.6). (iii) Proteins are substances that contain carbon, hydrogen, oxygen and nitrogen (Module 8.6).

9.5 METAL AND NON-METAL ELEMENTS

Aims:

To enable pupils to classify elements as metals and non-metals
To make pupils aware of the distinguishing physical properties of metals.
To make pupils aware of some common metals and their uses, and that these uses are related to relevant properties.
To make pupils aware of some common non-metal elements.

Activities:

Discussion of metal and non-metal elements and their general properties, starting from pupils' existing knowledge.
Looking at and handling samples of metal and non-metal elements.
Discussion of a few common metals and non-metals and their uses.

Answers:

Q1. Brittle means not flexible - incapable of bending very much. When something brittle breaks, it usually shatters into several pieces.
Q2. Sodium is Na, potassium is K, and silicon is Si.
Q3. (i) Steel is use in building bridges because it is strong. (ii) Zinc is used for galvanising iron because it protects the iron from rusting. (iii) Gold is used for making jewellery and coins because it does not corrode (and it looks nice and is valuable too!).

9.6 COMPOUNDS AND FORMULAE

Aims:

To introduce pupils to the ideas of compounds, molecules and formulae and make them familiar with a few common examples.
To make pupils aware that the properties of compounds are completely different from those of their constituent elements.
To inform pupils that compounds of metals consist of arrays of ions instead of molecules.

Activities:

Discussion of the ideas of compounds, molecules and formulae with reference to a few familiar examples.
Demonstration/practical activity with models of some of the molecules discussed.
Discussion of compounds of metals as exemplified by NaCl, including demonstration of a model of the NaCl crystal lattice if possible.
Practice in writing and interpreting formulae.

Answers:

Q1. (i) A compound is a substance that contains two or more elements combined together. (ii) A molecule is a small group of atoms joined together.
Q2. (i) 3 (ii) 45 (iii) there are no molecules in NaCl.
Q3. The properties of a compound are NOT related to the properties of the elements that it contains.

9.7 ORGANIC AND INORGANIC COMPOUNDS

Aims:

To enable pupils to distinguish between organic and inorganic compounds.
To give pupils some awareness of the huge range of organic compounds.
To introduce pupils to a few conventions used in naming simple inorganic compounds.
To make pupils familiar with the formulae of a few well known inorganic compounds.

Activities:

Discussion of the distinction between organic and inorganic compounds.
Discussion of the huge range of organic compounds.
Looking at samples of any available organic compounds.
Discussion of the naming of simple inorganic compounds and learning their formulae.
Looking at samples of any of the compounds named in the text, and similar compounds.
Exercises giving practice with names and formulae of a small range of simple inorganic compounds.

Answers:

Q1. (i) A carbohydrate is a compound containing carbon, hydrogen and oxygen only. (ii) The formula for table sugar is C12H22O11.
Q2. There are millions of carbon compounds because carbon atoms join with one another, and with a few other non-metal atoms, to form chains and rings.
Q3. (i) Silicon dioxide, SiO2 (ii) water, H2O
Q4. (i) Iron oxide is Fe2O3 and its common name is rust. (ii) Calcium carbonate is used for making carbon dioxide (see Modules 6.5/6) - it is also used as a stone for building and to manufacture cement. (iii) KNO3 is called potassium nitrate.

9.8 CHEMICAL CHANGES

Aim:

To make pupils aware of the nature of chemical change and help them distinguish between chemical change and other kinds of change.

Activities:

Discussion of the nature of chemical change, relating this to familiar examples of changes that are, and are not, chemical changes.
Demonstration or practical activity using examples given in the textbook if possible. Burning a piece of paper, observing the combustion, and examining the products would be useful as an introduction. (The orange crystals in activity 3 in the textbook are ammonium dichromate. This gives off steam and nitrogen on heating and leaves green chromium oxide. Solid iodine for activity 5 can be obtained by putting a few drops of ‘Betadine’ (povidone iodine solution) in a test tube, then leaving it to evaporate in a warm dry place).

Answers:

Q1. (i) In respiration the reactants are: food (carbohydrate) + O2, the products: CO2 + H2O. (ii) In photosynthesis the reactants are: CO2 + H2O, products: food (starch) + O2. (iii) In the preparation of CO2 the reactants are: CaCO3 + hydrochloric acid, products: solution + CO2.
Q2. Activities 1, 3 and 5 are chemical changes. In all these cases new substances are made. The new substances made are: Activity 1 - magnesium oxide, Activity 3 - smoke and a green powder, and Activity 5 - carbon and water (steam). In Activity 2 the zinc oxide changes colour but no new substance is made. In Activity 4 the iodine changes state twice – black solid to purple gas, then purple gas to black solid again - but no new substance is made.

9.9 PURE SUBSTANCES AND MIXTURES

Aims:

To summarise some of the ideas in this chapter by introducing pupils to a simple way of classifying matter.
To make pupils aware that most everyday materials are mixtures rather than pure elements or compounds.
To make pupils aware of some of the characteristics of mixtures and review some familiar ways of separating them.

Activities:

Discussion of the classification of matter as presented in the text book, relating to pupils' own knowledge.
Practical activity looking, with hand lenses, at mixtures such as rocks, sand and soil.
Demonstration/practical activity separating mixtures as described in the text book.
Discussion of the idea of 'pure' substances and some possible candidates.

Answers:

Q1. (i) Minerals are inorganic compounds found in rocks and soil. (ii) Components are the parts of anything such as a mixture or a machine. (iii) Bacteria are minute, unicellular organisms, some cause disease but many are useful. (iv) A solute is a substance that is dissolved, a solvent is the substance that dissolves it, and a solution is the resulting mixture. (v) The filtrate is the liquid that passes through a filter paper. (vi) Distillation is the process of evaporating a liquid and then condensing its vapour to obtain the liquid again, usually in a very pure state.
Q2. (i) Rice and sand are best separated using a sieve. The sieve holds the rice, but allows the sand to pass through. (ii) Salt can be obtained from the solution by evaporating the water. (iii) Pure water can be obtained from sea water by distillation.