CHAPTER 13 – ATOMS AND CHEMISTRY

CONCEPT MAP - ATOMS

1

INTRODUCTORY NOTES

Chapters 13 to 18 are aimed at students in the third or final year of junior secondary or middle school. For some students, this will be the last year in which they will study science. With this in mind, I have tried to cover those aspects of science and technology that I believe all citizens in a scientifically and technologically literate society ought to know and understand, or at least be aware of. For other students, this year will be a preparation for higher level studies in science and I have tried to keep their needs in mind too. Teachers should be aware that students who are not going to be future scientists may find some aspects of some topics to be difficult. For these students it will often be sufficient to aim forawareness of the ideas and phenomena concerned, rather than mastery of them, and to adopt a qualitative rather than a quantitative approach.

Chapter 13 will be the last chapter in this book to deal mainly with chemistry. For some students it will not be an easy chapter and Chapter 9 should be regarded as a pre-requisite. The first seven modules relate to atoms and are somewhat theoretical. It maynot be practicable for most teachers to provide hands-on activities for their students, however the use of clear drawings (on a board or on home-made wall charts) and simple models (for example with plasticine or small fruits joined with toothpicks) can help to make ideas easier to understand.Teachers should try to relate the work to students’ existing knowledge, experience and interests whenever possible.For example, students may have been X-rayed or may be aware of controversies regarding nuclear contamination and so on. Plenty of ongoing oral interaction with your students, and between studentsworking together in small groups, will help to clarify difficult ideas and make them more accessible.By contrast, the next three modules concerning acids and bases represent the sort of practical school chemistry familiar in traditional chemistry courses. All the suggested practical activities should either be carried out by the students themselves or demonstrated by the teacher. Finally, the last eight modules are intended to make students aware of the vital role that the chemistry of carbon plays in living things and of how the chemical industry usessubstances obtained from the Earth’s crust to manufacture the many useful materials that we take for granted in the modern world including fuels, plastics, metals, ceramics and concrete. A supplementary module(13.17) is not part of the program; it has been added as additional reading for students who may be interested in chemistry.

13.1ATOMIC STRUCTURE – PROTONS, NEUTRONS AND ELECTRONS

Aims:

  • Toreview relevant prior knowledge from Chapter 9 concerning elements and atoms.
  • To inform students about the basic structure of atoms and the nature of protons, neutrons and electrons.
  • To give students an overview of the coming chapter.

Activities:

  • Review previous work on elements and atoms, especially Modules 9.4/5 and Appendix B. Stress the small size of the atom and the fact that interactions between atoms are at the core of all the chemical reactions. Emphasise that all life processes and all technological process depend on these reactions.
  • Introduce the basic ideas of atomic structure. Use any suitable models and charts that may be available but do not overelaborate on the simple model given in the textbook. Make sure that students understand that like charges repel one another and unlike charges attract one another. Stress that (i) almost all the mass of an atom resides in the nucleus (a proton or a neutron has a mass almost 2000 times greater than that of an electron), (ii) every element has its own fixed number of protons in the nucleus, (iii) for every proton in the nucleus there is an orbiting electron, and (iv) electrons control the chemistry of elements.
  • Give students a very general overview of the work they will cover in the remainder of this chapter.
  • Encourage students to try the questions at the end of the module. Follow up by discussing their attempts. Make sure they are aware of, and understand, the correct answers.

Answers:

  • Q1. (i) The nucleus of an atom is the central part of the atom. It consists of protons and neutrons and accounts for almost the whole of the mass of the atom. (ii) A proton is a tiny, heavy, sub-atomic particle with a positive electric charge; it is found in the nucleus of an atom. (iii) A neutron is a tiny, heavy, sub-atomic particle with no electric charge; it is found the nucleus of most atoms. (iv) An electron is a tiny, light, sub-atomic particle with a negative electric charge; it orbits around the nucleus of an atom. (v) A proton or a neutron is almost 2000 times heavier than an electron. (More exactly, a proton has the mass of about 1836 electrons, and a neutron has the mass of about 1839 electrons!).
  • Q2. (i) The nucleus (which contains 7 heavy particles) is mainly responsible for the mass of the lithium atom. (ii) The electrons orbiting the nucleus are mainly responsible for the way the lithium atom interacts with other atoms (that is, for the chemical reactions of lithium).

13. 2 THE NUCLEUS 1 – ATOMIC NUMBER, ATOMIC MASS AND ISOTOPES

Aim:

  • To help students understand the meanings of atomic number (represented by Z), atomic mass (represented by A), and isotope.

Activities:

  • Discuss thoroughly the material presented in the textbook, section by section, adding additional examples that you think will interest your students. Use simple drawings/charts/models as available.
  • Encourage students to try the questions at the end of the module. Questions 4 and 5 are important and would be particularly suitable for discussion by students in small groups. Follow up by discussing their attempts. Make sure they are aware of, and understand, the correct answers.

Answers:

  • Q1. (i) The atomic number of an element is the number of protons in the nucleus of one atom of the element. (ii) The atomic mass of an element is the mass of one atom of the element compared to the mass of one atom of hydrogen. (Alternatively, it is the mass of one atom of the element on a scale such that the atomic mass of the common isotope of carbon is exactly 12). (iii) Isotopes are atoms of the same element with different atomic masses (owing to different numbers of neutrons).
  • Q2. Hydrogen is the element with the fewest neutrons in its atoms; 99.9% of hydrogen atoms have no neutrons at all!
  • Q3. Atomic number Z=1 for hydrogen; Z=6 for carbon; Z=8 for oxygen; Z=14 for silicon and Z=79 for gold (this information can be obtained from Appendix B).
  • Q4. If you subtract the atomic number Z, from the atomic mass A, you will get he number of neutrons in the nucleus. (Z is the number of protons in the nucleus and A corresponds to the total number of protons and neutrons in the nucleus).
  • Q5. If you subtract the atomic number of zinc (30) from its atomic mass (65.4) the answer ( 35.4) should be the number of neutrons. However you can’t have .4 of a neutron! Therefore zinc must have more than one common isotope and 65.4 must be the average atomic mass. [Students may be interested to know that zinc is unusual in having as many as five significant isotopes. They have atomic masses of 64 (48.6% of atoms), 66 (27.9%), 67 (4.1%), 68 (18.8%) and 70 (0.6%) which average out to 65.4].

13.3 THE NUCLEUS 2 – RADIOACTIVITY

Aims:

  • To help students to know and understand the basic facts about radioactivity and ionising radiation.
  • To inform students about radio-isotopes and the ways in which they are used.

Activities:

  • Discuss thoroughly the material presented in the textbook, section by section, adding additional examples that you think will interest your students. Some aspects of the work (for example ionising radiation, radioactive decay, and the uses of radio-isotopes) could be allocated to small groups of students to discuss, follow up and report back to the class on.
  • Encourage students to try the questions at the end of the module. Follow up by discussing their attempts. Make sure they are aware of, and understand, the correct answers.

Answers:

  • Q1. (i) Alpha rays are fast moving alpha particles consisting of two protons and two neutrons bound together. Beta rays are fast moving electrons. Gamma rays are very energetic electromagnetic rays. (ii) Ions are electrically charged atoms or groups of atoms. (iii) A Geiger counter is a device that detects ionising radiation by making clickswhen radiation enters a window at the end of a tube. (iv) Background radiation is the natural ionising radiation that we are exposed to from the world around us. (v) Radioactive decay is the breakdown of the nucleus of a radioactive isotope, usually by emitting an alpha or a beta particle. (vi) Radioactive tracers are isotopes used to follow the movement of atoms in an organism or other system.
  • Q2. Ionising radiation is very dangerous because the energy it carries and the ions it creates damageor destroy living cells. People can be protected by (i) keeping sources of ionising radiation in lead containers, (ii) using ionising radiation behind lead screens, (iii) wearing protective clothing, and (iv) wearing monitoring devices such as film badges or scintillation detectors that can warn them if they are in a danger from radiation.
  • Q3. 7/8 of the 234Pa will have decayed after 3 days. (After one day, half will decay; after 2 days, half of the remaining half will decay making ¾ in total; and after 3 days, half of the remaining quarter will decay making 7/8 in total).
  • Q4. Animal remains can be dated using radio-carbon dating. While the animal is alive, it constantly replaces the natural proportion of radioactive 14C atomsin its body through the food chain when it eats plants (or other animals that have eaten plants). When it dies, the 14C atoms are no longer replaced and they decay with a half-life of 5730 years. By measuring how much of the 14C has decayed, scientists can estimate how long ago the animal died.

13.4 THE NUCLEUS 3 – NUCLEAR FISSION AND NUCLEAR FUSION

Aims:

  • To help students to know and understand some basic information about nuclear fission (including the example of 235U), the idea of a chain reaction, and the use of nuclear fission in the atomic bomb and in nuclear power stations.
  • To make students aware of basic idea of nuclear fusion as the process that generates energy in stars.

Activities:

  • Discuss thoroughly the material presented in the textbook, section by section, adding additional examples that you think will interest your students. As regards nuclear fission, stress the concept of the chain reaction; an analogy with human interactions may be helpful (where ‘positive feedback’ means that one thing leads to another and another and another so a small event blows up out of all proportion!).
  • As regards nuclear fusion, stress the potential value of controlled fusion as a cheap source of energy.
  • Encourage students to try the questions at the end of the module. Follow up by discussing their attempts. Make sure they are aware of, and understand, the correct answers.

Answers:

  • Q1. Similarities as between nuclear fission and fusion: both are reactions of nuclei; both reactions produce large amounts of energy in the form of heat, light and gamma rays; both reactions result in the production of isotopes of new elements. Differences as between nuclear fission and fusion: fission involves breaking down the nucleus producing isotopes of lighter elements but fusion involves the joining of nuclei producing isotopes of heavier elements; fusion produces more energy than fusion; (we can control fission reactions in nuclear power stations, but we cannot yet control fusion reactions).
  • Q2. A chain reaction is a reaction whose products initiate further reactions of the same kind (it is anexample of positive feedback). An atom bomb uses enriched uranium with an increased proportion of 235U; when this isotope captures a neutron, fission occurs producing more neutrons which lead to an uncontrolled chain reaction. In a power station, the chain reaction is controlled by using (i) uranium that contains a lower proportion of 235U (so there are less nuclei to capture neutrons) and (ii) control rods to absorb some of the neutrons (so there are less neutrons to be captured).

13.5 THE ROLE OF ELECTRONS – IONIC AND COVALENT BONDING

Aims:

  • To help students understand how the chemical behaviour of atoms is controlled by electrons (at a qualitative level and without details of electronic configurations), referring to (i) the metallic lattice and electrical conduction, (ii) the contrasting behaviour of metal and non-metal elements, and (iii) ionic and covalent bonding in simple compounds.
  • To reinforce the distinction between mixtures of variable composition as exemplified by alloys, and compounds of fixed composition that can be represented by definite formulae.

Activities:

  • Start by reminding students of the basic ideas previously studied in Modules 9.4 – 9.7. Make sure they are familiar with the idea of elements, with the symbols for the most common ones, and with the distinction between metal and non-metal elements. Get them to name the non-metal elements from their symbols in the box at the top right of the page. Go on to emphasise the idea that the electrons on the outside of atoms determine how they interact with one another - chemistry is controlled by electrons.
  • As regards metals, demonstrate electrical conduction (use the suggestions given in Module 4.3) and stress the idea that an electric current is a flow of electrons, that they carry a negative charge, and that the actual direction of flow is from negative to positive. Show them examples of common alloys such as brass and stainless steel and discuss the properties and uses of each. You could also discuss how the malleability (and ductility) of metals is explained by the fact that the positive ions tend to repel one another. They are only held together by the ‘cloud’ of negative electrons surrounding them and are relatively easy to displace and rearrange by hammering (or stretching).Balls of plasticine could be used to model lattices.
  • Important points to stress are the nature of ionic and covalent compounds, including their definite composition (and formulae) which should be contrasted with that of mixtures such as alloys. Show students examples of salt crystals and encourage them to make models of the NaCl lattice and of a few covalent molecules from balls of coloured plasticine, or small fruits with toothpicks for bonds.
  • Encourage students to try the questions at the end of the module. Follow up by discussing their attempts. Make sure they are aware of, and understand, the correct answers.

Answers:

  • Q1. (i) A lattice is a regular arrangement of particles (such as atoms, ions or molecules) in a solid. (ii) An alloy is a mixture of two or more metals in varying proportions; a solid solution of one or more metals in another. (iii) An ionic compound is a compound of fixed composition composed of positive metal ions and negative non-metal ions. (iv) A molecule is a small group of atoms held together by covalent bonds. (v) A covalent bond is one in which two atoms share a pair of electrons.
  • Q2. An electric current is a flow of negatively charged electrons. The electrons flow from a negative terminal to a positive terminal.
  • Q3. Metal atoms can only form compounds by giving away electrons, creating positive metal ions and negative non-metal ions. Non-metal atoms can gain electrons from metals creating positive metal ions and negative non-metal ions, or they can share a pair of electrons with another non-metal atom creating a covalent bond.

13.6 THE NAMES, FORMULAE AND COMPOSITION OF SIMPLE COMPOUNDS

Aims:

  • To introduce students to the naming of simple compounds, to a simple concept of valency as combining power related to the exchange or sharing of electrons, and to show them how to use valency to work out the formulae of simple chemical compounds.
  • To introduce a simple concept of chemical radicals and to a few common examples of these.
  • To show students how to calculate the percentage composition by mass of simple chemical compounds using chemical formulae and tables of atomic masses.

Activities:

  • Review Modules 9.6/7 and introduce students to the naming of simple, inorganic compounds including the four radicals named in the textbook. Show students examples of as many compounds as possible. Stress that radicals do not exist alone, only as parts of compounds. Give students a list of formulae and ask them to name the compounds.
  • Introduce the concept of valency as combining power and relate this to the behaviour of electrons. Show them how to use valency to work out formulae and make sure that they understand the significance of the numerical subscripts in formulae. Help them to practice writing formulae from the names of simple compounds. Stress that compounds always have a fixed composition.The simple jigsaw game below may help students to understand how valency works.

  • Sow students how to calculate the percentage composition of a compound by mass, using the example given in the text book. Give them additional examples to try, for example the percentage of iron in rust as Fe2O3(70%), or the percentage of calcium in limestone (40%).
  • Encourage students to try the questions at the end of the module. Follow up by discussing their attempts. Make sure they are aware of, and understand, the correct answers.

Answers:

  • Q1. HCl, MgO, KNO3, AlCl3, Na2CO3, Cu(NO3)2, SO2, SF6, Al2O3
  • Q2. Ca 40%, C 12%, O 48% (see the calculation in table below)