8. Structures & Bonding

13.1 What happens when elements react? Scientists believe that when elements react, their atoms are joining together or bonding. To understand the ways in which this can happen we need to know how atoms themselves are constructed. When the particles of a substance gain or lose energy, the substance may change its state. If energy is supplied to a solid, its particles vibrate more violently; they may separate from each other and become free to move. This is melting. The temperature at which a solid melts is called its melting point. Heating a liquid makes its particles move around more quickly. Particles which have enough energy may overcome attractive forces and escape from the liquid and become a gas. This is evaporation. When the temperature is higher, more particles have enough energy to escape so evaporation is faster. If the temperature is high enough, a liquid will boil. The temperature at which a liquid boils is called its boiling point. All substances are made of atoms. There are about 100 different sorts of atoms. A substance which contains only one sort of atom is called an element. ! Candidates should be able, when provided with appropriate information, to explain why the idea of atoms only became generally accepted by scientists after Dalton re-introduced the idea about 200 years ago. Atoms have a small central nucleus made up of protons and neutrons around which there are electrons. The relative masses of protons, neutrons and electrons, and their relative electric charges are as shown: Mass Charge proton 1 +1 neutron 1 0 electron negligible 1 In an atom, the number of electrons is equal to the number of protons in the nucleus. Atoms have no overall electrical charge. All atoms of a particular element have the same number of protons. Atoms of different elements have different numbers of protons. The number of protons in an atom is called its atomic number (proton number). The total number of protons and neutrons in an atom is called its mass number. Atoms of the same element can have different numbers of neutrons; these atoms are called isotopes of that element. Candidates should be able to represent and interpret atomic diagrams. Electrons occupy particular energy levels. Each electron in an atom is at a particular energy level (in a particular shell). The electrons in an atom occupy the lowest available energy levels (innermost available shells). (N.B. Though only energy levels are referred to throughout this syllabus, candidates may answer in terms of shells if they prefer.] Candidates should be able to represent the electronic structure of the first twenty elements of the periodic table in the following forms: Compounds are substances in which atoms of two, or more, elements are not just mixed together but chemically combined. Chemical reactions between elements involve either the giving and taking, or sharing, of electrons in the highest occupied energy levels of atoms. When atoms form chemical bonds by gaining and losing electrons they form electrically charged atoms called ions. Atoms can also form bonds by sharing electrons. The atoms which lose electrons become positively charged ions and the atoms which gain electrons become negatively charged ions. These ions now have the electronic structure of a noble gas. Candidates should be able to represent the electronic structure of the ions in sodium chloride, magnesium oxide and calcium chloride. An ionic compound is a giant structure of ions. Substances with giant structures have high melting points and boiling points. Ionic compounds are held together by strong forces of attraction between oppositely charged ions. This is the ionic bond. Atoms which share electrons often form molecules. The atoms in molecules are held together because they share pairs of electrons. The strong bonds between the atoms are called covalent bonds. Candidates should be able to represent the covalent bonds in water, ammonia, hydrogen, hydrogen chloride, methane and oxygen. Substances which are made of molecules have low melting points and boiling points. 13.2 How can we explain the different properties of different types of substances? Different types of substances have different properties because of differences in the forces between the particles from which they are made. Higher Tier Simple molecular compounds are gases, liquids or solids which have relatively low melting points and boiling points and do not conduct electricity. This is because: - the forces between the molecules (intermolecular forces) are weak; - the molecules do not carry an overall electric charge. Atoms which share electrons can also form giant structures. Diamond and graphite (forms of carbon) and silicon dioxide (silica) are giant covalent structures (lattices) of atoms. Because of the large number of covalent bonds in their structures, they have very high melting points. In diamond each carbon atom forms four covalent bonds in a rigid, giant covalent structure. In graphite each carbon atom forms three covalent bonds and the carbon atoms form layers which are free to slide over each other. In graphite there are free electrons which allow graphite to conduct electricity. Ionic compounds form regular structures (giant ionic lattices) in which the strong forces between oppositely charged ions result in these compounds having high melting points and high boiling points. When they are melted or dissolved in water, ionic compounds conduct electricity because the ions are free to move. Metals consist of giant structures in which the electrons from the highest occupied (outer) energy levels of metal atoms are free to move through the whole structure. These free electrons: - hold the atoms together in a regular structure; - allow the atoms to slide over each other; - allow the metal to conduct heat and electricity. 13.3 How can chemical elements be grouped into families? It is easier to remember the properties of more than 90 naturally occurring elements if we can group them into families of elements with similar properties. Chemists arrange these family groups of elements in a special way which is known as the periodic table. The chemical elements can be arranged in order of their relative atomic masses. This list can then be arranged in rows so that elements with similar properties are in the same columns, known as Groups. The resulting table is known as the periodic table. Although most elements are in appropriate Groups, a few are not. Argon atoms, for example, have a greater relative atomic mass than potassium atoms but argon is better placed before the potassium in the periodic table so that it is in Group 0 and potassium is in Group 1. In the modern periodic table elements are arranged in order of their atomic (proton) number. All elements are then in the appropriate Group. The periodic table can be seen as an arrangement of the elements in terms of their electronic structure. From left to right, across each horizontal row (period) of the periodic table, a particular energy level is gradually filled up with electrons; in the next period, the next energy level is filled with electrons. The similarities and differences between the properties of elements in the same group of the periodic table can be explained by the electronic structure of their atoms. ! Candidates should be able, when provided with appropriate additional information, to explain: - how attempts to classify elements in a systematic way, including those of Newlands and Mendeleev, have led through the growth of chemical knowledge to the modern periodic table; - why scientists regarded a periodic table of the elements first as a curiosity, then as a useful tool and finally as an important summary of the structure of atoms. The elements in Group 1 of the periodic table are known as alkali metals because they form hydroxides which dissolve in water to give alkaline solutions. They react with non-metals to form ionic compounds in which the metal ion carries a charge of +1. Fewer than one quarter of the elements are non-metals. Non-metal elements are found in the Groups at the right hand side of the periodic table. The elements in Group 7 and Group 0 have the typical properties of non-metals: - they have low melting points and boiling points (at room temperature all the Group 0 elements are gases, the first two Group 7 elements are gases and the third, bromine, is a liquid); - they are brittle and crumbly when solid; - they are poor conductors of heat and electricity even when solid or liquid. The elements in Group 7 of the periodic table (known as halogens): - have coloured vapours; - consist of molecules which are made up of pairs of atoms; - form ionic salts with metals in which the chloride, bromide or iodide ion (halide ions) carries a charge of 1; - form molecule compounds with other non-metallic elements. The elements in Group 0 of the periodic table (known as noble gases); - are all chemically very unreactive gases; - exist as individual atoms rather than as diatomic gases like other gaseous elements; - are used as inert gases in filament lamps and in electrical discharge tubes. The first element in the Group, helium, is much less dense than air and is used in balloons. In Group 1, the further down the group an element is: - the more reactive the element; - the lower its melting point and boiling point. When a piece of lithium, sodium or potassium is placed in cold water the metal floats, may melt and moves around the surface of the water. The metal reacts with the water to form a metal hydroxide solution and hydrogen gas. The more reactive the metal, the more vigorous is the reaction with water. A simple laboratory test for hydrogen is that when a test tube of hydrogen is held to a flame the hydrogen burns in the air with a squeaky explosion. In Group 7, the further down the group an element is: - the less reactive the element; - the higher its melting point and boiling point. A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. 13.4 How can the similarities between elements in the same group be explained? Once again, these similarities and differences can be explained in terms of the atomic structure of the atoms concerned. Higher Tier Elements in the same group have similar properties because they have the same number of electrons in the highest occupied (outer) energy level. The higher the energy level: - the more easily electrons are lost; - the less easily electrons are gained. These ideas explain the trends in the reactivity of elements in Groups 1 and 7 of the periodic table. Group 0 elements (noble gases) are unreactive and monatomic because their highest occupied energy level is full so that atoms have no tendency to gain, to lose or to share electrons. 13.5 How do metal halogen compounds compare with the elements from which they are made? What use are these compounds? Compounds of metals and halogens have very different properties than the elements from which they are made. The use we make of these compounds depends on these different properties. Sodium chloride (common salt) is a compound of an alkali metal and a halogen. It is found in large quantities in the sea and in underground deposits. The electrolysis of sodium chloride solution (brine) is an important industrial process. Chlorine gas is formed at the positive electrode and hydrogen gas at the negative electrode. A solution of sodium hydroxide is also formed. Each of these products can be used to make other useful materials: - chlorine is used to kill bacteria in drinking water and in swimming pools, and to manufacture hydrochloric acid, disinfectants, bleach and the plastic (polymer) known as PVC; - hydrogen is used in the manufacture of ammonia and margarine; - sodium hydroxide is used in the manufacture of soap, paper and ceramics. A simple laboratory test for chlorine is that it bleaches damp litmus paper. Silver chloride, silver bromide and silver iodide (silver halides) are reduced to silver by the action of light, X-rays and the radiation from radioactive substances. They are used to make photographic film and photographic paper. Hydrogen halides are gases which dissolve in water to produce acidic solutions. 13.6 What do all these chemical symbols, formulae and equations mean? Once you understand them, chemical symbols, formulae and equations are a very convenient way of describing what elements chemical compounds contain and what is happening in chemical reactions. Each element is represented by a different symbol. The chemical formula for a compound shows which elements are in the compound. A chemical reaction can be described using a word equation: reactants products Candidates should be able to write word equations for all reactions referred to in the tier of the specification for which they are entered. The symbols for elements are used to write chemical formulae for compounds which show the ratios of atoms from different elements which are combined to form the compounds. Candidates should be able to write down the correct formulae for simple ionic compounds. [See Data Sheet for the formulae of, and charges on, common ions.] Candidates should be able to recall the formulae of all simple covalent compounds referred to in the relevant tier of the specification. Candidates should be able to interpret chemical formulae or symbolic representations of molecules in terms of the elements present and the ratios of their atoms. Chemical reactions can be represented using the chemical formulae for the reactants and the products. Candidates should be able to interpret supplied symbol equations, which may include the state symbols (s), (l), (g) and (aq). The total mass of the product(s) of a chemical reaction is always equal to the total mass of the reactant(s). This is because the products of a chemical reaction are made up from exactly the same atoms as the reactants. Symbol chemical equations must, therefore, always be balanced. The total number of atoms of each element on the reactants side of the equation must be equal to the total number of atoms of the same element on the products side of the equation. Candidates should be able to : - balance supplied symbol equations; - write a balanced symbol equation from a supplied word equation. Higher Tier During electrolysis, ions gain or lose electrons at the electrodes. Electrically neutral atoms or molecules are released. Candidates should be able to complete and balance supplied half equations for the reactions occurring at the electrodes during electrolysis. The supplied equations will include information about the charge of the ion and the atomic or molecular nature of the product. For example, when supplied with Cl- - e- Cl2 Candidates should be able to produce 2Cl- - 2e- Cl2