Topic 3 National Chemistry Summary Notes. Bonding, Structure and Properties of Substances. Covalent Bonds

Transcription

1 Topic 3 National Chemistry Summary Notes Bonding, Structure and Properties of Substances LI 1 Covalent Bonds Most atoms do not exist as single atoms. They are mainly found combined with other atoms in compounds. They are held together in these compounds by chemical bonds. Atoms prefer to be bonded to each other because this gives them a stable electron arrangement. A stable electron arrangement is achieved when the outer energy level (electron level or electron shell) is full. The noble gases all have full outer energy levels and are very stable. Remember you were told in the last topic that chemical reactions are all to do with outer electrons? If the outer energy level is full then the atom is un-reactive. Most atoms need 8 electrons in the outer energy level to be full (the exceptions are Hydrogen and Helium who only need 2). This is sometimes described as a stable octet. You can use target diagrams, like the one shown below for sodium, to show the arrangement of electrons in an atom: The electron arrangement for sodium is : 2,8,1 You will need to be able to draw similar diagrams for the first 20 elements. 1

2 In this first part of the topic we are going to look at how two non-metal atoms form bonds called covalent bonds. When two or more non-metal atoms are joined by covalent bonds we call them molecules.to help us we are going to use simple diagrams showing just the outer electrons of atoms rather than target diagrams. When non-metal atoms (like the example for hydrogen shown below) form covalent bonds they do this by overlapping their outer energy levels in order to share one pair of electrons. This gives each atom a full outer energy level (electron level). Note: In some textbooks energy levels are also called electron shells. LI 2 E Covalent Bonds in Elements There are a group of 7 elements in the Periodic Table that exist as two atoms joined together by covalent bonds in a Diatomic Molecule. They are called The Diatomic Elements and are shown below: Iodine - In Oxygen - October Chlorine - Children Bromine - Buy Nitrogen - Nasty Hydrogen - Halloween Fluorine - Food 2 A diatomic molecule is a molecule containing two non-metal atoms joined together by covalent bonds. A diatomic element is a when a diatomic molecule is formed from two identical atoms

3 When you look at the outer energy levels of each element you can see how many electrons their atoms would have to share in order to fill up their outer energy level. Hydrogen has room for one electron to fill up its outer energy level, so it will share one electron pair. Chlorine, Bromine, Iodine and Fluorine all have 7 electrons in the outer energy level and therefore have room for one more, so they will also share one electron pair. Every time a pair of electrons is shared a covalent bond is made. The bonds between these atoms are described as single bonds because only one pair of electrons are shared: The single covalent bond can also be represented using element symbols and lines: Cl-Cl, H-H, Br-Br, I-I, F-F one dash represents a single bond. 3

4 Double and Triple Covalent Bonds Oxygen atoms have room to share two electron pairs. This means that a double covalent bond is formed between two oxygen atoms. You can draw an overlapping orbital diagram to show this sharing: O=O Nitrogen atoms have 5 outer energy level electrons so they need to share 3 pairs of electrons in order to fill the level. This requires a triple bond: How would you show two nitrogen atoms joining up to form a diatomic molecule using symbols and dashes? Answer: 4

5 LI 3 Covalent Bonds in Compounds Note: You should remember from earlier work that a compound is a substance made up of two or more different elements joined together. As a general rule if a compound is made up of non-metal elements only then it will have covalent bonding. Examples: Methane- CH 4 or Ammonia NH 3 or Water H 2 O or E Note: Covalent bonds are very strong. A covalent bond is when the two positive nuclei involved in the bond are held tightly together by their common attraction for the shared pair of electrons. 5

6 LI 4 E Shapes of Covalent Molecules Molecules are three dimensional structures with specific shapes. We need to know the shapes of some of the simple two element molecules: Molecule Arrangement of Atoms Shape Hydrogen oxide (water) Bent (and flat or planar) Hydrogen fluoride Linear Nitrogen hydride (ammonia) Pyramidal Carbon hydride (methane) Tetrahedral The arrangement of atoms in methane and ammonia are trying to represent the real shape using a perspective diagram. 6

7 LI 5 E Discrete Molecular Covalent versus Giant Network Structures Most covalent substances exist as molecules with a definite number of atoms present eg methane CH 4. These are called discrete covalent molecular structures. Some covalent substances, however, take the form of huge repeating structures, called giant covalent networks. Example 1 : Diamond a covalent network element Diamond contains only carbon atoms joined together by covalent bonds. The carbon atoms are held very tightly giving Diamond great hardness and a very high melting point. Example 2 : Quartz a covalent network compound Quartz contains atoms of silicon and oxygen in the ratio of one silicon to two oxygen. Its formula is SiO 2 and it s proper name is silicon dioxide. The arrangement of atoms in quartz makes the structure very rigid and gives it a high melting and boiling point. To melt a covalent network you have to break that network of covalent bonds which are very strong!!! 7

8 LI 6 Ionic Bonds Ionic bonds are a special bond that forms between a metal atom and a non-metal atom. The bonds form when a metal atom transfers electrons(s) to a non-metal atom. This means the atoms are able to achieve the same stable, full outer energy level arrangement as the nearest noble gas. Ions Atoms which have lost or gained electrons become charged particles called ions. How do you know what kind of ion an atom will become? Atoms in groups 1,2 and 3 all LOSE electrons to become positively charged. Atoms in groups 5,6 and 7 all GAIN electrons to become negatively charged. Atoms in group 4 do not usually form ions!! In General Metals lose electrons forming positive ions. Non-metals gain electrons forming negative ions. Activity!! Complete the following: Group Number Charge on ion + example Na + 8

9 LI 7 How Does An Ionic Bond Form? Your teacher will show you how to use target diagrams to illustrate how metal atoms react with non-metal atoms to form ionic bonds. The sodium atom gives away its outer electron to the chlorine atom. The sodium atom is now a positive ion with a full outer energy level. The chlorine atom is now a negative ion with a full outer energy level Atoms are electrically neutral (no charge) but electrons are negatively charged. When metal atoms lose electrons becoming positively charged and non-metal atoms gain electrons to become negatively charged these oppositely charged ions attract each other forming a strong bond. E This is often described as an electrostatic force of attraction between positive and negative ions. 9

10 LI 8 E Ionic Lattice Structures A tiny amount of an ionic solid compound contains millions of ions. These millions of ions are held together by ionic bonds, that is : The electrostatic force of attraction between positive and negative ions These forces are so great that the ions group together with positive ions surrounding negative ions and negative ions surrounding positive ions. This produces a regular, geometric structure called an Ionic Lattice. The lattice structure of a sodium chloride crystal. In an ionic lattice the ions are held tight and cannot move. Ionic compounds form lattice structures of oppositely charged particles. 10

11 LI 9 Melting Points of Substances The bonding present in a substance has a big effect on a number of different properties. Melting points (and boiling points) are an important property. How easy it is to melt or boil a substance determines what the physical state will be at room temperature. ( room temperature is aprox 25 o C ) Physical States of Ionic Compounds Have a look at page 6 of the Data Booklet. Find the melting points and boiling points of the following substances and then decide what state they will be at room temperature: Name of Compound Melting Point o C Boiling Point o C Physical State at Room Temperature Barium chloride Calcium oxide Magnesium chloride Potassium iodide As you can see all these ionic compounds are solids at room temperature. In fact all ionic compounds are solids at room temperature. E The high melting and boiling points are due to the strong ionic bonds which need to be broken. A lot of heat energy has to be put in to allow the ions to break free from each other and move around. Therefore all ionic substances are solids at room temperature. 11

12 Physical States of Covalent Compounds Most covalent compounds have low melting points and can be found as gases, liquids and soft solids at room temperature. Name of Compound Melting Point o C Boiling Point o C Physical State at Room Temperature Water (hydrogen oxide) Methane (carbon hydride) Carbon dioxide Silicon dioxide ** ** E Discrete Covalent Molecular compounds all have low melting and boiling points but The Giant Covalent Network compounds like silicon dioxide have very high melting points because very strong covalent bonds have to be broken in order to melt the compound. To melt a Discrete Molecular Covalent compound there are only weak forces of attraction between molecules that are easy to break. LI 10 Comparing Electrical Conductivity of Substances A substance can conduct electricity if it can allow some kind of charged particles to flow through it. An electric current (electricity) is a flow of charged particles. From practical work in science you may remember that all metals conduct electricity but what about non-metal elements, ionic compounds and covalent compounds? Using a simple electrical circuit with a light bulb to indicate whether the substance conducts electricity you will test a range of solids first and fill in the following table: 12

13 The solid to be tested has to be attached in series in the circuit using wires and crocodile clips. Electrical Conductivity of Solid Substances: Name of Substance Sulphur Metallic Bonding (please tick) Covalent Bonding (please tick) Ionic Bonding (please tick) Does it Conduct Yes/No Copper Iron Tin Carbon (graphite) Wax (contains carbon and hydrogen) Sodium Chloride Lead bromide Sugar (carbon, hydrogen and oxygen) 13

14 Summary of Conductivity of Solids 1. All the metals do conduct electricity. 2. The only non-metal that can conduct is carbon in the form of graphite. 3. Ionic compound do not conduct as solids. 4. Covalent compounds do not conduct as solids. LI11 Electrical Conductivity of Liquid Substances A range of liquids were tested. These included solutions of both ionic and covalent substances, some pure liquids and a teacher demonstration of two melts. Melts simply means solids that have to be heated first to melt them before they were tested using a simple circuit. They can also be described as molten. Substance Ionic Bonding (please tick) Covalent Bonding (please tick) Does it Conduct Yes/No sugar solution (carbohydrate) sodium chloride solution copper chloride solution hexane (carbon hydride) nickel sulphate solution molten wax molten lead bromide ethanol 14

15 Summary of Conductivity of Liquids 1. Ionic substances DO conduct when in solution. 2. Ionic substances DO conduct when molten. 3. Covalent solutions, pure liquids and melts do NOT conduct. Problems With Testing Conductivity of a Melt!! Testing conductivity of a melt can often give false positive results. After watching the teacher demonstration, draw a labelled diagram of the apparatus used and clearly indicate and explain the possible source of error. Circuit to Test Conductivity of a Melt: Explanation: 15

16 E LI12 Explanation of Conductivity of Substances Why do covalent substances not conduct in any state or in solution? Covalent substances do not contain any charged particles. They are made up of neutral atoms joined together. Electricity cannot flow unless there are charged particles able to flow (move). Why do ionic substances only conduct in solution or melt? Ionic substances do contain charged particles but they are not able to move in a solid, therefore ionic solids cannot conduct. Ionic solutions do conduct because dissolving the substance has broken down the ionic lattice. The ions are now free to move. Ionic melts conduct because the addition of heat energy has allowed the ions to vibrate more, overcoming the electrostatic forces holding them together and shake free of each other. The ions are free to move. Why do metals conduct in any state? We will be looking at metals in more detail later in the course but they have a special structure where their outer energy level electrons are loosely shared between atoms in a metallic lattice. These electrons are easy to move and since they are charged particles their movement allows electricity to flow through easily. Some text books describe these electrons as a sea of delocalised electrons. Metallic lattice 16

17 Summary of Physical State and Conductivity of Structures Type of Structure Ionic Physical State at Room Temperature High melting point solids Liquid Conductivity Solid Discrete Covalent Molecular Low melting point, gas, liquids and soft solids Giant Covalent Network Very high melting point solids Metallic Wide range of melting points but all solids except mercury 17

18 Topic 3 Pupil Self-evaluation E Note: if the success criteria is in italics then this is extension level work. Learning Intention Success Criteria I can: 1 I am going to find out how draw target diagrams to show electron arrangements of atoms. atoms can join together in a draw outer electron diagrams to show overlapping energy levels. covalent bond. state that a covalent bond is a shared pair of electrons. state that atoms share electrons to get a full, stable, outer energy level. state that non-metal atoms form covalent bonds. state that when two or more non-metal atoms are joined together by covalent bonds then a molecule if formed. 2 I am going to find out about name the 7 diatomic elements the diatomic molecules. describe what diatomic element and diatomic molecule means. draw diagrams to represent single, double and triple covalent bonds. 3 I am going to find out about draw simple diagrams of two element compounds. how compounds are held state that covalent bonds are very strong. together by covalent bonds. describe what holds a covalent bond together. 4 I am going to find out about the shapes of simple describe the shapes of water, methane, ammonia and hydrogen fluoride molecules. covalent molecules. draw perspective diagrams of these molecules. 5 I am going to find out about different covalent describe what a Giant Network Covalent and a Discrete Molecular Covalent structure is like. structures. give an example of a compound that exists as Giant Network Covalent state that melting point and boiling point are related to structure. 6 I am going to find out how state that ionic bonds form between a non-metal atom and a metal 18

19 atoms can change in order to join together in ionic bonds. 7 I am going to find out how an ionic bond forms. 8 I am going to be able to describe an ionic lattice. 9 I am going to be able to use the data booklet to find out the melting and boiling points of substances and relate this to physical state and type of bonding. 10 I am going to find out how electrical conductivity of a solid substance relates to its bonding. 11 I am going to find out how electrical conductivity of a liquid substance is related to the bonding. 12 I am going to be able to describe why a substance conducts. atom. state that atoms become ions by losing or gaining electrons. state what charge an ion will have from groups 1 to 0. draw diagrams to explain how an ionic bond forms. explain that an electrostatic force of attraction holds the ionic bond together. explain that an ionic solid has a regular, geometric structure called an ionic lattice and describe it. use the data booklet to find the melting and boiling points of selected substances. state that all ionic substances are solids at room temperature. explain why ionic substances are all solids. describe how I could test a solid substance for electrical conductivity. state the conductivity of metal solids. state the conductivity of covalent or ionic solids. state the conductivity of ionic substances when in solution. state the conductivity of covalent substances as solution or as liquid. state that ionic melts conduct but covalent melts do not conduct. explain a problem that can occur when testing melts for conductivity. explain that for an ionic substance to conduct then the ions need to be free to move. describe conductivity in a metal as a flow of electrons. 19