CHEMICAL BONDING PART 2 COVALENT AND METALLIC BONDING

CHEMICAL BONDING PART 2 COVALENT AND METALLIC BONDING © Boardworks Ltd 2001

COVALENT BONDING © Boardworks Ltd 2001

Covalent Compounds • • • Covalent compounds are formed when nonmetal atoms react together. As these atoms come near their outer electrons are attracted to the nucleus of both atoms and become shared by the atoms. The shared electrons count towards the shells of both atoms and therefore help fill up incomplete electron shells. © Boardworks Ltd 2001

Covalent Bonds • • • Covalent compounds are held together by this sharing of electrons. A pair of electrons shared in this way is known as a covalent bond. It is sometimes represented in full bonding diagrams (Fig 1) often these bonds are just shown as a pair of electrons (xx) or even just a line FX F X - F F © Boardworks Ltd 2001

Small Covalent Structures • Sometimes just a few atoms join together in this way. This produces small covalent molecules – often known as simple molecular structures A simple molecular structure Covalent bonds © Boardworks Ltd 2001

Giant Covalent Structures • • Covalent bonds Sometimes millions of atoms are joined together by covalent bonds. This produces to rigid 3 -D network called a giant lattice A giant lattice © Boardworks Ltd 2001

Covalent Bonding and Electron Structures • • • The driving force for covalent bonding is again the attainment of electron shells that are either completely full or empty. This is achieved by sharing electrons where the shared electrons count towards the outer shells of both atoms. Sometimes this is achieved with equal numbers of each type of atom. Sometimes it is not! H Cl Cl H C H H H N H Cl H H © Boardworks Ltd 2001

Covalent Bonding in Chlorine • Chlorine (2. 8. 7) needs 1 more electron to attain a full electron shell. Cl Cl (2, 8, 7) Cl-Cl Cl (2, 8, 8) © Boardworks Ltd 2001

Activity • Both fluorine and chlorine needs 1 more electron to attain a full electron shell. F (2, 7) Cl Copy this diagram and add the electron arrangements that could exist in fluorine chloride (FCl) (2, 8, 7) F (2, 8) Cl (2, 8, 8) © Boardworks Ltd 2001

Covalent Bonding in Hydrogen Chloride • Both hydrogen (1) and chlorine (2. 8. 7) needs 1 more electron to attain a full outer shell. H Cl (1) (2, 8, 7) H-Cl H Cl (2) (2, 8, 8) © Boardworks Ltd 2001

Covalent Bonding in Water • Hydrogen (1) needs 1 more electron but oxygen (2. 6) needs 2 more. We need 2 hydrogens H O H H © Boardworks Ltd 2001

Activity • Hydrogen (1) needs 1 more electron. • How many does nitrogen (2. 5) need? 3 • How many hydrogens per 1 nitrogen? 3 • Draw bonding diagrams for ammonia H N H H © Boardworks Ltd 2001

Activity • Hydrogen (1) needs 1 more electron. • How many does carbon (2. 4) need? 4 • How many hydrogens per 1 carbon? 4 • Draw bonding diagrams for methane H H H C H H © Boardworks Ltd 2001

Activity • Copy the atoms below. • Complete the diagram showing how each atom can achieve full shells O H O H H © Boardworks Ltd 2001

Covalent Bonding Multiple bonds • • Mostly electrons are shared as pairs. There are some compounds where they are shared in fours or even sixes. This gives rise to single, double and triple covalent bonds. Again, each pair of electrons is often represented by a single line when doing simple diagrams of molecules. Cl-Cl O=O N=N Single bond Double bond Triple bond © Boardworks Ltd 2001

Covalent Bonding in Oxygen • Oxygen (2. 8. 6) needs 2 more electrons to attain a full electron shell. O O 4 electrons O O=O O © Boardworks Ltd 2001

Activity • • Nitrogen (2. 8. 5) needs 3 more electrons to attain a full electron shell and forms a triple bond Draw a bonding diagram of nitrogen. 6 electrons N N=N N © Boardworks Ltd 2001

Activity • 1. 2. 3. 4. Draw dot and cross type bonding diagrams for each of the following: Hydrogen fluoride (HF) Hydrogen sulphide (H 2 S) Ethane (C 2 H 6 and the carbons are joined by a single covalent bond) Carbon dioxide (CO 2 and the carbon oxygen bonds are double bonds) H F H H S H H H C C H H H O C O © Boardworks Ltd 2001

Giant Covalent structures 1. Carbon atoms form giant structures. 2. What is interesting is that there is more than one possible arrangement for the atoms. 3. Although this does not affect the chemical properties it can make a huge difference to the physical properties such as hardness, slipperiness, melting point and density. Different arrangements of the same element are called allotropes C © Boardworks Ltd 2001

Giant Covalent structures: Diamond • One form of carbon is diamond. • Each diamond consists of millions of carbon atoms bonded into a single giant structure. • It is very hard. Diamond Strong covalent bonds carbon atoms © Boardworks Ltd 2001

Giant Covalent structures: Graphite • A more common form of carbon is graphite. • Millions of carbon atoms bonded into a giant structure but within this structure the layers are only weakly joined. Graphite brac © Boardworks Ltd 2001

Giant Covalent structures: Carbon Footballs! • During the last 20 years new forms of carbon have been discovered some of which have “closed cage” arrangements of the atoms. • These are large but not really giant molecules. • One of them contains 60 carbon atoms and bears remarkable similarities to a football! © Boardworks Ltd 2001

Giant Covalent structures: Sand • Sand is an impure form of silicon dioxide. • Although it is a compound it has a giant covalent structure with certain similarities to diamond. silicon atoms oxygen atoms © Boardworks Ltd 2001

Activity Copy this choosing words from below to fill the gaps. giant covalent • • • molecules share double Non-metals electrons Formed when Non-metals _____ react. The atoms share ___ electrons to achieve full covalent bonding. outer electron shells by _______ Non-metals may join into small _______ molecules or they may form _____ Giant structures. A single covalent bond consists of a pair of shared _______. electrons double. Covalent molecules may also have _____ bonds consisting of a group of 4 electrons © Boardworks Ltd 2001

METALLIC BONDING © Boardworks Ltd 2001

Metallic Bonding • • • Metal atoms form a giant lattice similar to ionic compounds. The outermost electrons on each metal are free to move throughout the structure and form a “sea of electrons”. Having released electrons into this “sea” the metal atoms are left with a + charge. Metallic bonding is the attraction of + metal ions for the “sea of electrons. ” = Positively charged metal ion © Boardworks Ltd 2001

BONDING AND PHYSICAL PROPERTIES © Boardworks Ltd 2001

Bonding and physical properties • • The type of structure that substances have has a huge effect upon physical properties. These are things such as: Density Conductivity Malleabilty/ brittleness Melting point The next few slides illustrate just a few of the general patterns. © Boardworks Ltd 2001

Bonding and physical properties • • Ionic compounds are very brittle. It is the opposite charges that attract + - - + + ++ - + the ions to stay together. + - + + -- ++ If the substance is struck a blow a layer - + + - + of atoms will slip to the next position along. Attraction becomes: + - + repulsion! Blow - + - + - + © Boardworks Ltd 2001

Bonding and physical properties • • • Metals are not brittle. The metal atoms are the same and exist in simple structures. If one is struck by a blow it simply moves to the next layer along Blow © Boardworks Ltd 2001

Bonding and physical properties • • Covalent substances do not conduct electricity. Metals conduct electricity. This is because in covalent substances the outer electrons are fixed (localised) between specific atoms In metals the electrons can, given a potential, move anywhere throughout the structure H H H C C H H H Electrons fixed in covalent bonds Electrons free to move © Boardworks Ltd 2001

Bonding and physical properties • • • Ionic substances do not conduct electricity as solids. When molten or dissolved they will conduct (and also undergo electrolysis. ) This is because the electricity is carried through the solution by the ions which have to be free to move (mobile). - ++- + -+ - -+ + -- ++ -+ +- -+ + + Solid – not free to move. Doesn’t conduct + - + - Molten – mobile. Does conduct © Boardworks Ltd 2001

nd • ing Generally giant and phsubstances ysical prowith pert ies structures have • high melting points and boiling points. Small molecules have melting points and boiling points that increase as the size of the molecule increases. all Molecules tend e gas, liquid or low ing solids. Weak forces between In Giant Structures all the atoms are tightly bonded together. Usually high melting point solids - ++- +-+- + -+ - -+ + -- ++ -+ + + © Boardworks Ltd 2001

nd • ing. Generally and phsubstances ysical prowith pertgiant ies structures do not • dissolve easily (although many ionic compounds dissolve in water for a special reason). Again this is because in giant structures separating the particles involves breaking chemical bonds Small molecules usually Giant St dissolve in a range of ructures generally don’t solvents. We just dissolve separate one molecule easily. from another Strong bonds between the Weak atoms/ions forces between + - ++ +- - + molecules - -+ + -- ++ - + + © Boardworks Ltd 2001

nd • ing. Thean density d phofyssubstances ical prodepends pertiesupon how closely the • • atoms are packed together. Giant structures, metals especially, tend to be dense because all atoms/ions are pulled tightly together. Small molecules often have lower densities Small molecules tend Giant to have low densities Structures generally have high because of space densities wasted between the molecules Atoms / ions held closely together Weak forces + + between- +- + - + +- -- + molecules + -- + ++ -+ - -+ © Boardworks Ltd 2001

Activity • Copy the Table and fill in the blank columns Substance Soluble in petrol Melting Point Conduct as solid Conduct when molten Small or giant? Metal Ionic Small Mol Giant Mol A No High Yes Giant Metal B Yes Low No No Small Mol C No High No No Giant Mol D No High No Yes Giant Ionic E No Low Yes Giant Metal Substance E is peculiar: Can you suggest an actual substance that E could be? Can you explain the low melting point? © Boardworks Ltd 2001

Activity What type of structure do I have? • • metallic I conduct as a solid and a liquid. ionic I conduct as a liquid but not as a solid. I am a gas. Small molecule I am a brittle solid that dissolves in water. ionic I am malleable. metallic I don’t conduct but am hard to melt. Giant molecule I am a solid but I melt easily Small molecule © Boardworks Ltd 2001

Activity • What type of bonding will the substances have? Substance Brass (Alloy copper + zinc) Copper oxide Sulphur dioxide Iron Sodium fluoride Nitrogen chloride Bonding Metallic Ionic Covalent © Boardworks Ltd 2001

Which of the following will have covalent bonding? A. B. C. D. Sodium chloride Iron Bronze Nitrogen dioxide © Boardworks Ltd 2001

Which of the following will have metallic bonding? A. B. C. D. Copper chloride Graphite Bronze Phosphorus chloride © Boardworks Ltd 2001

Which is a true statement about covalent bonds? A. Usually formed between metals and nonmetals B. Involve transfer of electrons between atoms. C. Form full electron shells by sharing of electrons. D. Always involve 2 electrons per atom. © Boardworks Ltd 2001

Which of the following exists as a giant molecular structure? A. B. C. D. Water Carbon dioxide Sodium chloride Diamond. © Boardworks Ltd 2001

What will be the formula of the compound formed by hydrogen and sulphur? A. HS B. H 2 S C. HS 2 D. H 2 S 2 1 16 H S 1 1 32 2. 8. 6 © Boardworks Ltd 2001