BONDING AND PROPERTIES 4.5 lessons Read over Introduction to Bonding Powerpoint to remember the three bonding types and the placement of electrons forming the bonds for each. Remember, ionic bonding involves metal and non-metal atoms combined together. Covalent bonding involves two non-metal atoms and has two bonding types (inside the molecule and between molecules). Metallic bonding is metal atoms only. This is summarised on the table on the next slide. Review of bonding types (from previous section - Introduction to bonding) Ionic Bonding Metal + nonmetal Metallic Bonding Metals Covalent Molecular Nonmetal + nonmetal Covalent network Nonmetal atoms Bonding Electron(s) are transferred from one atom to another causing formation of anion and cation. Many of these form a strong lattice structure. It occurs between a metal and a non-metal atom The valence electron(s) of metallic atoms become detached and move freely. Bonding consists of the nuclei (cations) in a 3D lattice surrounded by a sea of electrons. Inside a molecule electron(s) Electron sharing as for are shared between two atoms covalent molecular, but large as they attempt to have a full numbers of atoms are made outer shell (8 electrons). One into a network structure. electron pair is a single covalent Carbon makes up most bond (strong intramolecular examples of network solids bond) Between molecules weak intermolecular bonds exist. Naming Use the name of the element for pure substances First write the name of the more metallic element then use the name of the less metallic element with an ‘ide’ on the end.(ie. In the order left to right that they are shown on the periodic table) Add ‘di’ to show two atoms and ‘tri’ to show three and ‘tetra’ for four atoms. First write the name of the cation then use the name of the anion with an ‘ide’ on the end. Balance the charges (use swap and drop method in text if you choose) to make a neutral compound diamond graphite graphene Ionic, covalent and metallically bonded substances have particular physical properties resulting from the bonding types and electron placement. Many trends can be distinguished however we will focus on melting point and boiling point, electrical conductivity, solubility and strength/brittleness/hardness. 1. MELTING POINT AND BOILING POINT – STRONG BONDS CAUSE HIGH MP/BP Ionic bonding is particularly strong and forms a rigid 3-dimensional structure where ions are held by electrostatic attraction. This means that the MP and BP of ionic substances is high – a large amount of energy is needed to break these bonds and convert a solid to liquid OR a liquid to gas. Ions that have a 2+ or 3+ charge form stronger ionic bonds than 1+ ions. Also, the lattice of polyatomic ions is less symmetrical (and weaker) than monoatomic lattices. For example, NaCl compared to NaMnO4. Covalent substances have strong intramolecular bonds but weak intermolecular bonds. This means that these bonds are easily broken allowing the molecules to separate form each other (a covalent gas has separated molecules NOT separated atoms). So, covalent compounds have low MP and BP, and are generally gases at room temperature. Polarity in a covalent molecule increases the intermolecular forces and so these compounds have higher MP and BP (see Unit 2 Topic 1 Intermolecular forces). Covalent network substances have very high MP/BP due to their stable regular structure Metallic bonding is generally strong but there are numerous exceptions to this trend. Metallic 2. ELECTRICAL CONDUCTIVITY – FREE ELECTRONS OR CHARGED PARTICLES ALLOW ELECTRICAL CONDUCTIVITY Ionic bonding involves the formation and association of charged particles (ions) but these are tightly held by electrostatic forces in its rigid 3-dimensional structure. This means that electrons and charged particles cannot move through the solid, and electricity will not be conducted. However, when the structure is broken by melting or dissolving the charged ions are able move, and therefore in these situations electricity is conducted. In covalent substances there are no charged particles and therefore electricity is not conducted. Metallic bonding generates cations in a sea of electrons (delocalised electrons). The movement of these electrons promotes electrical conductivity and so metal will conduct electricity very effectively. 3. SOLUBILITY – ABILITY TO ATTACH TO WATER MOLECULES CONFERS SOLUBILITY Ionic substances are highly soluble in water as the anions and cations in the ionic structure attract the partially charged areas of the water molecule. These interactions are strong enough for the ionic lattice to be broken, and the ions to be dispserde (dissolved) through the water. Some covalent molecules show polarity (are a dipole – see intermolecular forces section later). Only these compounds are soluble because the opposite charges in the molecule and water attract. Non-polar covalent substances are not soluble in water. Metallic substances are not soluble in water as the bonding is strong enough to resist breakdown by water molecules. 4 STRENGTH, BRITTLENESS AND HARDNESS Ionic substances are hard but they do not have strength because when an applied force alters the shape of the lattice so that two similarly charged ions are next to each other, there is strong repulsion and the crystal shatters. The weak intermolecular bonds in covalent substances means that these compounds are soft solids, liquids or gases. They are not strong brittle or hard. Metallic bonding is relatively strong and so metals are hard. Also, the structure has delocalised electrons (cations in a sea of electrons) allows some flexibility, and so many metals are malleable (their shape can be changed by force) and ductile (they can be drawn into a wire). SUMMARY OF LINKS BETWEEN BONDING AND PROPERTIES (NEXT SLIDE) Ionic Bonding Metallic Bonding Covalent Bonding Covalent network Melting point and boiling point high medium to high. low high Electrical conductivit y no (unless dissolved/melted) yes no Solubility soluble (mostly) insoluble some no (except nanomaterials and graphite) insoluble brittle, hard hard, malleable and ductile liquid gas or soft solids). Properties very hard lattice SUMMARY OF LINKS BETWEEN BONDING AND PROPERTIES (NEXT SLIDE) (AS YOU CAN SEE, THESE PROPERTIES ARE EASILY REMEMBERED BY HAVING A GOOD UNDERSTANDING OF THESE THREE BONDING TYPES) Covalent Bonding Covalent network Melting High melting and boiling points The structure is strong but with some point and because electrostatic attraction is movement. Melting points and boiling boiling very strong and a regular 3D points are medium to high. point lattice is formed Lower melting points and boiling points because the intermolecular forces between molecules are low High melting and boiling points because bonding is very strong and a regular 3D lattice is usually formed Electrical conductivi ty No electrical conductivity High electrical conductivity because because ions are very tightly there are delocalised valence electrons bound (not movable) free to move Electrical conductivity will occur when dissolved or melted because charged particles are able to move No electrical conductivity because there are no charged particles present No electrical conductivity because valence electrons are contained in stable covalent bonds (not movable) (except nanomaterials and graphite) Solubility Soluble (mostly) because partial insoluble charges on water molecules are attracted to ions Ionic Bonding Properties Brittle because changing the shape of the lattice forces like charges together and it splits Hard because the bonding is strong Metallic Bonding Covalent molecule which have partial charge (polarity) will dissolve in water. Those with no partial change will not dissolve (see polar and nonpolar) Most are liquid or gas (some are soft Metals are hard, malleable and ductile solids). (their shape can be changed). Bonding is strong but the ‘sea’ of electrons allows some movement of cations in the lattice.. Insoluble Very hard lattice, giant molecule or network solid. Many examples are comprised of carbon PRACTICE QUESTIONS Properties of metals Module 6.1 review pg 142 Properties of ionic substances Module 7.5 review pg 191 Properties of covalent substances Module 8.1 review pg 199 Complete Skills book worksheet WS 1.1.2 WS 1.2.5 ALKANES ARE A GROUP OF MOLECULES CONTAINING C AND H OF VARYING LENGTH. THEY ARE STABLE STRUCTURES BUT WILL UNDERGO COMBUSTION REACTIONS. ALKANES ARE KNOWN AS SATURATED MOLECULES BECAUSE ALL CARBON ATOMS ARE BONDED TO THE MAXIMUM NUMBER OF OTHER ATOMS (4) THE MOLECULES OF THE LARGER ALKANES (SUCH AS OCTANE NONANE OR DECANE) HAVE FORCES BETWEEN THEM (SEE LATER) THAT CAUSE THESE LARGER MOLECULES TO HAVE HIGHER BOILING POINTS ALKENES ARE A FAMILY OF MOLECULES OF DIFFERENT SIZES THAT HAVE AT LEAST ONE DOUBLE BOND. THIS MEANS THAT THE MOLECULE IS UNSATURATED AND CAN REACT WITH HYDROGEN ATOMS. AFTER THIS REACTION THE MOLECULE IS NOW SATURATED Another covalent molecule with interesting properties is benzene. Its ring structure and electron sharing confers stability. Some organic compounds based on benzene have become persistent organic pollutants in environmental situations where they have been used. Syllabus Bonding and properties (3 hours) • recognise that the properties of ionic compounds, including high melting point, brittleness, and ability to conduct electricity when liquid or an aqueous solution, can be explained by modelling ionic bonding as ions arranged in a crystalline lattice structure with strong electrostatic forces of attraction between oppositely charged ions (metallic lattice, giant covalent networks, allotropes — carbon) • understand that the type of bonding within ionic, metallic and covalent substances explains their physical properties, including melting and boiling point, thermal and electrical conductivity, strength and hardness • understand that hydrocarbons, including alkanes (saturated), alkenes (unsaturated) and benzene, have different chemical properties that are determined by the nature of the bonding within the molecules • analyse and interpret given data to evaluate the properties, structure and bonding of ionic, covalent and metallic compounds.
