Metallic, Ionic And Covalent Bonding

An investigation into the relationship between bond types and properties of substances The well known properties of materials in everyday life can be explained by the interaction of atoms and molecules of which they are comprised with their neighbours. The different types of bonds between atoms produce different properties such as melting point, electrical conductivity and density in compounds like Titanium, Sodium Chloride and Water. The melting point of an element or compound is the temperature at which the substance changes state from the solid form to the liquid form. For a substance to melt it would have to be heated to such a degree that the bonds holding its atoms and molecules together would be severed. The melting point of a substance depends on the strength of the bonds holding the particles together. Sodium Chloride (NaCl) has a high melting point of around 800 o C. NaCl is bonded through ionic bonding. An ionic bond is a type of chemical bond where there is an attraction between two oppositely charged ions, usually a metal and a nonThe ionization of salt metal. The metal donates one or more electrons to the non-metal to form a positively charged cation, while the non-metal becomes a negatively charged anion. This exchange of electrons leads to the 2 atoms having a stable electron configuration. In sodium chloride (NaCl), the sodium atom loses an electron to Figure 1 become a cation and the chlorine atom gains an electron to become a chloride anion. These two ions are attracted to each other in a 1:1 ratio to form the compound sodium chloride (Figure 1). NaCl forms crystals with a cubic symmetry. The large Cl- ions are arranged in a cubic close-packing, while the small Na+ ions fill the octahedral gaps between them. Each ion is surrounded by 6 oppositely charged ions and this arrangement is called cubic close packed. The ionic bonds in sodium chloride are very strong, so it would take a lot of energy to break these bonds apart. These bonds are so strong because the attractive forces between the two oppositely charged ions are very powerful. It could therefore be supposed that this could lead to NaCl having a high melting point. Titanium has an even higher melting point of around 1660º C. Titanium atoms are held together by forces of attraction called a metallic bond. A metallic bond is one where there is an electrostatic attraction between delocalized electrons and metallic The structure of titanium ions with metals. Metallic bonds are non-polar because there is no or hardly any difference in the electronegativities of the atoms involved in the bonding relations, and the electrons concerned are delocalized throughout the crystalline structure of the metal. Metal atoms contain few electrons in their valence shells and as such these electrons easily become delocalized to form a sea of electrons. The freedom of these electrons to move allows the metal atoms or layers of atoms to slide past each other and this gives rise to such metallic properties such as malleability and ductility. The Figure 2. negative electrons and positive metal ions have a strong attractive force between them. Therefore, metals have high melting and boiling points as a lot of energy is required to overcome these forces. Water (H2O) has a melting point of 0oC. The atoms in water are bonded together through a covalent bond. A covalent bond is a form of chemical bonding where there is a sharing of pairs of electrons between two non-metallic atoms or The dipole-dipole bonding in H2O between non-metallic atoms and other covalent bonds to attain complete outer shells of electrons. In a molecule of water two hydrogen atoms are covalently bonded to a single oxygen atom. The Figure 3

reason why water is in a liquid state is because it is more electronagative than most other compounds. In H2O the Oxygen atom has a higher electronegativity than the Hydrogen atom and this leads to water being a polar molecule. Oxygen has a slightly negative charge and the Hydrogen has a slightly positive charge because the electrons orbiting the atoms tend to stay closer to the O2 atom rather than the H2 atom (Figure2). This gives the molecule a dipole effect. The electrical conductivity is the ability of a substance to conduct an electrical current. Electricity is conducted when electrons travel from an area where there is a negative electrode to a region where it is positive. Water is a fairly good conductor of electricity. However this is due to the presence of certain ions of mineral salts and CO2. Pure, distilled water contains no ions so there is almost no electrical conductivity. When salts dissolve in water they give off electrically charged ions which conduct electricity. Water does not conduct electricity because the ions which make up the H2O molecule are held together by extremely strong covalent bonds. These covalent bonds do not allow the H2 and O2 ions to become disjoint and therefore no movement of ions takes place to conduct the electricity. Ionic substances do not make good electrical conductors. NaCl in its solid state is a bad conductor of electricity. However, when it is dissolved in water the ions that make up the NaCl molecule can move around freely so that sodium chloride in its NaCl in solid and saturated form molten or dissolved state is a good conductor of electricity. When NaCl is saturated it decomposes into sodium and chlorine and these atoms are surrounded by water molecules (Figure3). Ionic substances when molten conduct electricity because the ions are free to move and because the charged particles (the ionic substances) are mobilized they can flow from a negative electrode to a positive one. Sodium chloride is a bad conductor of electricity Figure 4 because it is bonded through ionic bonds. In an ionic bond, since the ions are held together they are not able to move around to conduct the electricity. It could therefore be presumed that this could be a reason as to why ionic and covalent substances are not good conductors of electricity. Metals are very good conductors. This is because metals are bonded through metallic bonding. In a metallic bond, delocalized electrons weave through positively charged cations. Electrostatic forces of attraction between the positive cations and the negative delocalized cells hold the lattice together. When an electric field is applied, a slight imbalance develops and electrons are free to flow. The electrons that are free to move about the cations conduct electricity and this makes metals good conductors. However, not all metals are good conductors of electricity. The atomic structure of a metal strongly influences its electrical behavior. Titanium is not a good conductor of electricity. If copper had a conductivity of 100%, Titanium would only have a conductivity of around 3%. This is because Titanium can only lose up to four electrons and these electrons weave through the metal structure. It could therefore be presumed that with the more loss or release of electrons from the atom of that element the better it conduct electricity as there are more electrons intertwining through the structure. The density of a substance is the ratio of the amount of matter compared to its volume. It is, essentially, a measurement of how tightly matter is packed together. One of the most common uses of density is in how different materials interact when mixed together. Wood floats on water because it has a lower density, while an anchor sinks because the metal has a higher density. Helium balloons float because the density of the helium is lower than the density of the air. Density is a property of matter that is unique to each substance. The density of NaCl is 2.16 g per cubic cm. This means that in every 1cm3 of its volume sodium chloride has a mass of 2.16g. The density of titanium is 4.95g/cm3. This is because it is bonded through a metallic bond. As Ti loses up to four electrons and can become positively charged, the negative delocalized electrons and positive metals ions are attracted to each other strongly. It can safely be presumed that with the

fewer electrons lost to obtain full outer shells, the lower the density of that substance is. This is because the atoms are held together less strongly. 3 The Hydrogen bonding between water molecules Water has a density of 1g/cm . This means that water weighs 1 gram for every cubic centimeter of its volume. The reason that water is not so dense is because the molecules are held together by hydrogen bonding. Hydrogen bonds are dipole-dipole forces which are relatively weak compared to metallic, ionic and covalent bonds. As a result, the water molecules are not held as close to each other as other substances and this results in water having a low density.

The bonds which hold the particles of a substance together affect the properties of these substances. Substances of metallic, covalent and ionic bonds each have different properties as a consequence of these bonds. Figure 5 Such properties as the melting point, density and electrical conductivity can all be linked to the way bonds hold particles of a substance together.