CHEM11 Bonding

Ionic bonding 

  • A type of chemical bonding that involves the electrostatic force of attraction between oppositely charged ions. 

Ionic Compounds 

  • A type of chemical compound that involves the electrostatic attraction between oppositely charged ions. 

 

  • Ionic compounds are made of the combination of atoms, or groups of atoms, where electron(s) are transferred from one to another. In doing so, the particles become ions, hence ionic compounds are compounds from ions. 

Properties of ionic compounds 

  • Have high melting and boiling points and they are solids at room temperature 

  • Are hard but brittle 

  • Do not conduct electricity in the solid state 

  • Are good conductors of electricity in the liquid state or when dissolved in water 

  • Vary from very soluble to insoluble 

Ionic bonding model 

  • Large numbers of cations and anions combine to form a three-dimensional lattice. 

  • The three-dimensional lattice is held together by strong electrostatic forces of attraction. The electrostatic force of attraction holding the ions together is called ionic bonding. 

High melting points  

  • To melt an ionic compound energy is required to allow the ions to break free and move. The high melting points indicate that a large amount of energy is required to overcome the electrostatic attraction between oppositely charged ions and allow them to move freely. Therefor the ionic bonds between the positive and negative ions must be strong, which explains why a high temperature is required to melt solid ionic compounds. 

Hardness and Brittleness 

  • There are strong electrostatic forces of attraction between ions in an ionic compound, so a strong force is needed to disrupt the crystal lattice. Therefore, one of the properties of ionic compounds is that they are hard. 

Electrical conductivity 

  • When ionic compounds are solid the lattice is held together and the atoms can't move freely 

  • When solid ionic compounds melt, the ions become free to move, enabling the cations and anions in the molten compound to conduct electricity. 

  • When ionic compounds dissolve in water, ionic bonds in the lattice are broken and the ions are separated and move freely in solution. 

Solubility 

  • Some ionic compounds are very soluble in water whereas others are insoluble. When a soluble ionic compound is added to water the ions break away from the ionic lattice and mix with the water molecules. If an insoluble compound is added to water, the ions remain bonded together in the ionic lattice and do not form a solution. 

Modifying metals 

A metal can be modified by: 

  • Heat treatment 

  • Alloy production 

  • Formation of nano-sized structures 

Alloys 

  • A homogenous mixture when other elements (either metal or non-metal) are added to a metal to alter its properties and therefor its possible use 

  • Generally, alloys are harder than pure metal due to the different sized particles of the metal being added disrupting the original orderly lattice structure. This prevents the easy movement of atoms within the alloys. 

  • Interstitial 

Substitutional alloy 

  • The mixture formed when atoms of the added element replace some of the metal cations 

  • In general, the metallic elements added to make these alloys have fairly similar chemical properties to form cations of similar size to the main metal 

  • Examples include manganese, chromium, nickel, cobalt 

Work hardening and heat treatment 

  • The way a metal object is prepared also affects how it behaves. Many metals are prepared in the liquid state and then cooled. The rate at which a metal is cooled affects the properties of the solid. 

  • A sample of solid metal consists of many small crystals. Each crystal is a continuous arrangement of cations surrounded by a sea of delocalized electrons. 

Work hardening 

  • Hammering or working cold metals causes the crystals to rearrange as they are pushed and deformed. This can result in the hardening of the metals as the crystals are flattened out and pushed closer together. 

Heat treatment 

  • Physical properties of a metal can be altered by controlled heating then cooling. 3 main methods of heat treatment are annealing, quenching and tempering. 

Forms of metallic nanomaterials 

  • Particles-spherical particles that range from 1-100nm in size. 

  • Rods-nanoscale rods in which each dimension ranges 1-100nm 

  • Wires-diameter is measured on the nanoscale, but length is unrestricted 

  • Tubes 

Gold nanoparticles 

  • These can be attached to molecules of a tumor killing agent known as a tumor necrosis factor. The nanoparticles hide the molecule from the body’s immune system. 

  • The nanoparticles carrying the TNF tend to accumulate in cancer tumors allowing the TNF to destroy tumors. The nanoparticles do not accumulate in other parts of the body. 

  

  • Metals from group 1 are more reactive in water than those in group 2 

  • Going down a group, the reactivity of the metals in the water increases 

Reactivity with acids 

  • The reactivity of different metals with acids follows the same general pattern as the reactivity of metals with water. Metals are normally more reactive with acids than with water. More metals react with acids and the reactions tend to be more energetic. 

Reactivity with Oxygen 

  • Many metals react with oxygen. The group 1 metals all react rapidly with oxygen. 

  • Group 2 metals also react with oxygen to form oxides although not as rapidly as group 1 metals. Heat is usually required to start the reaction. 

  • The transition metals are less reactive with oxygen than the metals in group 1 and 2, their reactions are also important. Many transition metals needed by society are found as oxides. 

Reactivity series of metals 

  • Chemists have used experimental data to produce a reactivity series of metals. Group 1 metals are at the top of the series while transition metals are at the bottom. 

Reasons for different Reactivities 

  • In general, the reactivity of main group metals increases going down a group and decreases across a period in the periodic table. The trend in reactivity can be explained in terms of the relative attractions of valence electrons to the nucleus or atoms. 

  • When metals react, their atoms tend to form positive ions by donating one or more of their valence electrons to other atoms. The metal atoms that require less energy to remove electrons tend to be most reactive. The most reactive metals to be those with the largest atomic radii and therefore the lowest ionization energies, which are found in the bottom left-hand corner of the periodic table, such as cesium and francium.

Metals 

  • Metals make up over 80 percent of the periodic table  

Properties of Metals 

  • Range of melting points with relatively high boiling points 

  • Good conductors of heat and electricity 

  • Generally, high densities 

  • Malleable and ductile 

  • Lustrous or reflective when cut or polished 

  • Often hard with high tensile strength 

  • Low ionization energy and electronegativity 

Properties of Transition Metals 

  • Found in center of periodic table and are typically: 

  • Harder 

  • More dense 

  • Higher melting points 

  • Strongly magnetic in many instances 

  • Brightly colored compounds 

Metallic bonding model 

  • Electrons are able to move within the lattice of metal atoms. Negatively charged electrons can be lost from the outer shell of metals, forming positive atoms (cations). The delocalized electrons form a ‘sea’ of electrons throughout the entire metal structure and are strongly attracted to the cations. 

  • In the metallic bonding model, positive meta cations are surrounded by a sea of delocalized electrons.