Chemistry 11 Bonding Environment – Types of Substances Answer Key Ionic Bonding 1. Ionic bonds are usually formed between what two types of elements? Ionic bonds occur between elements that are classified as metals and nonmetals. 2. What happens to the atoms and electrons involved when an ionic bond is formed? In an ionic bond, the metal will lose its valence electrons become a positively charged ion (cation). These electrons are being donated to the nonmetal. The nonmetal will accept the electrons becoming a negatively charged ion (anion). The attraction between these positive and negative ions, due to their opposite charges, creased the ionic bond. 3. What type of shape is formed in an ionic bond? Are these known as strong or weak types of bonds? Why? These anions and cations that create the ionic bond do so in a 3-D structure called a crystal lattice. These bonds are considered to be strong because of the strong attraction between positive and negative ions. Also, the tight packing of ions in the lattice will make the overall structure very strong. The actual strength of the bond depends on the size of the ion, charge of the ion and the lattice packing. Ions with higher charge, smaller size and tighter packing will make a stronger ionic bond. 4. Substances conduct electricity when a charge can be carried to complete the circuit. (a) Will an ionic compound conduct electricity as a solid? Explain using your own words. As a solid, an ionic compound will not conduct electricity. This is because the ions are in a fixed lattice (solid structure) with no movement of ions. It is these ions (with negative and positive charges) that will conduct the electricity. Thus, with no moving ions there will be no conduction of electricity. (b) Will an ionic compound conduct electricity when dissolved or when melted? Explain. When an ionic compound is dissolved or melted, the ions are now free moving. These ions are now capable of conducting electricity by carrying the charge. 5. Does and ionic compound have a high or low melting point? High or low boiling point? Explain why. Ionic compounds have a high melting point and a high boiling point. This is because of the strong ionic bonds in the arrangement of the lattice. The anions and cations will have a strong attraction to each other and this holds them tightly together. A significant amount of thermal energy is needed to overcome this attraction and break these bonds. 6. Are ionic compounds known as being brittle or malleable? Explain discussing the bonds. Ionic compounds are said to be brittle. This is because of the arrangement of the ionic lattice. This arrangement is specific to alternate the positive and negative ions and keep similar charges far apart. When it is altered, the like charges will become close to each other and will repel each other. This repulsion shatters the lattice causing the compound to break. Covalent Bonding 1. Covalent bonds are mainly formed between which types of elements? Covalent bonds occur between nonmetal elements. 2. What is happening to the atoms and electrons involved when a covalent bond is formed? In a covalent bond the unpaired valence electrons of adjacent atoms will pair up and share the paired electrons between the two atoms. The covalent bond is the strong force of attraction between the nuclei of two atoms and their shared electrons (intramolecular force). 3. Do compounds with covalent bonds conduct electricity? Explain why or why not. Covalently bonded compounds do not conduct electricity, regardless of the state they are in. This is because electrons are being shared during the bond, not donated or accepted. Because there is no transfer of electrons, there are no ions. Ions are what conduct electricity and thus, without ions, the compounds cannot conduct electricity. Also, covalent compounds are uncharged molecules with the electrons held tightly together. This also means there are no free electrons, or charged ions, to conduct electricity. The exception to this rule is when a covalent compound reacts with water. When this occurs, ions are formed and the compound is able to conduct electricity. Examples of this are acids and ammonia. 4. Do covalent compounds have a high or low melting point? High or low boiling point? Explain the reasoning behind this. Covalent compounds have low melting and boiling points. This is because the forces of attraction between molecules are very weak and do not require a lot of thermal energy to break them. 5. What are intermolecular forces? Intermolecular forces are the week forces of attraction between molecules. They are also called Van der Waals forces or Dispersion forces. 6. Are all types of sharing equal? Explain this in terms of electrons and state the name for this type of bond. No, all types of sharing are not equal. Electrons that are shared equally will spend an equal amount of time around each of the atoms involved in the sharing. When they are not shared equally, the electrons will spend more time around one atom than the other. This causes the atom that has the electrons for more of the time to have a partially negative charge. The atom that has the electrons for less of the time will have a partially positive charge. This results in a polar covalent bond, as each part of the molecule has a partial charge. Metallic Bonding 1. What is an alloy? An alloy is a blend of two or more different metals. Examples are brass (which is made up of copper and zinc) and bronze (which is usually made up of copper and tin) 2. Metallic bonds form between which types of elements? A metallic bond is the bond formed between two metal elements 3. What happens to electrons when metallic atoms bond? In general, metals will lose their valence electrons in order to have a complete octet. This causes them to become positively charged ions (cations). As two metals combine, both will want to release their electrons, neither wanting to gain anymore. Therefore, these released valence electrons become delocalized, meaning they can move anywhere throughout the metallic sample. This creates what is commonly called a “sea of electrons” 4. What two factors affect the strength of metallic bonds? The strength of a metallic bond depends on two things. One, the size of the metallic ion. The smaller the ion size, the stronger the metallic bond. Two, the number of valence electrons. More valence electrons mean more electrons are the “sea” making the overall metallic bond stronger. 5. How can metallic bonds conduct electricity and heat? Metallic bonds are poor conductors of electricity and heat, but conductors nonetheless. They can conduct electricity and heat because of the delocalized electrons, with a negative charge, allowing for the carrying of heat and electricity. 6. What makes pure metals and alloys malleable and ductile? Discuss this in terms of atoms and electrons. The sea of electrons means that the electrons are flowing freely throughout the molecule. There is no competition between oppositely charged ions or need to keep them far apart from each other. Everything is moving and flowing freely. For this reason, it is easy for metals to move, bend and shape as these things do not disrupt the flow of electrons or the bonding. Network Solids 1. What are network solids? Network solids are arrangements of covalently bonded atoms. They are giant molecular lattice structures. 2. Do giant covalent network structures a high or low melting point? High or low boiling point? These structures have high melting and boiling points. This is because the networks are a large series of covalent bonds making them very hard to break or disturb. This requires a large amount of thermal energy in order to overcome the strength of these bonds. 3. What are allotropes? What are the four allotropes of carbon? Allotropes are elements that can exist in two or more different physical forms. For example carbon can exist in four different forms; diamond, graphite, Buckminster fullerene and carbon nanotubes. 4. Name three properties of diamond. Diamond is very hard, has a high melting point and is not easily compressed. Also, when diamond is cut, it reflects light. 5. How is the bonding of graphite different than the bonding of diamond? In diamond, each carbon atom is bonded to four other carbon atoms in a tetrahedral arrangement. Alternatively, in graphite each carbon atom is bonded to three other carbon atoms, forming one double bond and two single bonds. Graphite is arranged in a two dimensional layer with carbon atoms arranged in a series of hexagons. 6. Where is a common place that Buckminster Fullerene is found? Describe this type of bonding with the carbon atoms. Buckminster Fullerene is a structure containing 60 carbon atoms. It is commonly found in the soot of a blown out match. Each carbon atom is bonded to three other carbon atoms, two are singly bonded and one is a double bond. The arrangement of these carbon atoms is in the shape of a sphere, consisting of a series of pentagons and hexagons (think of a soccer ball). 7. How are carbon nanotubes unique in their bonding compared to the other allotropes of carbon? Carbon nanotubes consist of carbon atoms bonded in a series of pentagons and hexagons. These are arranged into a flat sheet that is folded together end-to-end to create a tube. These tubes have a hollow center in which some other compounds can pass through. Extra Thought Questions… Why is graphite able to conduct electricity? Due to many experiments, it was discovered that the three bonds between carbon atoms are identically, suggesting that they are all single bonds. This results in an extra pair of electrons that is not associated with a single atom or bond. This extra pair of “free” electrons will allow for the conductivity of electricity as they are negatively charged and can carry this electrical charge. What are two other, non-carbon, examples of allotropes? Explain the bonding within each example. One example of an allotrope is quartz. This consists of silicon and oxygen atoms. Each silicon atom is singly bonded to four other oxygen atoms and each oxygen is bonded to two silicon atoms. This creates the three dimensional network. A second example is clay or asbestos. In these examples, silicon atoms are singly bonded to four other oxygen atoms however each oxygen atom is only bonded to one silicon atom. This results in some silicon atoms having a charge, becoming a silicate ion. In clay, the silicate ions form a two dimensional network. In asbestos, the silicate ions join to form chains. To further your understanding of melting points, boiling points, electrical and thermal conductivity read through pages 216-221 in your textbook.