Unit 2 Bonding

• Graphite and diamond have high melting points because they're giant structures with strong covalent bonds.
• The ionic bonds break under high temperatures, or when dissolved in water with less electrostatic forces of attraction. For instance NaCl and Magnesium Oxide are both ionic compounds. In ionic compounds, the intermolecular forces aren't broken because they're made of ions. They have high boiling points due to the high heat energy required to break the strong electrostatic forces of attraction.
• The 3 allotropes of carbon that are all giant covalent structures are:

  1. Diamond - is the only one bonding to 4 covalent compounds (this case carbons), instead of 3.
  2. Graphite - although these are all covalent compounds, graphite is special because it is a non-metal that can conduct electricity with free electrons.
  3. Fullerene - shaped like a football and only one having a chemical formula: C60
• When you melt simple molecular substances (Hydrochloric Acid, Oxygen), the intermolecular forces of attraction break. That means the covalent bonds don't break because they're too strong. In simple covalent structures, you break the bond in between the molecules, not the atoms; there's no chemical change, just a physical change of it's state.

  NOTE: electrostatic forces (similar to intermolecular forces) are also in covalent compounds (NOT just ionic).
• Ionic compounds do not conduct electricity when solid, but do conduct electricity when molten and in aqueous solution due to the presence of free flowing electrons. Covalent compound's don't conduct electricity at all; molecules are neutral with no charge.
• The maximum number of electrons for:

  
  

  Shell 1 is 2

  Shell 2 is 8

  Shell 3 is 18
• In 4 Mark Questions for Ionic Bonding you must say:

  
  

  1)Number of electrons in outer shell of both elements, meaning the atoms are unstable.

  2)? atom transfers it's _ electrons giving _ to each ? atom, meaning the atoms have full outer shells and are stable.

  3)Consequently, ? has _ charge and ? has _ charge.

  4)These atoms form ionic compounds, and the electrostatic attraction between the atoms cancel each other out, leaving an overall neutral charge.
• To determine a formula for ions, you can use your remembered knowledge of ions and look at the periodic table. In chemical formulas, we don't write the charges because we assume they're already balanced.
• Ionic compounds are always empirical formulas. This is because in a solid state, they are in giant crystal lattice structures containing many ions, which is why the simplest ratio of ions is shown.
• Relative atomic mass (Ar) = The average mass of all the different isotopes in an element =(atomic mass x %abundance) (+ continue as needed for amount of isotopes)

  The units are in amu.
• Graphite is an allotrope of carbon that can conduct electricity because it can make 3 covalent bonds and has a structure in layers. Consequently, it can slide and move to conduct electricity from one delocalised electron (AKA free electrons) per carbon atom (making it soft).

  
• In covalent compounds, the (electrostatic) attraction are between the shared pairs of electrons and nuclei (plural for nucleus).

  A covalent bond consists of the simultaneous attraction of two nuclei for one or more pairs of electrons. The electrons located between the two nuclei are bonding electrons.

  
• Why does salt dissolve in water?

  Because NaCl is bonded to positive sodium ions and negative chloride ions, the positive parts of the water molecule attract the negative chloride ions. Or in other words, salt dissolves in water because of electrical charges (both water and salt compounds are polar - attractive with opposite charges).

  
  

  As a result, these attractive forces means that the water molecules pull the sodium and chloride ions apart. Now, the ionic bond that held NaCl is broken, and the salt is dissolved, as shown in the diagram.

  
• An atom is neutral when the number of protons and electrons are the same.
• In a combustion reaction (reacted with oxygen), carbon dioxide and water are always formed as the products. Carbon monoxide is only formed during incomplete combustion (lack of oxygen).

  
• Relative Formula Mass (Mr) finds the average mass of entire compounds; has more than 1 type of element. Calculate Mr by adding the relative atomic mass of all atoms in a compound. The relative atomic mass is the same atomic mass you see in the nuclear symbol box.
• Elements in the same group have similar chemical properties because:(Physical & chemical properties) depend on number of valence electrons (electrons in outer shell), which elements in the same group have the same of. Valence electrons are first to come in contact with each other; they determine how an atom will react in a chemical reaction.
• The periodic number tells you how many shells there are.
• The group number tells you how many electrons are in the outer shell.
• To count (extremely small) atoms, Avogadro's number: 6.02x10^23 is used, equivalent to one mole.
• Isotopes are atoms that have the same number of protons, but different numbers of neutrons in their nuclei.