Unit 1 – Day 3 Bonding The Stable Octet • Atoms are most stable when they have a full outer shell of electrons. This is why noble gases, which have a full shell, do not react. • For other elements, they will need to gain or lose electrons to obtain a full outer shell. Ions • When an atom gains or loses electrons, it becomes a charged ion. • If an atom loses negative electrons, it becomes positive. • If an atom gains negative electrons, it becomes negative. • Example: Na Na+ + eS + 2e- S2- Metals • Metals are elements on the left side of the periodic table, like sodium or iron. • They have less than 4 electrons in their outer shell. • They tend to lose electrons and become positive ions. Non-metals • Non-metals are elements on the right side of the periodic table, like oxygen or chlorine. • They have more than 4 electrons in their outer shell. • They tend to gain electrons and become negative ions. Ionic Bonds • Metals tend to give their electrons to nonmetals, so they can both get a full outer shell. • The metal and non-metal ions are opposite charges, and they attract. • This attraction is called an ionic bond. Properties of Ionic Compounds • An ionic bond is very strong. • As well, the ions arrange themselves so that molecules attract to other molecules nearby. Properties of Ionic Compounds • Because of this, it is very difficult to pull apart ionic molecules. • This affects many properties. • Melting Point: – Ionic compounds are very difficult to melt, because in order to make a liquid, molecules need to move farther apart. Properties of Ionic Compounds • Smell – Ionic compounds have no smell, because in order to enter the air, molecules would need to leave the surface. • Hardness – Ionic compounds are hard to crush, because the network of molecules supports each other in a crystal. Properties of Ionic Compounds • Solubility – Ionic compounds dissolve in substances like water. The charges from a water molecule pull apart the ions. Properties of Ionic Compounds • Electrical Conductivity – When melted or dissolved, ionic compounds separate into ions. These ions can carry electrical charges. Covalent Bonds • Non-metals need to gain electrons to fill their outer shell, so they sometimes share electrons with each other. • When two non-metals share electrons, it is called a covalent bond. Pure Covalent Bonds • If the atoms are the same, they share equally. This is called a pure covalent bond. • Ex: H2, O2, N2 , CH bonds • Pure covalent compounds have no charges, so the molecules don’t attract each other as well as ionic compounds do. Pure Covalent Properties • Pure covalent compounds have the following properties: – Strong odour (molecules escape into the air easily) – Crush easily (no crystal support) – Do not dissolve in water (no charges to attract) – Do not conduct electricity (no ions) – Low melting pt. (molecules are easy to separate) Polar Covalent Compounds • Sometimes when atoms share electrons, the electrons are closer to one atom than the other. • This results in one atom having a slight negative charge, and the other having a slight positive charge. Polar Covalent Properties • Pure covalent compounds have the following properties: – No odour (slight charge holds molecules in place) – Hard (charges have some crystal support) – Dissolve in water (slight charges attract water) – Do not conduct electricity (no ions) – Average melting point (molecules are easier to separate than ionic, but not as easy as pure cov.) Table of Properties • Use your notes to summarize into a table the properties of the three types of compounds.