Science 9: Unit B

1. Science 9: Unit B Topic 6: Chemical Compounds

2. What is a compound? • A compound is when two or more element atoms combine together by either giving up electrons or sharing them. A compound is usually much more stable than the individual elements by themselves. • There are two kinds of chemical compounds: ionic and molecular.

3. Ionic Compounds • 1. Ionic Compounds - form when a metal atom gives up one or more valence electrons to a nonmetal atom. The two atoms become oppositely charged and therefore become bonded to each other. This is called an ionic bond.

4. Molecular Compounds • 2. Molecular Compounds – form when two or more non-metal atoms share their valence electrons. The bonds in a molecular compound are weaker than in an ionic compound so the compound is not as stable as an ionic compound.

5. Molecular Compounds • A molecule is the smallest unit of a molecular compound. A molecular compound is made up of many molecules bonded to each other. Eg. Water is made up of many H2O molecules bonded together. • The bond between molecules is weaker than the bond within the molecule. That is why it takes less energy to split a piece of wood than to burn wood. • Molecular compounds, unlike ionic compounds are poor conductors of heat and electricity and are usually gases and liquids instead of solids.

6. Naming Molecular Compounds • II. Naming Molecular Compounds • Write the name of the first element • Write the name of the second element, but change the ending (after the first syllable) to –ide. So oxygen would be oxide, chlorine would be chloride, hydrogen be hydride, etc. • Write the correct Greek prefix in front of each element name to indicate how many atoms of that element there are. • Do not use the prefix mono- for the first element name, just write the element name as is.

7. Molecular Compound Naming System – The Greek Prefixes For example: H2O = dihydrogen monoxide (di because there are 2 hydrogen atoms and mono because there’s just the one oxygen atom). And NH3 = nitrogen trihydride (NOT mononitrogen trihydride).

8. Ionic Compounds - Naming • Ionic compounds form when a metal gives up an electron to a nonmetal atom. Now the metal is positively charged and the nonmetal is negatively charged. Since they have opposite charges they bond together and form a stable ionic compound. • There is no sharing of electrons here, the metal actually gives up its electrons to the nonmetal. • When an atom gives or takes in extra electrons we call it an ion. Metal ions (positive charge) are called cations, nonmetals ions (negative charge) are called anions.

9. Ionic Compounds Cont’d • A common example of an ionic compound is table salt: NaCl(s). Sodium ions give up one electron to a chlorine atom to form an ionic compound, now they both are more stable noble-like atoms. • Since ions have electric charges they can conduct electricity this property is called conductivity. Molecular compounds usually do not have this property so an easy way to tell them apart is to see if they conduct electricity or not.

10. Naming Ionic Compounds • Step 1: Write name of metal first • Step 2: Write name of nonmetal second and change ending to –ide. • Step 3: For transition metals (metals in the middle area of the periodic table), you must write down the roman numeral (in brackets) for the ion charge of the cation. Eg. iron(II)chloride is a different compound than iron(III)chloride.

11. Odds and Ends • Diatomic Compounds – Special kind of molecular compound where two of the same non-metals elements will form a molecular bond together and share their electrons. Eg. oxygen gas is O2 . HONI Brushed Clean Fingersis mnemonic device to help you remember which elements form diatomic compounds. • Because atoms in molecular compounds are sharing electrons they are not as stable as atoms in ionic compounds which actually give up or take in extra electrons. Sharing electrons means that some of the time the electrons are in another atom, this makes the atom unstable during that time. In an ionic compound all atoms are stable all the time. • The stability of ionic compounds is shown when you try to perform reactions with them. You can split water into hydrogen and oxygen (chemical reaction) with a small electric current, but it takes 900°C just to melt a typical ionic compound like NaCl(s) (a physical change).