Chapter 19 Notes
Objectives: Describe how a compound differs from its elements.
Explain what a chemical formula represents.
Explain chemical bonding.
Describe ions, ionic bonding, and the product.
Describe covalent bonding and its product.
Distinguish between polar and nonpolar molecules.
Explain how to determine oxidation numbers.
Write chemical formulas and names.
Describe hydrates.
Section 1 Stability in Bonding
The properties of a compound will be different from the elements that make it up. An example is salt.
Na (soft metal explosive in water) + Cl (toxic green gas) = NaCl (white crystal)
A chemical formula tells what kind and how many atoms of each element are found in a compound.
Atoms form compounds when the compound is more stable than the element.
Noble gases (Group 18) are more chemically stable than any other group of elements because they have a completely filled outer level.
Atoms can gain, lose, or share electrons to become more stable.
An atom wants to have eight electrons in its outer energy level.
A chemical bond is the force that holds atoms together in a compound.
An atom is chemically stable when its outer energy level is filled.
Section 2 Types of Bonding
An ion is a positively or negatively charged particle.
A positive ion is called a cation.
A negative ion is called an anion.
When an atom loses an electron it becomes positively charged.
When an atom gains an electron it becomes negatively charged.
A superscript (written above) tells the charge that an atom has.
Ionics Bonds.
1) transfer electrons
2) occurs between metal and a non-metal
3) produces an ionic compound (usually a crystalline solid at room temp)
Covalent Bonds
1) shares electrons
2) occurs between two non-metals
3) produces a molecule (usually a liquid or gas at room temperature).
Covalent bonds are based on the number of shared pairs of electrons.
One pair - single bond
Two pair - double bond
Three pair - triple bond
In a molecule electrons are pulled stronger to the larger nucleus.
A polar molecule has opposite ends (1 partially negative, 1 partially positive).
A non-polar molecule has equal sharing of electrons, no opposite ends.
Section 3 Writing formulas and Naming Compounds
Chemists use symbols from the periodic table to write formulas for compounds.
A binary compound is composed of 2 elements.
An oxidation number tells how many electrons an atom gains, loses, or shares.
When writing formulas:
1. Use oxidation numbers and their least common multiples
2. Write the cation then the anion.
3. Use subscripts to make the charges balance out.
4. Make the compound neutral.
When naming compounds:
1. Use the name of the first element.
2. Then use the root name of the 2nd element and the suffix –ide.
A polyatomic ion is a positively or negatively charged covalently bonded group of atoms.
The compound contains three or more elements.
To write names:
Write the positive ion first.
Write the negative ion.
To write formulas:
Use the oxidation numbers, their least common multiple, and put parentheses around the polyatomic ion before adding a subscript.
A hydrate is a compound with water chemically attached to its ions.
Name covalent compounds using prefixes to indicate how many atoms of each element are in the compound.