Section 2.6: Molecules and Molecular Compounds

Most Matter is composed by molecular compounds (composed of molecules) or ionic compounds (composed of ions).
Molecules are made of atoms of nonmetal elements bound by a strong "force" called covalent bond.
Molecules are finite substances (they have a limited amount of atoms) and they may be solid, liquid or gaseous.
A few elements occur naturally as diatomic molecules: H2, N2, O2, F2, Cl2, Br2, I2. Therefore when we say nitrogen, we mean the molecule N2.
Molecules are represented by a chemical formula: e.g. H2O is the molecular formula of water, and it shows that one molecule of water contains 2 atoms of hydrogen and one atom of oxygen.
Molecular formulas show exactly the number and types of atoms in a molecule.
Empirical formulas show the smallest possible whole-number ratios.
e.g.: HO is the empirical formula of H2O2 (hydrogen peroxide)
Do: Write the molecular formulas of two compounds which have the same empirical formula, CH2. 
A molecule may be represented by a chemical formula, a structural formula, perspective drawing, ball-and-stick model and space-filling model. See page 51.

Section 2.7: Ions and Ionic compounds
Q: Which part of atoms are unchanged by chemical processes?
Q: What particles do atoms may lose or gain?
Q: What happens to an atom if it loses an electron? 
Q: What does it become?
Q: Does it increase or decrease in size?
Q: What happens to an atom if it gains three electrons? 
Q: What does it become?
Q: Does it increase or decrease in size?
Q: Why are ionic compounds electrically neutral?
Ionic compounds have extremelly large, three-dimensional structures.We use formula unit to represent the minimal composition of an ionic compound. Something like an empirical formula.