分类在下列化合物中，哪些化合物是离子化合物，哪些是分子化合物?
1. NaCl    2. CH4    3. CO2     4. H2O


NaCl, CH4, CO2, H2O
1. NaCl is an ionic compound.
2. CH4 is a molecular compound.
3. CO2 is a molecular compound.
4. H2O is a molecular compound.