1. Molecular Formulas & Relative Atomic Mass The Use of Avogadro’s Hypothesis

2. Avogadro’s Hypothesis (1811) Equal volumes of gases contain equal numbers of particles.

3. Water’s Formula Two volumes of hydrogen combine with precisely one of oxygen. This meant water’s molecular formula was H 2 O. Molecular formula – how elements combine and in what ratios to form compounds.

4. The Mole Mole = an amount of a substance Another way to measure atoms 2 moles of H combine with 1 mole of O to form 1 mole of water

5. Relative Atomic Mass Avogadro’s hypothesis also provided a way of determining relative atomic masses. H atom – relative mass of 1 O atom – relative mass of 16 O atoms weighed 16 times that of H atoms.

6. By 1860, a list of 60-some elements had been developed including their…  Properties  Relative atomic masses  Known molecular formulas that they formed It was found that…  Oxygen usually combined with 2 atoms of another element  Nitrogen with 3  Carbon with 4

7. The Periodic Table Dmitri Mendeleev tried to make sense of this list & organize all the elements into a useful table. But, it was no easy task.