Atomic Structure and Properties

Ever wonder what everything around you is made of? It all comes down to atoms - the fundamental units of matter that can't be broken down further while keeping an element's properties. When atoms bond together, they form molecules, which make up compounds.

One way to count atoms is using the mole, equal to Avogadro's number $6.022×10^23$. Just like a dozen always means 12 of something, a mole always means 6.022×10^23 particles of any substance. The molar mass tells you how many grams one mole of a substance weighs, and it's measured in grams/mole.

To find out how many moles are in a substance, use this formula: Number of moles = weight in grams ÷ molar mass

Remember This! Converting between grams and molecules is a two-step process: first convert grams to moles using molar mass, then convert moles to molecules using Avogadro's number.

Scientists use mass spectroscopy to measure different atoms in a sample. This technique charges molecules, separates them using magnetic fields, and plots the results showing the abundance of each atom type. This might show up on your AP exam when determining the relative abundance of isotopes.

Pure substances maintain consistent characteristics and can be elements (made of a single type of atom) or compounds (made of one type of molecule). According to the law of definite proportions, a pure compound will always contain the same ratio of elements, no matter where it comes from - like how water is always hydrogen and oxygen in the same proportions.