Chemistry is the study of the properties and [?] of matter. In order to understand our world, we need to understand how the 100 or so [?] combine. All elements are composed of [?]. The properties of matter relate to composition (the kinds of atoms) and to (the arrangement of atoms). All molecules are formed by at least two or more [?] which are joined together in specific [?]. This is why chemists concentrate on two worlds: the world that can be seen without a microscope (the [?] world) and the world that can only be seen with a microscope (the [?] world).