The world around you seems to consist of solid material, but if you could look closely enough you would find that "solid" is an illusion. Matter is made up of atoms, tiny little objects made up mostly of space and separated by a lot more space. When atoms were first conceived, they were imagined as each being tiny versions of a solar system, with an important but relatively tiny star (the nucleus) in the middle and a number of even tinier planets (the electrons) circling at set distances. Atoms don't really behave like little solar systems, but the image is good for getting across just how little material and how much space an atom is made up of. Where the image fails is in showing movement: except when they are held in place by connections to neighbors (in molecules and in "solids"), these little atoms are zipping around their small world at high speeds. What makes materials solid, liquid, or gas is the freedom of the atoms there: atoms and molecules in solids are more tightly packed and somewhat connected to each other, while in liquids and gases they move about more freely, bumping into each other and off the barriers presented by solid objects.
When atoms actually interact, it's less like the bumping of balls and more a matter of attraction and repulsion; at atomic levels, mass (the important part of gravity) is tiny, and so much less an important consideration than charges, which are electrical: positive, negative, or neutral (balanced). Much of what atoms and molecules do is based upon their charged particle parts: they attract, they repel, they share bits, all to become more stable. Most of the terms associated with atoms: molecules, radicals, ions, elements, isotopes, and others, are defined by the particles found inside the atoms and by what levels of stability they produce.
How small is an atom?
How atoms relate to elements. (Video)
The parts of atoms. (Song on Video)
Clickable list of elements on a periodic table.
Cool Periodic Table.