In layman’s terms, an atom can only be one atom of one element on the periodic table. You can have an atom of oxygen, or hydrogen. Anymore than one atom; even of the same type (2 oxygens, or 2 hydrogens) is a molecule. One oxygen, one hydrogen and one hydrogen combine to make water, a molecule.
Atoms are basic units of matter, consisting; in most cases, of protons, neutrons and electrons (most hydrogen has no neutrons). Protons are positively charged, meaning they exert a positive force when near other objects of matter. Electrons are negatively charged. Neutrons carry no charge. Think of the ends of batteries. When you insert batteries into a device, the ends must oppose each other. The little nubbin on the top of a battery is the positive end, and the flat end is the negative end. If you are placing two batteries into your, let’s say, xbox 360 remote, one battery will be positive side up and the other will be negative side up. Like charges repel each other whereas opposite charges attract each other. When batteries are placed in this matter, electricity is pulled through the entire device to complete a circuit allowing you to kill that dragon on screen.
Atoms can gain or lose electrons, not protons. Therefore, an atom or molecule that is negatively charged has gained electrons and and one that is positively charged has lost electrons. This is where understanding ions can be a bit confusing: a gain in electrons is denoted by a minus (-) symbol and a loss in electrons is noted by a (+) symbol. The most common examples of ions are H+ and OH- and/or Na+ and Cl-. H+ is hydron. It is an anion of hydrogen (“an” means without, so anions are minus electrons resulting in a positive charge). It is also a measure of acidity on the Ph scale. OH- is hydroxide. It’s a polyatomic cation (“ca” means with, so cations are with extra electrons resulting in a negative charge). OH- is also a measure of alkalinity on the Ph scale. One H+ and one OH- combine to form water; a neutral molecule.
A molecule is two or more atoms combined into a neutral group. Molecules have the same number of protons and electrons so they hold neither a positive or negative charge. Two or more atoms joined together that hold a positive or negative charge are called polyatomic ions (the prefix “poly” means many, but in this specific case, it means two or more).
In summary, I could go into minute details about the nature of atoms and molecules, but to simplify, atoms are single units of one element on the Periodic Table of Elements. Molecules are two or more atoms, of the same or different element bonded together to make a neutral grouping.