Physics for beginners/05-atoms and molecules

This is perhaps the simplest of many possible representations of an atom. Lithium always has three protons in the nucleus, and most lithium has four neutrons. Neutral (unionized) lithium has three electrons.
Physics often uses contradictory or even mutually exclusive models. Here the two electrons of a helium atom are shown as what might be called an "electron probability cloud. The red spot in the center does not represent the nucleus becuse it would not be visible in this figure: It is about $10^{6}$ (or a million) times smaller than the the atom's electron cloud.
A representation of the water molecule, with one oxygen atom attached to two hydrogen atoms. Note how the oxygen atom "steals" a small portion of the electron cloud from the two hydrogen atoms. This renders the the hydrogen parts slightly positive and the oxygen portion slightly negative in charge.