All matter is made up of tiny atoms, so how do we get larger substance, like water, sugar, or iron? These very small atoms can bond together into bigger compounds, either ions or molecules.
Based on different relationships between elements, there are different types of bonds. When metals and nonmetals join, the bond type is ionic. An electron from one element is transferred to the outer electron level, or valence, of another element. The compounds formed in this way are ions, rather than molecules, because the bonded atoms change their amount of electrons and thus become electrically unbalanced.
Molecules consisting of nonmetals are joined by covalent bonds; their electrons are shared by pairs of atoms, not transferred, so the bond between them tends to be very tight.
In molecules consisting of metals, the bond type is called metallic. The name scientists use to explain the electron relationship in these molecules is called the electron-sea theory. Like in molecules with covalent bonds, the electrons are shared; but they are shared with all of the atoms together, not between individuals. The valence electrons (those that are in the outer electron level) become 'free' and mobile in the middle of the compound, hemmed in by the positive charges of the protons of the joined atoms.
Molecules have different shapes, depending on the types of atoms bonded together. The Valence Shell Electron Pair Repulsion (VSEPR) theory explains this relationship as, molecules will form whatever shape will keep the valence electrons in the central atom as far apart from each other as possible.
Compounds of atoms can exist in three different states. Solids are formed by slow-moving molecules. Liquids are formed by faster-moving molecules; the attracting forces between atoms are partly overcome by the motion. In gases, molecules are moving very quickly, and the attracting forces are completely overcome. Heat causes molecules to move faster, which is why ice, a solid, will melt into water, a liquid, when heated. If you boil the water over the stove, it will evaporate as it gets hotter, turning into a gas. Usually liquids made of molecules that have a high atomic weight take longer to boil, because the molecules take longer to start moving.
To help you visualize how atoms bond together into molecules, experiment with our molecular model set.