When two or more atoms combine, they form a molecule or a compound. Molecules make up almost everything we interact with on Earth, be it life or our atmosphere. A compound is similar to a molecule, but it has its own subset of rules and restrictions for classification.
Circumstances for Forming
A molecule is formed when two or more atoms share their outer shell electrons. This process of sharing electrons is known as a "bond," and, specifically for molecules, it is a covalent bond. The overall charge of a molecule is neutral, which means the total number of electrons in the molecule is the same as the total number of protons. If a molecule has more than two atoms, it is not necessary for each atom to be bonded to one another (e.g. in H2O, the two hydrogen atoms are bonded to the central oxygen, but not directly to one another).
Molecules take different shapes depending on the atoms that are combined. The shape of a molecule can be either linear, bent or take on a geometric appearance (e.g. a tetrahedral, such as in CH4). A molecule's shape is determined by the strength of repulsion of the electrons of the atoms that are not directly attached (known as a "lone pair") and by those directly attached (known as a "bond pair"). The more linear a molecule appears, the stronger the repulsion is between the electrons.
Organic molecules are a specific group of molecules that contain carbon. However, not all molecules that contain carbon are necessarily organic; an organic compound must be involved in the process of a living system to be identified as an organic molecule. Examples of organic molecules include carbohydrates, proteins, sugars and fats. Organisms obtain organic molecules primarily from their food, and all organisms on Earth are known as "carbon-based organisms" because they require organic molecules to survive.
Molecules are commonly confused with compounds. Some molecules are compounds (and vice versa), but not all of them. A compound is defined as any chemical bond between two or more different types of atoms. For example, oxygen gas (O2) is not considered a compound because it is not a bond between two different types of atoms. Additionally, compounds do not have the restriction of having a net neutral charge, and thus can form different types of bonds besides covalent bonds (e.g. ionic bonds).