Strictly defined, an atom is the smallest unit of an element that 1) retains all the element’s distinctive properties and 2) can enter into a chemical reaction. In other words, anything less than an atom of carbon (C) is no longer carbon. We could split a carbon atom into its component parts (see Figure 10), but the resulting subatomic particles (positively-charged protons, non-charged neutrons, and negatively-charged electrons) would not reflect the properties of carbon. Though their number and arrangement vary from element to element, subatomic particles alone tell you nothing about the atoms from which they came. A proton from a carbon atom is identical to an oxygen (O) proton.

Subatomic particles are important, however, in that they determine one of the defining characteristics of any given atom: its atomic weight, or the total mass of the protons and neutrons within its nucleus (orbiting electrons are of negligible weight and don’t figure into this total). For example, the nucleus of a hydrogen (H) atom contains one proton and no neutrons, so its atomic weight is 1. Carbon is composed of six protons and six neutrons, for an atomic weight of 12.

Each individual atom is also distinguishable by the one or more energy bonds it can form with neighboring atoms. This ability to combine is known as valence, and the amount of valence varies with each element. For example, an atom of hydrogen has a valence of 1, meaning it can form only one such energy bond. Oxygen has a valence of 2 and carbon has a valence of 4, meaning they can form two and four bonds, respectively. To be more precise, atoms need to form these energy bonds to be “satisfied” or “stable.” The interaction of differing valences is what allows a group of atoms to join together into a molecule.

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