A. Overview of Elements and Atoms

The difference between elements and atoms is often confused in casual conversation. Both terms describe matter, substances with mass. Different elements are different kinds of matter distinguished by different physical and chemical properties. In turn, the atom is the fundamental unit of matter…, that is, of an element.

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The number of positively charged protons and neutral neutrons in an atomic nucleus account for most of the mass of an atom. Each negatively charged electron that orbits a nucleus is about 1/2000th of the mass of a proton or neutron. Thus, they do not add much to the mass of an atom. Electrons stay in atomic orbits because of electromagnetic forces, i.e., their attraction to the positively charged nuclei. Nuclear size (mass) and the cloud of electrons around its nucleus define structure of an atom. And that structure dictates the different properties of the elements.

Recall that atoms are chemically most stable when they are electrically uncharged, with an equal number of protons and electrons. Isotopes of the same element are atoms with the same number of protons and electrons, but a different number of neutrons. Therefore, isotopes are also chemically stable, but they may not be physically stable. For example, the most abundant isotope of hydrogen contains one proton, one electron and no neutrons. The nucleus of the deuterium isotope of hydrogen contains one neutron and that of tritium contains two neutrons. Both isotopes can be found in water molecules. Deuterium is stable. In contrast, the tritium atom is radioactive, subject to nuclear decay over time. Whether physically stable or not, all isotopes of an element share the same chemical and electromagnetic properties and behave the same way in chemical reactions.

The electromagnetic forces that keep electrons orbiting their nuclei allow the formation of chemical bonds in molecules. We model atoms to illustrate the average physical location of electrons (the orbital model) on one hand, and their potential energy levels (the Bohr, or shell model) on the other. Look at the models for helium illustrated below.


Up to two electrons move in a space defined as an orbital. In addition to occupying different areas around the nucleus, electrons exist at different energy levels, moving with different kinetic energy. Electrons can also absorb or lose energy, jumping or falling from one energy level to another.

A unique atomic number (number of protons) and atomic mass (usually measured in Daltons, or Da) characterize different elements. A unique symbol with a superscripted atomic number and a subscripted atomic mass number defines each element. Take the most common isotope of carbon (C) for example. Its atomic number is 6 (the number of protons in its nucleus) and its mass is 12 Da (6 protons and 6 neutrons at 1 Da each!). Remember that the mass of the electrons in a carbon (C) atom is negligible!

Find the C atom and look at some of the other atoms of elements in the partial periodic table below.

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