What are Atoms and Isotopes?


Atoms, molecules, elements, isotopes…BORING! Well, it may be for some people, but personally, I think it’s rather fascinating. For one thing, everything in the world is made from these invisible particles. Although you might think of atoms when imagining the smallest “thing” in the world, when asking yourself what an atom is made of, it becomes clear there are smaller particles (see below).

What is an Atom?

Explaining Anions and Cations

Atoms are made out of a combination of electrons, protons, and neutrons – subatomic particles. How many of these three subatomic particles an atom contains depends on what chemical element it belongs to. Atoms are categorized by the unique number of protons within its nucleus – this is its atomic number. A stable atom must have an equal number of protons and electrons. (Click image for larger view.)

Protons have a positive electric charge while electrons have a negative one. Therefore, if there are more protons than electrons, you have a positively charged ion, also known as a cation. Conversely, if you have more electrons than protons, you have a negatively charged ion, also known as an anion. Atoms with a net electrical charge as described can be made so artificially from a neutral state by ionizing radiation.

So we covered protons and electrons, but what does the neutron do? Well, you can think of neutrons as the glue that binds the protons together. Why do they need to be bound together? As mentioned above, protons and electrons are electrically charged, and as such, will naturally repel particles of the same sign. This is why groups of protons need neutrons to hold them together. Hydrogen-1 does not have any neutrons as it only has one proton.

What is an Isotope?

So we know that the atomic number is derived from the number of protons in an atom’s nucleus, but what about isotopes? An isotope is defined by the number of neutrons in an atom’s nucleus. Within a given chemical element, there are often several of these isotopes. For example, hydrogen has 1 proton, but depending on what hydrogen isotope it is, the number of neutrons vary.

Illustration of lithium atom

Lithium-7 (Red:Protons; Blue:Neutrons; Black:Electrons)

Isotopes are named by their given chemical element, followed by their atomic mass, as in hydrogen-1, hydrogen-2, and so on. This means that although the number of protons remains constant, the number of neutrons changes. Thus hydrogen-2 will have a neutron as well as a proton, while hydrogen-3 will have 2 neutrons and a proton. Hydrogen-1 has only 1 proton and no neutrons.

By subtracting the atomic number from the mass number, you get the number of neutrons. Isotopes can be recognized in writing by an element name followed by a distinct mass number such as hydrogen-3 or iodine-131. When speaking of various isotopes, radioactive or not, they will be identified by name. Therefore, familiarizing yourself with the structure of atoms and related terminology can be helpful.

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