Perhaps the most famous diagram in science and a chemist’s best friend is the periodic table, created in 1869 by Russian chemist, Dmitri Mendeleev. The periodic table organizes every fundamental atom according to their valence electron numbers, size, type, electronegativity, and much more. The first 92 elements on the periodic table are naturally occurring, the rest are made in laboratories.

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To start, let’s look at how to read an element on the periodic table. Many elements, like hydrogen (H), carbon (C), chlorine (Cl), or argon (Ar), have abbreviations/symbols derived from either the first letter or the first two letters of the element’s name. Other elements, like sodium (Na) or gold (Au), have elements derived from their Latin/Greek origins. Sodium’s Latin name is natrium, thus its symbol is Na.

Let’s take a closer look at an element:

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The atomic number indicates the number of protons in the nucleus of an atom. The number of protons identifies the element, if an atom changes the number of protons, it changes into a new element. This is not true of any other subatomic particle (e.g. electrons and neutrons), if an atom changes in its number of neutrons it becomes a new isotope (see: the article what are isotopes) of that particular element and if an atom loses or gains electrons such that the number of protons and electrons no longer equal, the atom becomes an ion. For carbon, the number of protons in its nucleus is 6.

Below the element’s symbol is the atomic mass. Before explaining this, it is important to understand what a mole is. You might be thinking that a mole is an animal, which it is, but in chemistry it is a specific quantity sort of like a dozen. In this case, a mole is equal to 6.0221023 particles (electrons, protons, neutrons, atoms, ions, or molecules). This is a very large number and it is used to quantify atoms, ions, molecules, or any other particle on the atomic or subatomic level. On the periodic table, one mole of a certain atom is equal to the atomic weight, also called the molar mass. For example, 1 mole of carbon is equal to 12.01 grams, 1 mole of oxygen is equal to 16.00 grams, 1 mole of uranium is equal to 238.03 grams. 2 moles of carbon is equal to 24.02 grams. 1 mole of O2 is equal to 32.00 grams.

The molar mass is equal to the mass of 1 mole of a particle in grams, but it is also equal to 1 single particle in atomic mass units (amu). A mole of carbon is equal to 12.01 grams and 1 atom of carbon is also equal to 12.01 amu.

The mass of one proton (in amu) is approximately 1 amu. The mass of one neutron (in amu) is approximately 1 amu. The mass of one electron (in amu) is equal to approximately 0 amu. For atoms with atomic numbers less than 20, the number of protons equals the number of neutrons. For atoms with atomic numbers greater than 20, there should be a rough approximation of about 5 more neutrons than protons. For smaller atoms with an atomic number less than 20, because neutrons and protons make up the vast majority of mass within the atom and also because they have equal proton-neutron numbers, shouldn’t their atomic weight be equal to the number of protons plus the number of neutrons? Why is carbon’s (atomic- number=6) atomic weight equal to 12.01 and not 12? This is because of isotopes. Remember how if an atom changes its neutron number it is still the same element, well it turns out atoms of the same element exist with different numbers of neutrons in the nucleus. Every atom is an isotope of an element, they have a certain number of neutrons in their nucleus that may differ from another isotope of that element. The atomic weight is an average of all isotopes known to exist. Some isotopes are more common than others.

An example of isotopes is hydrogen. All hydrogen atoms have one proton in their nuclei. However, some isotopes of hydrogen have 1 neutron, some have 2 neutrons, and the majority of hydrogen isotopes have no neutrons.

The three main isotopes of hydrogen, when indicating the number of neutrons in an atom, the notation is to write the element’s symbol, put the number of protons (atomic number) in the bottom left corner of the symbol, and the number of protons plus the number of neutrons in the top left corner of the symbol.

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