What is an Atomic Number and What is the Atomic Mass of an Atom?
In ancient Greece, most people believed everything in the world was made from the elements earth, water, fire, or water. Modern scientists know that there are far more than four elements; currently 118 elements have been discovered.
Elements are what we call different types of atoms. We know what element an atom is based on the number of protons it has in its nucleus. The number of neutrons and electrons in an atom may impact some of the atom’s properties, but they don’t change what element it is.
The number of protons in an atom’s nucleus is known as its “atomic number”. Under normal circumstances, this number is also equal to the number of electrons that an atom has, but it’s the number of protons that officially determines the atomic number.
Atomic number is important, because it’s used to organize the Periodic Table of Elements. The elements in the Periodic Table are put in order from Hydrogen, with an atomic number of one, to Oganesson, with an atomic number of 118.
The first scientist to determine the atomic numbers of all the currently known elements was Henry Moseley. Moseley also noted that when the elements were organized according to their atomic number, elements with similar chemical and physical properties reoccurred in a predictable way.
This pattern, called the Periodic Law, is part of what makes the Periodic Table so useful; elements with similar properties are grouped together. For example, the noble gasses are all odorless, colorless, and slow to react due to the stable number of electrons they have, so they form a column on the far right side of the Periodic Table.
In addition to atomic number, atomic mass is another important property of atoms. Atomic mass is the mass of a specific atom expressed in atomic mass units or amu. One atomic mass unit is equal to the mass of a single proton and very close to the mass of a single neutron.
The atomic mass of an atom is equal to the total number of protons and neutrons in that atom. This is because electrons are so lightweight that the vast majority of an atom’s mass is made up by protons and neutrons.
Atomic weight is similar to atomic mass, but slightly different because it takes isotopes into consideration. Isotopes are atoms with the same number of protons but a different number of neutrons. These atoms still have the same atomic number and are the same element, but have a slightly different atomic mass. Atomic weight is the average mass of a particular type of atom, including all isotopes.
ExplorersInternational (artist). (2017). Science Periodic Table Elements [digital art]. Retrieved from