This picture explains the idea of "atomic mass". The carbon atom (14
C) nucleus on the top has 6 protons plus 8 neutrons. It has an atomic mass of 14. Tritium (3
H), an isotope of hydrogen, is shown on the bottom. It has 1 proton plus 2 neutrons in its nucleus. Tritium has an atomic mass of 3.
Click on image for full size
Original artwork by Windows to the Universe staff (Randy Russell).
The nucleus of an atom has protons and neutrons in it. Each element (like carbon or oxygen or gold) has a different number of protons in its atoms. Scientists have a special name for the number of protons in an atom. They call it the "atomic number".
Why is the atomic number important? Normal atoms have the same number of electrons as protons. The number of electrons is what makes each element behave a certain way in chemical reactions. So the atomic number, which is the number of protons and thus of electrons, is what makes one element different from another.
Hydrogen atoms have 1 proton, and thus an atomic number of 1. Carbon has 6 protons and an atomic number of 6; oxygen has 8 protons and thus and atomic number of 8. The atomic number of uranium is 92!
Atoms of the same element and same atomic number can have different numbers of neutrons. All carbon atoms have 6 protons. Most carbon atoms also have 6 neutrons, but some carbon atoms have 7 or even 8 neutrons. Scientists call these different kinds of carbon atoms "isotopes" of carbon.
Scientists also talk about the "atomic mass" of an atom. The nucleus of an atom contains nearly all (more than 99%) of an atom's mass. Neutrons and protons have almost exactly the same mass. So, to calculate atomic mass, we just add up the number of protons plus the number of neutrons. A carbon atom with 6 protons and 8 neutrons has an atomic mass of 14 ( = 6 + 8). Sometimes scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.
Shop Windows to the Universe Science Store!
Our online store
includes issues of NESTA's quarterly journal, The Earth Scientist
, full of classroom activities on different topics in Earth and space science, as well as books
on science education!
You might also be interested in:
An element (also called a "chemical element") is a substance made up entirely of atoms having the same atomic number; that is, all of the atoms have the same number of protons. Hydrogen, helium, oxygen,...more
Everything you see around you is made of tiny particles called atoms. There are many different types of atoms, each with a special combination of protons , neutrons and electrons . These different types...more
Isotopes are different "versions" of an element. All atoms of an element have the same number of protons. For example, all hydrogen atoms have one proton, all carbon atoms have 6 protons, and all uranium...more
One way scientists measure the size of something is by its mass. Scientists can even measure very, very tiny things like atoms. One measure of the size of an atom is its "atomic mass". Almost all of the...more
Carbon-14 is an isotope of the element carbon. All carbon atoms have 6 protons in their nucleus. Most carbon atoms also have 6 neutrons, giving them an atomic mass of 12 ( = 6 protons + 6 neutrons). Carbon-14...more
Looking for online content that can be used for a climate change education course or module? Pages linked below can be used to support an introductory climate change education for either a unit or a full...more
Atoms and the tiny particles from which they are made strongly influence the world around us. The fields of atomic physics and particle physics help us understand the life cycles of stars, the forms of...more