Atomic Number: Definition & Symbol

Instructor: Nathan Crawford

Nathan, a PhD chemist, has taught chemistry and physical science courses.

This lesson focuses on an important concept in chemistry and physics known as the atomic number. Atomic number is defined, and examples from the periodic table of the elements are provided to help students apply the concept.

Atomic Identity

If you've read about or studied the concept of matter, you have undoubtedly read about how matter is made up of atoms, like carbon, hydrogen, oxygen, or many others. Have you ever wondered what makes each type of atom unique? In other words, what makes a carbon atom a carbon atom, or what makes a gold atom a gold atom? The key to answering these questions can be found if you grasp a simple concept, the atomic number.

Graphite, a substance made from only carbon atoms
Graphite, a substance made from only carbon atoms

Pure gold that consists of only gold atoms
Pure gold that consists of only gold atoms

Before moving further into the subject of atomic numbers, a brief explanation of atoms and their structure is needed here. Atoms, the fundamental building blocks of all substances, are actually made up of three smaller subatomic particles. At the center of the atom lies the nucleus, the part of the atom that makes up more than 99% of the mass of the atom. Within the nucleus, two types of particles are found, including positively charged particles called protons and particles with virtually no charge, called neutrons. Outside of the nucleus, the rest of the atom is mostly empty space that is home to negatively charged particles, the electrons.

The particles within an atom
The particles within an atom, protons are red, neutrons are green, and electrons are yellow

The number of protons in the nucleus of an atom answers the previous question about the unique identity of each atom that is encountered in the universe. The number protons determine the total number of electrons in an atom because each proton requires a negatively charged electron to balance out the positive charge of the proton. Due to the crucial role that the protons play in atomic structure, the number of protons within the nucleus of the atom is communicated as the atomic number.

Finding Atomic Numbers

The atomic numbers for atoms can be found easily by consulting the periodic table of the elements. The periodic table of the elements, or periodic table for short, is a graphical arrangement of all of the elements, substances made of only one type of atom, that are known to humankind. Each element is depicted with a symbol, atomic number, and other pieces of information that vary from one version to the next.

The periodic table of the elements
The periodic table of the elements

Regardless of the type of periodic table, the atomic number is found above the symbol for the element. For example, copper has the symbol Cu, and its atomic number, 29, is found above the symbol Cu. The atomic number for copper means that a copper nucleus has 29 protons, and a copper atom has a total of 29 electrons.

The symbol for the element copper with the atomic number above the symbol
The symbol for the element copper with the atomic number above the symbol

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