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Inside an Atom

In the early 1900s, scientists began to identify the particles that make up atoms (subatomic particles). These new discoveries would require changes to be made to Mendeleev's periodic table. Below is a picture of the basic parts of the atom.  

Every atom has a core called a nucleus, where the majority (99.9%, to be exact) of an atom's mass is held. Although the nucleus contains the majority of the mass of the atom, the nucleus is very small compared to the size of the whole atom, because most of the atom is empty space surrounding the nucleus. Within the nucleus are two types of smaller particles called protons and neutrons. The third type of particle that makes up the atom, electrons, orbit around the nucleus.

Let's look at each particle in further detail:


Protons are positively charged particles found inside the nucleus of an atom. Every atom of a particular element contains the same number of protons. In fact, the number of protons is unique to each element. Each element has a unique atomic number, or a unique number of protons in its nucleus. Proton number never changes for any given element. For example, oxygen has an atomic number of 8. That tells us that oxygen always has 8 protons.


Neutrons are the other particle found in the nucleus of an atom. Unlike protons and electrons, however, neutrons carry no electrical charge. Therefore, neutrons are "neutral." Atoms of a given element do not always contain the same number of neutrons. Atoms of an element that have a different number of neutrons in the nucleus are called isotopes of each other.


Electrons are negatively charged particles that orbit around the outside of the nucleus. The mass of an electron is about 1/2000th of the mass of a proton or a neutron. The sharing or exchange of electrons between atoms forms chemical bonds, which is how new molecules and compounds are formed.

Atomic Number:

An atom's atomic number tells you how many protons are in that atom's nucleus. For example, oxygen has an atomic number of 8, meaning that there are 8 protons in the nucleus of an atom of oxygen. Copper's atomic number is 29, meaning that there are 29 protons in the nucleus of an atom of copper. Later, you'll see how the periodic table conveniently tells you each element's atomic number.

Atomic Mass:

Because atoms are so small, their masses cannot be measured in grams or milligrams. Instead, scientists have created the atomic mass unit (amu) to measure mass of subatomic particles. The mass of a proton or a neutron is about 1 amu. The mass of an electron, however, is about 1/2000 amu. To find the atomic mass of an atom, add the number of protons and neutrons in the nucleus.

Example: If an atom has 3 protons, 4 neutrons, and 3 electrons, the atomic mass is 7 amu, because you do not count the very small mass of the atom's electrons (1/2000 amu). Only add the number of protons and neutrons (each has a mass of 1 amu) in the nucleus.

Question: Find the atomic mass of an atom that has 10 protons, 8 neutrons, and 10 electrons.

Answer: 18 amu. Just add the protons and neutrons.

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This page was last updated: 11/04/02