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| Section 2: The Structure of Atoms |
Key Ideas | What is the difference between protons, neutrons, and electrons? |
| What do atoms of an element have in common with other atoms of the same element? |
| Why do isotopes of the same element have different atomic masses? |
| What unit is used to express atomic mass? |

Why It MattersRadioisotopes emit energy when they decay. Todiagnose and treat diseases, doctors use this property of radioisotopes to track where in the body certain atoms go. |
Less than 100 years after Dalton published his atomic theory, scientists determined that atoms consisted of smaller particles, such as the electron. In this section, you will learn more about the particles inside the atom.
What Is in an Atom?Atoms are made up of varioussubatomic particles. To understand the chemistry of most substances, however, we need to study only three of these particles. The three main subatomic particles are distinguished by mass, charge, and location in the atom. Figure 1 compares these particles.
Figure 1 Subatomic Particles Particle | Charge | Mass (kg) | Location in the atom |
Proton | +1 | 1.67 × 10-27 | in the nucleus |
Neutron| 0 | 1.67 × 10-27 | in the nucleus |
Electron | –1 | 9.11 × 10-31 | outside the nucleus |

At the center of each atom is a small, dense nucleus. The nucleus is made of protons, which have a positive charge, and neutrons, which have no charge. Protons and neutrons are almost identical in size and mass. Moving around outside the nucleus is a cloud of very tiny, negatively chargedelectrons. The mass of an electron is much smaller than that of a proton or neutron. | |
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Each element has a unique number of protons.
A hydrogen atom has one proton. A helium atom, shown in Figure 2, has two protons. Lithium has three protons. As you move through the periodic table of the elements, this pattern continues. Each element has a unique number of protons. In fact, an element isdefined by the number of protons in an atom of that element.

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Figure 2 Helium atoms, including the ones in this helium blimp, are made up of two protons, two neutrons, and two electrons.
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Principio del formulario
Which of these particles defines the element as helium? | | | | | |
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Final del formulario
Unreactedatoms have no overall charge.
Even though the protons and electrons in atoms have electric charges, most atoms do not have an overall charge. The reason is that most atoms have an equal number of protons and electrons, whose charges exactly cancel. For example, a helium atom has two protons and two electrons. The atom is neutral because the positive charge of the two protons exactly cancels thenegative charge of the two electrons, as shown below.

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If an atom gains or loses electrons, it becomes charged. A charged atom is called an ion.

The electric force holds the atom together.
Positive and negative charges attract each other with a force known as the electric force. Because protons are positive and electrons are negative, protons and electrons are attracted to one anotherby the electric force. In fact, the electric force between protons in the nucleus and electrons outside the nucleus holds the atom together. On a larger scale, this same force holds solid and liquid materials together. For instance, electric attractions hold water molecules together.

Atomic Number and Mass Number
Atoms of different elements have their own unique structures. Because these atomshave different structures, they have different properties. Atoms of the same element can vary in structure, too. Atoms of each element have the same number of protons, but they can have different numbers of neutrons.

The atomic number equals the number of protons.
The atomic number of an element, Z, tells you how many protons are in an atom of the element. Remember that most atoms are...