1. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu What’s in an Atom? Atoms are made of protons, neutrons, and electrons. At the center of each atom is a small, dense nucleus with a positive electric charge. The nucleus is made of protons (a subatomic particle that has a positive charge) and neutrons (a subatomic particle that has no charge). Moving around outside the nucleus is a cloud of electrons: subatomic particles with negative charges. Section 1 Atomic Structure Chapter 4

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Parts of the Atom Section 1 Atomic Structure Chapter 4

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu What’s in an Atom? continued Unreacted atoms have no overall charge. Although atoms are made of charged particles, they do not have an overall charge because they have an equal number of protons and electrons whose charges exactly cancel. To the right is shown a helium atom, which is made of two protons, two neutrons, and two electrons. Section 1 Atomic Structure Chapter 4

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Models of the Atom Bohr’s model compares electrons to planets. In 1913, the Danish scientist Niels Bohr suggested that electrons in an atom move in set paths around the nucleus much like the planets orbit the sun in our solar system. In Bohr’s model, electrons can only be in certain energy levels. Bohr’s model of electrons is illustrated on the following slide. Section 1 Atomic Structure Chapter 4

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Building Model Section 1 Atomic Structure Chapter 4

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. ResourcesChapter menu Bohr Model of the Atom Section 1 Atomic Structure Chapter 4