4.2 The Structure of an Atom
Reading Focus
Key Concepts
What are three subatomic particles?- What properties can be used to compare protons, electrons, and neutrons?
- How are atoms of one element different from atoms of other elements?
- What is the difference between two isotopes of the same element?
Vocabulary
proton
electron
neutron
atomic number
mass number
isotopes
Reading Strategy
Monitoring Your Understanding Before you read, copy the table. List what you know about atoms and what you would like to learn. After you read, list what you have learned.
What I Know About Atoms |
What I Would Like to Learn |
What I Have Learned |
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Figure 9 This 45-foot-tall steel sculpture of a clothespin is in Philadelphia, Pennsylvania. Claes Oldenburg made the clothespin in 1976 from 10 tons of steel. If a proton had a mass of 10 tons, then an electron would have a mass of about 5 kilograms.
Beams like the ones Thomson produced create the images on many television screens. When a beam sweeps across the screen, spots on the screen light up in the same way the screen in the gold-foil experiment lit up when struck by an alpha particle. In a color television, there are three beams, one for each primary color of light—red, green, and blue. The particles in these beams are subatomic particles.
Properties of Subatomic Particles
By 1920, Rutherford had seen evidence for the existence of two subatomic particles and had predicted the existence of a third particle.
Protons, electrons, and neutrons are subatomic particles.
Protons
Based on experiments with elements other than gold, Rutherford concluded that the amount of positive charge varies among elements. Each nucleus must contain at least one particle with a positive charge. Rutherford called these particles protons. A proton is a positively charged subatomic particle that is found in the nucleus of an atom. Each proton is assigned a charge of 1+. Some nuclei contain more than 100 protons.
Electrons
The particles that Thomson detected were later named electrons. Electron comes from a Greek word meaning “amber.” An electron is a negatively charged subatomic particle that is found in the space outside the nucleus. Each electron has a charge of 1−.
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