What is the Structure of an Atom?    Lesson 3.1

Key Concepts
  • What is the structure of an atom?

Key Terms
  • protons
  • neutrons
  • electrons
  • atomic number
  • isotopes
  • mass number
  • model
Particles of Matter
  • Scientists once thought that atoms were the smallest particles of matter.
  • Now Scientists know more.
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Atoms
  • Atoms are made of even smaller particles called protons, neutrons, and electrons.
  • An atom is made up of a nucleus surrounded by one or more electrons.
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Nucleus
  • The nucleus is the very small center core of an atom.
  • It is made up of smaller particles called protons and neutrons.
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Subatomic Particles
  • Protons have a positive electric charge.
  • Neutrons have no charge.
  • Electrons move rapidly around the nucleus and have a negative electric charge.

  Balanced Charge

  • In an atom, the number of protons equals the number of electrons.
  • An atom is neutral because the positive and negative electric charges are balanced.
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Electron Cloud
  • Electrons move around the nucleus in a space shaped somewhat like a sphere.
  • Scientists describe this space as a cloud of negative charge.
  • Electrons may be anywhere within this cloud.
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Relative Mass
  • Protons and neutrons are about equal in mass.
  • Electrons are much smaller.
Relative Space
  • It takes almost 2,000 electrons to equal the mass of one proton.
  • Electrons, however, take up much more space in the atom than does the nucleus.

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  Atomic Mass Units
  • Atoms are so small their masses are measured in atomic mass units (amu).
  • A proton or a neutron has a mass about equal to one amu.