Atomic Structure (College Board AP® Chemistry)

Atomic Structure

The Atom

• The atom is the smallest building block of matter

• It consists of three subatomic particles

  • Protons are positive particles found in the nucleus

  • Neutrons are neutral particles found in the nucleus

  • Electrons are negative particles that orbit the nucleus in electron shells

• The electrons determine the chemical properties of the element

• Neutral atoms have the same amount of protons and electrons

Atomic Number and Mass Number

• The atomic number (Z) of an element is the number of protons in its nucleus

  • It defines the element's identity and it always a whole number in the periodic table

    • Z = Number of protons

• The mass number (A) is the sum of protons and neutrons in the nucleus

  • It represents the atom's mass

  • Electrons do not contribute to the mass of the atom because they are too light compared with protons or neutrons

    • A = Number of protons + Number of neutrons

• Isotopes are atoms of the same element with the same atomic number but different mass numbers. Therefore, protons are the same but neutrons are different

• Elements are usually represented with the A/Z notation

  

The beryllium atom

The beryllium atom contains 4 protons, 5 neutrons and 4 electrons

The Rutherford Experiment

• Rutherford performed an experiment to understand the structure of the atom

• He bombarded a gold foil with tiny positive particles, called alpha particles

• Most alpha particles passed through the foil, but some were significantly deflected, and a few of them bounced back

• Since particles with the same charge repel, Rutherford concluded that the atoms should contain an small, dense, and positively charged core at the center

  • This core was named nucleus

  • The nucleus contain protons and neutrons

The Rutherford Experiment

The Rutherford experiment showed atoms contain a small, dense, and positively charged core at the center