Atomic Structure (College Board AP Chemistry)

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

${}_{\mathrm{Z}}{}^{\mathrm{A}} \mathrm{Atomic} \mathrm{symbol}$

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