Atomic Structure

The basics of atoms and atomic structure.

Atomic Structure

The Structure of an Atom

Atom Diagram

Diagram of a helium atom, showing the subatomic particles.

Every atom is composed of three basic subatomic particles – protons, neutrons, and electrons.

Protons. A subatomic particle, located in the nucleus of an atom, with a positive charge. Accounts for nearly 50% of the total atomic mass.

Neutrons. A subatomic particle, located in the nucleus of an atom, with a neutral charge. Accounts for nearly 50% of the total atomic mass.

Electrons. A subatomic particle, located in clouds surrounding the nucleus of an atom, with a negative charge. Mass is negligible.

Atomic Properties

Atomic Mass. The atomic mass of an atom is the mass of its protons plus the mass of its neutrons.

Atomic Number. The atomic number of an atom is the number of protons in the atom.

Isotopes. Isotopes are atoms with a different number of neutrons. They are unstable versions of the element.

Quantum Numbers. There are four quantum numbers for each electron in an atom.

  1. Principal Quantum Number (n). Designates the shell level of the electron.
  2. Azimuthal Quantum Number (l). Designates the subshell of the electron.
  3. Magnetic Quantum Number (m). Designates the precise orbital of the electron.
  4. Electron Spin Quantum Number (ms). Designates the spin of the electron.

Ions are elements that have a different number of electrons than protons. Cations are atoms with fewer electrons that protons, resulting in a positive charge. Anions are atoms with more electrons than protons, resulting in a negative charge.

Useful MCAT Tips

  • A mole is simply a number of items. Memorize that a mole of something is 6.022 x 10^23 units of that something.
  • Memorize the symbols for each element on the periodic table.
  • Know the periodic table of elements. You are given access to a periodic table during the exam that you can reference. Memorizing the atomic weights of some of the most common elements used in chemistry is helpful, although not necessary.
  • You should be able to quickly and easily convert atomic weights to moles and vice versa.

Next: The Periodic Table of Elements

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