Atoms, Bonding, and the Periodic Table

An atom: Atoms are the basic units of matter and the defining structure of elements.

The term "atom" comes from the Greek word for indivisible, because it was once thought that atoms were the smallest things in the universe and could not be divided. We now know that atoms are made up of three particles (which are composed of even smaller particles such as quarks):

  • protons
  • neutrons
  • electrons

*The earliest atoms were primarily hydrogen and helium, which are still the most abundant elements in the universe!

Atomic particles

  • Protons and neutrons are heavier than electrons and reside in the nucleus at the center
  • The electrons are extremely lightweight and exist in a cloud orbiting the nucleus called the electron cloud which has a radius 10,000 times greater than the nucleus
  • Protons and neutrons have approximately the same mass
  • One proton weighs more than 1,800 electrons
  • Atoms always have an equal number of protons and electrons, and the number of protons and neutrons is usually the same as well
  • Adding a proton to an atom makes a new element, while adding a neutron makes an isotope, or heavier version, of that atom.

Nucleus

Virtually all the mass (heaviness) of the atom resides in the nucleus. The protons and neutrons that make up the nucleus are approximately the same mass (the proton is slightly less).

Protons

Protons are positively charged particles found within atomic nuclei. Rutherford discovered them in experiments with cathode-ray tubes conducted between 1911 and 1919. Protons are slightly smaller in mass than neutrons with a relative mass of 0.9986 (as compared with the mass of the neutron being 1) or about 1.673x10-27kg.

The number of protons in an atom defines what element it is. For example,carbonatoms have six protons,hydrogenatoms have one andoxygenatoms have eight. The number of protons in an atom is referred to as the atomic number of that element. The number of protons in an atom also determines the chemical behavior of the element. ThePeriodic Table of the Elementsarranges elements in order of increasing atomic number.

Electrons

Electrons are tiny compared to protons and neutrons, over 1,800 times smaller than either a proton or a neutron. Electrons have a relative mass of 0.0005439 (as compared with the mass of a neutron being 1) or about 9.109x10-31kg.

J.J. Thomson, a British physicist, discovered the electron in 1897. Originally known as "corpuscles," electrons have a negative charge and are electrically attracted to the positively charged protons.

Neutrons

The neutron is used as a comparison to find the relative mass of protons and electrons (so it has a relative mass of 1) and has a physical mass of 1.6749x10-27kg.

Isotopes

The number of neutrons in a nucleus determines the isotope of that element. For example, hydrogen has three known isotopes: protium, deuterium, and tritium. Protium, symbolized as1H, is just ordinary hydrogen; it has one proton and one electron and no neutrons. Deuterium (D or2H) has one proton, one electron and one neutron. Tritium (T or3H) has one proton, one electron and two neutrons.

In-class activity: Choose an atom from the periodic table and draw it in the blank page underneath. Make sure to name the parts of it, such as protons, electrons, neutrons, nucleus, and electron cloud. Once you are done, you can design with your group a 3d version of it.