• 2.1: Atoms and Elements

    Let's begin by discussing elements. You are probably familiar with many elements, such as sodium, oxygen, and helium, from everyday life. Elements correspond to the symbols you see listed on the periodic table. All atoms are made of the subatomic particles protons, neutrons, and electrons.

    Protons and neutrons exist in the nucleus, or the high-density center of the atom. Electrons exist as an electron cloud around the nucleus.

    • Protons are positively charged
    • Neutrons are not charged
    • Electrons are negatively charged

    Different elements are defined by the number of protons in the nucleus. For example, all hydrogen atoms have one proton in their nucleus. All helium atoms have two protons in their nucleus.

    • Atoms, Elements, and the Nucleus Page

      Read this text, which gives some of the history of the development of atomic theory. In the section, "Atoms Become Real", pay close attention to the law of conservation of mass-energy and the law of definite proportions. These laws define how we describe chemical reactions. Also, pay close attention to Dalton's Atomic Theory. John Dalton was the first person to propose a cohesive theory for how atoms make up matter.

    • The Nuclear Atom and Atomic Mass Page

      As stated above, we define the elements by their number of protons. We define the atomic number, Z, as the number of protons in an atom. For a neutral atom (not a charged ion), the number of electrons must equal the number of protons. However, the number of neutrons can vary within atoms of a given element.

      Atoms of the same element with different numbers of neutrons are called isotopes. Most elements have multiple isotopes. For a given isotope, we define the mass number, A, as the atomic number plus the number of protons. We can write this as A = Z + N where A is mass number, Z is atomic number, and N is number of neutrons.

      Read this section, which explains how we write atomic symbols with atomic numbers and mass numbers.

      When we see the mass of an element on the periodic table, we are seeing the weighted average of the masses of all isotopes of that element. Then read the section "Isotopic Mixtures and Abundances" near the bottom of the page. This section describes how to determine the average atomic mass of an element if we know the isotope masses and their relative abundance. Try the practice problems to do these types of calculations yourself.