The Periodic Table of Elements

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The periodic table of elements. Image courtesy of TexAmple. 

In this unit of the course, you will constantly be hearing about elements - the first elements after the Big Bang, the elements that make up stars, and the elements that form the various planets in our solar system. What are elements? How are they arranged on the periodic table of elements? The periodic table of elements was created to show the characteristics and organization of the elements in our universe. The elements are arranged "periodically", that is, according to properties that repeat in a regular, predictable pattern. That arrangement makes the table very useful because if you know the location of an element on the table, you can predict its properties.

Elements are a type of matter made up of only one kind of atom. For example, the element hydrogen is only composed of hydrogen atoms. An atom is the smallest particle of an element. An atom of any given element is made up of a certain number of protons, an equal number of electrons, and approximately the same number of neutrons. Protons and neutrons form the nucleus of an atom, while the electrons swarm around the nucleus. 

Elements are organized in a series of 18 columns and 7 rows. Each column is known as a group or family.  Each row is called a period. Elements in the same group have similar physical characteristics. For example, all the elements in group 1 (far left) react easily with other elements. Elements in the same period (row), however, do not share properties; rather, the properties of elements change as you move from left to right across the row. 

Rows in the periodic table are arranged according to atomic number, from left to right, top to bottom. The atomic number is found at the top left of the element's symbol (for example, He (helium) has an atomic number of 2). The atomic number is equal to the number of protons found in 1 atom of that element. For example, an atom of helium has 2 protons in its nucleus, so its atomic number is 2. Another property of elements is their atomic mass, which is the number of protons and neutrons. Electrons are not included in the atomic mass of an element because the mass of an electron is very small.

Elements that have a lower atomic mass are located on the left side of each period and increase across the row. For example, Lithium (Li) has a smaller mass than Beryllium (Be), which has a smaller mass than boron (B), and so on across the row. The next row down (which starts with sodium (Na)) contains elements that have increasingly larger atomic masses; this pattern continues down the periodic table of elements. 

The following video describes what atoms are, what elements are, and how elements are arranged on the periodic table of elements.