Clickable Periodic Table of the Elements

How To Read the Periodic Table of the Elements

Click on an element symbol to get detailed facts about each chemical element. The element symbol is a one- or two-letter abbreviation for an element's name.

The integer number above the element symbol is its atomic number. The atomic number is the number of protons in every atom of that element. The number of electrons can change, forming ions, or the number of neutrons can change, forming isotopes, but the proton number defines the element. The modern periodic table orders the element by increasing atomic number. Mendeleev's periodic table was similar, but the parts of the atom weren't known in his day, so he organized elements by increasing atomic weight.

The number below the element symbol is called the atomic mass or the atomic weight. It's the sum of the mass of protons and neutrons in an atom (electrons contribute negligible mass), but you may notice it isn't the value you'd get if you assumed the atom had an equal number of protons and neutrons. The atomic weight values may be different from one periodic table to another because it's a calculated number, based on the weighted average of the natural isotopes of an element. If a new supply of an element is discovered, the isotope ratio may be different from what scientists previously believed. Then, the number may change. Note, if you have a sample of a pure isotope of an element, the atomic mass is simply the sum of the number of protons and neutrons of that isotope!

Element Groups and Element Periods

The periodic table gets its name because it arranges the elements according to recurring or periodic properties. The groups and periods of the table organize elements according to these trends. Even if you didn't know anything about an element, if you knew about one of the other elements in its group or period, you could make predictions about its behavior.

Groups

Most periodic tables are color-coded so that you can see at a glance which elements share common properties with each other. Sometimes these clusters of elements (e.g., alkali metals, transition metals, non-metals) are called element groups, yet you'll also hear chemists refer to the columns (moving top to bottom) of the periodic table called element groups. Elements in the same column (group) have the same electron shell structure and the same number of valence electrons. Since these are the electrons that participate in chemical reactions, elements in a group tend to react similarly.

The Roman numerals listed across the top of the periodic table indicate the usual number of valence electrons for an atom of an element listed below it. For example, an atom of a group VA element will typically have 5 valence electrons.

Periods

The rows of the periodic table are called periods. Atoms of elements in the same period have the same highest unexcited (ground state) electron energy level. As you move down the periodic table, the number of elements in each group increases because there are more electron energy sublevels per level.

Periodic Table Trends

In addition to the common properties of elements in groups and periods, the chart organizes elements according to trends in ionic or atomic radius, electronegativity, ionization energy, and electron affinity.

Atomic radius is half the distance between two atoms that are just touching. Ionic radius is half the distance between two atomic ions that are barely touching. Atomic radius and ionic radius increase as you move down an element group and decrease as you move across a period from left to right.

Electronegativity is how easily an atom attracts electrons to form a chemical bond. The higher its value, the greater the attraction for bonding electrons. Electronegativity decreases as you move down a periodic table group and increases as you move across a period.

The energy needed to remove an electron from a gaseous atom or atomic ion is its ionization energy. Ionization energy decreases moving down a group or column and increases moving from left to right across a period or row.

Electron affinity is how easily an atom can accept an electron. Except that the noble gases have practically zero electron affinity, this property generally decreases moving down a group and increases moving across a period.

The Purpose of the Periodic Table

The reason chemists and other scientists use the periodic table rather than some other chart of element information is because the arrangement of elements according to periodic properties helps predict properties of unfamiliar or undiscovered elements. You can use the location of an element on the periodic table to predict the types of chemical reactions it will participate in and whether or not it will form chemical bonds with other elements.

Printable Periodic Tables and More

Sometimes it's helpful to print out a periodic table, so you can write on it or have it with you anywhere. I've got a large collection of periodic tables you can download to use on a mobile device or print. I've also got a selection of periodic table quizzes you can take to test your understanding of how the table is organized and how to use it to get information about the elements.