How Is the Periodic Table Organized Today?

The periodic table is one of the most valuable tools for chemists and other scientists because it orders the chemical elements in a useful way. Once you understand how the modern periodic table is organized, you'll be able to do much more than just look up element facts like their atomic numbers and symbols.

Chart Organization

The organization of the periodic table allows you to predict the properties of the elements based on their position on the chart. Here's how it works:

• Elements are listed in numerical order by atomic number. The atomic number is the number of protons in an atom of that element. So element number 1 (hydrogen) is the first element. Every atom of hydrogen has 1 proton. Until a new element is discovered, the last element on the table is element number 118. Every atom of element 118 has 118 protons. This is the biggest difference between today's periodic table and Mendeleev's periodic table. The original table organized the elements by increasing atomic weight.
• Each horizontal row on the periodic table is called a period. There are seven periods on the periodic table. Elements in the same period all have the same electron ground state energy level. As you move from left to right across a period, elements transition from displaying metal characteristics toward nonmetallic properties.
• Each vertical column on the periodic table is called a group. Elements belonging to one of the 18 groups will share similar properties. Atoms of each element within a group have the same number of electrons in their outermost electron shell. For example, elements of the halogen group all have a valence of -1 and are highly reactive.
• There are two rows of elements found below the main body of the periodic table. They are placed there because there wasn't room to put them where they should go. These rows of elements, the lanthanides and actinides, are special transition metals. The top row goes with period 6, while the bottom row goes with period 7.
• Each element has its tile or cell in the periodic table. The exact information given for the element varies, but there is always the atomic number, the symbol for the element, and the atomic weight. The element symbol is a shorthand notation that is either one capital letter or a capital letter and a lowercase letter. The exception is the elements at the very end of the periodic table, which have placeholder names (until they are officially discovered and named) and three-letter symbols.
• The two main types of elements are metals and nonmetals. There are also elements with properties intermediate between metals and nonmetals. These elements are called metalloids or semimetals. Examples of groups of elements that are metals include alkali metals, alkaline earths, basic metals, and transition metals. Examples of groups of elements that are nonmetals are the nonmetals (of course), the halogens, and the noble gases.

Predicting Properties

Even if you don't know anything about a particular element, you can make predictions about it based on its position on the table and its relationship to elements that are familiar to you. For example, you may not know anything about the element osmium, but if you look at its position on the periodic table, you'll see it's located in the same group (column) as iron. This means the two elements share some common properties. You know iron is a dense, hard metal. You can predict osmium is also a dense, hard metal.

As you progress in chemistry, there are other trends in the periodic table you'll need to know:

• Atomic radius and ionic radius increase as you move down a group, but decrease as you move across a period.
• Electron affinity decreases as you move down a group, but increases as you move across a period until you get to the last column. The elements in this group, the noble gases, have practically no electron affinity.
• The related property, electronegativity, decreases going down a group and increases across a period. Noble gases have practically zero electronegativity and electron affinity because they have complete outer electron shells.
• Ionization energy decreases as you move down a group, but increases moving across a period.
• Elements with the highest metallic character are located on the lower left side of the periodic table. Elements with the least metallic character (most nonmetallic) are on the upper right side of the table.