The periodic table is organized like a big grid. Each element is placed in a specific location because of its atomic structure. As with any grid, the periodic table has rows (left to right) and columns (up and down). Each row and each column has specific characteristics. In this activity, you will learn more about how elements in the periodic table are organized.

Two or three hundred years ago when scientists were experimenting with matter, they experimented to discover more about the properties of various substances. However, they had difficulty comparing the results of their experiments with each other because there was no organized system for describing the elements they were working with.

The first step in organizing the elements came when Jons Jacob Berzelius suggested using the first letter of the name of each element as its symbol. If more than one element started with the same first letter, the second letter of the element was also used.

   

The second step scientists took in organizing the elements occurred when Dmitri Mendeleev looked at the properties of the elements that had been discovered before and in his lifetime (he lived over 200 years ago) and realized that a pattern could be used to organize the elements based on their atomic number and atomic masses.

The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1.

Atomic mass is determined by how many protons, neutrons, and electrons atoms have. He found that elements with similar atomic masses shared other similar properties. Atomic mass is measured in atomic mass units (amu). In the example on the left, the element lithium (symbol = Li) has an atomic mass of 6.941 (the number of protons/electrons and neutrons the atom has).

	Remember that the elements we see on the periodic table are pure substances. An element cannot be broken down into any other substance — it is in its smallest form already.