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# Periodic Table Study Guide - Introduction & History

## Organization of the Elements

The periodic table of the elements is an essential chemistry resource.

Steve Cole, Getty Images
Introduction | Periods & Groups | More about Groups | Review Questions | Quiz #1 | Quiz #2 | Periodic Table

Introduction to the Periodic Table

People have known about elements like carbon and gold since ancient time. The elements couldn't be changed using any chemical method. Each element has a unique number of protons. If you examine samples of iron and silver, you can't tell how many protons the atoms have. However, you can tell the elements apart because they have different properties. You might notice there are more similarities between iron and silver than between iron and oxygen. Could there be a way to organize the elements so you could tell at a glance which ones had similar properties?

What is the Periodic Table?

Dmitri Mendeleev was the first scientist to create a periodic table of the elements similar to the one we use today. You can see Mendeleev's original table (1869). This table showed that when the elements were ordered by increasing atomic weight, a pattern appeared where properties of the elements repeated periodically. This periodic table is a chart that groups the elements according to their similar properties.

Why was the Periodic Table Created?

Why do you think Mendeleev made a periodic table? Many elements remained to be discovered in Mendeleev's time. The periodic table helped predict the properties of new elements.

Mendeleev's Table

Compare the modern periodic table with Mendeleev's table. What do you notice? Mendeleev's table didn't have very many elements, did it? He had question marks and spaces between elements, where he predicted undiscovered elements would fit.

Discovering Elements

Remember changing the number of protons changes the atomic number, which is the number of the element. When you look at the modern periodic table, do you see any skipped atomic numbers that would be undiscovered elements? New elements today aren't discovered. They are made. You can still use the periodic table to predict the properties of these new elements.

Properties and Trends

The periodic table helps predict some properties of the elements compared to each other. Atom size decreases as you move from left to right across the table and increases as you move down a column. Energy required to remove an electron from an atom increases as you move from left to right and decreases as you move down a column. The ability to form a chemical bond increases as you move from left to right and decreases as you move down a column.

Today's Table

The most important difference between Mendeleev's table and today's table is the modern table is organized by increasing atomic number, not increasing atomic weight. Why was the table changed? In 1914, Henry Moseley learned you could experimentally determine the atomic numbers of elements. Before that, atomic numbers were just the order of elements based on increasing atomic weight. Once atomic numbers had significance, the periodic table was reorganized.

##### Periodic Table Resources

Anne Marie Helmenstine, Ph.D.