Organize the Elements: Periodic Table

In the mid-1800s there were 64 known elements (today there are over 116 elements).  Scientists kept having difficulties organizing the information about the 64 elements.  They kept trying to organize them but found no pattern between them. Many chemicals have very similar colors, luster, and conductivity of electricity and heat.

Scientists were able to classify only two groups of elements at this point: metals and non-metals. Most metals were solid, shiny, and conducted electricity and heat easily. Most non-metals were gases and liquids, brittle, and poor conductors of heat and electricity. Scientists then came up with the idea that they could arrange the element by the atomic mass number.  Every element has its own unique atomic mass number because elements have different numbers of protons and neutrons.

MENDELEEV AND THE FIRST PERIODIC TABLE

A Russian scientist named Dmitri Mendeleev came up with a way to organize the elements and it’s a system we still use today. He found that certain elements had very similar physical and chemical properties.  He kept finding that certain elements could be grouped into categories based on their properties. He took elements with similar properties and placed them into vertical columns. 

Unlike scientists before him, he didn’t just use the atomic mass number to sort the elements. Mendeleev’s basic law stated the following: “If the elements are arranged according to their atomic mass number, a pattern can be seen in which similar properties Mendeleev’s occur regularly.” The table even had spaces in it for elements that had not yet been discovered.  As scientists discovered more elements, they found that the new elements fit perfectly into the spaces left by Mendeleev.

TODAY’S PERIODIC TABLE

As more was discovered about chemistry and elements, they discovered that the most important number about elements is not the atomic mass number.  The most important number is the atomic number (which tells us how many protons and electrons are in an atom). Therefore, a modern periodic law was written that stated: “If the elements are arranged according to their atomic number, a pattern can be seen in which similar properties occur regularly.”

Let’s now look at the periodic table and see what patterns exist: Everything to the left of the “staircase” is a metal.  Most metals are solids at room temperature. Everything to the right of the staircase is a non-metal.  Most non-metals are gases and liquids at room temperature. The vertical (up and down) columns are called “groups”.  There are 18 groups on the periodic table.

• Group 1 elements are called the Alkali Metals.  Every element in this group is very reactive.  These elements will light on fire when placed in water!
• Group 2 elements are called the Alkaline Earth Metals.  These elements are also very reactive.
• Group 17 elements are called the Halogens.  This group is also very reactive, however, they all exist as diatomic molecules (for example, chlorine exists as Cl2) and are therefore not as reactive.
• Group 18 elements are called the Noble Gases.  These elements are all unreactive and exist as gases.

The horizontal (across) columns are called “periods.”  There are 7 periods on the periodic table. The bottom two rows of elements actually fit into the periodic table, but were placed below the table to create more room. 

The first row at the bottom fits between elements 57 and 72.  These are called the “lanthanides” after element 57, which is lanthanum. The second row at the bottom fits between elements 89 and 104.  These are called the “actinides” after element 89, which is actinium.

All of the elements from group 3 to the staircase are called “transition metals.”  The transition metals include most of the metals you are familiar with such as gold, lead, copper, and tin. Every element that has an atomic number higher than 92 is a man-made element and is not found in nature.  These elements are the products of nuclear reactions and are all radioactive.  Some can stay in the environment for a long time while most break apart very quickly.  The largest discovered element to date is element 116.  Scientists are still discussing names for elements 112-116.