 In this video, we will be talking about the modern periodic table and how it works. If you've ever been in a chemistry classroom, you've probably noticed a big poster like this one hanging on the wall. This is called the periodic table, and it has its origins in the work of Dmitry Mendeleev, who in the 1800s came up with a way to arrange all the chemical elements in a table of rows and columns. We have discussed Mendeleev in a previous video. The periodic table has evolved over the years, but works on similar principles to the one brainstormed by Mendeleev. Depending on who you ask, there are 92 to 98 naturally occurring elements. There are several more elements that were developed in laboratories, and all the known elements are in the periodic table. Each element is represented by a square that looks like this. The top number is called the atomic number. The atomic number is related to the structure of the atom and the number of protons in it. The atomic number determines the identity of the atom and what element it is. The big letter in the middle of the square is the chemical symbol. Sometimes the letters in the boxes are similar to the first couple letters in the name of the element. For example, beryllium has the symbol BE. Lithium has the symbol LI. Oxygen has the symbol O. But then you get elements like silver. Silver has the symbol AG. Well, the AG stands for Argentum, which is the Latin word for silver. Lead also has a symbol that is different from what we would expect. It is PB, and that is because lead in Latin is plumbum. The symbol for gold is AU, and that's because the Latin word for gold was ORUM. The fact that the symbols for some elements are derived from Latin words reflects the fact that these elements have been known since ancient times. Under the symbol is the name of the element, and below the name is the atomic mass, which is the mass of the atoms in the element. One way you can tell the difference between the atomic number and the atomic mass is that the atomic mass is usually larger. It also has decimals. Each row of the table is called a period. As you move from left to right, the atomic number increases. As you move across a period, the elements generally get more massive and heavier, but there are a couple exceptions. The columns of the periodic table are called groups or families. Elements that are in the same column or family tend to have similar properties. And can participate in similar types of chemical reactions. Let's look at the elements in column one, which are called the alkali metals. These metals are so soft that you can cut them with a knife. They all tarnish very quickly after they're cut, and they conduct electricity. Their density is so low that they can float on water. They are also very reactive. If you look all the way over to the right of the periodic table at the noble gases, all of these elements are inert, which means they rarely ever participate in chemical reactions. However, if you run electricity through these gases, they will glow. This is where we get neon lights from. Elements in other families also have similar chemical and physical properties. So let's summarize. The periodic table arranges the elements according to a pattern. As you move from left to right across a period, the atomic numbers get larger. The elements in the columns, which are also called groups and families, all have similar chemical and physical properties.