The periodic table of the elements was understood by scientists centuries ago. In the late 19th century, a Russian chemist published his first attempt at grouping chemical elements according to their atomic weights. When the elements were organized by weight, there were certain types of elements that happened in regular intervals.
After the addition of four new elements in 2016, the number of elements in the periodic table is 118. The table starts with the simplest atom, hydrogen, and then organizes the rest of the elements by atomic number, which is the number of protons each contains. The increasing mass of each atom corresponds with the order of the elements.
There are seven rows and 18 columns in the table. The period number of an element indicates how many electrons it has. The third period is when a sodium atom has electrons in the first three energy levels. Periods are longer when moving down the table because it takes more electrons to fill the larger and more complex outer levels.
The elements are represented by the columns of the table. The elements in a group look and behave the same, because they have the same number of electrons in their shell. Group 18 elements are on the far right side of the table and rarely participate in chemical reactions.
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The dividing line between metal and nonmetal is fuzzy. Good conductors of heat and electricity are metal elements. The subgroup within the metals are based on the same characteristics. The Los Alamos National Laboratory says that our description of the periodic table uses commonly accepted groupings of elements.
The periodic table of elements has several broad groups.
The table's first column contains the alkali metals. The metals start with Li and end with Fr. They are extremely reactive and will burst into flame or explode on contact with water, so they are stored in oils or gases. The gas is considered a nonmetal in Group 1, but with its single electron.
Group 2 of the periodic table is made up of the alkaline-earth metals. Each of these elements has two electrons in its energy level, which makes them reactive, and they're rarely found alone in nature. They are not as reactive as the alkali metals. Their chemical reactions take less time and produce less heat.
The third group is too long to fit into the third column, so it is broken out and flipped sideways to become the top row of the island. The elements are lanthanum to lutetium. The elements in this group have a silvery white color.
The actinides line the bottom row of the island and comprise elements 89, actinium, through 103, lawrencium. Thorium and U are the only elements that occur naturally on Earth. All are radioactive. The actinides and lanthanides are part of the inner transition metals.
The rest of Groups 3 through 12 are the transition metals. When you hear the word metal, you typically think of elements that are hard, shiny, and good conductors. The greatest hits of the metal world, including gold, silver, iron and Platinum, are all here.
Ahead of the jump into the nonmetal world, shared characteristics aren't neatly divided along vertical group lines. Group 13 to Group 17 spans the post-transition metals of aluminum, indium, thallium, tin, lead, and Bi. The elements have some of the classic characteristics of the transition metals, but they are softer and conduct less well than other transition metals. A bolded "staircase" line below the diagonal connecting boron and astatine is found in many periodic tables. The lower left of the line has a cluster of metals.
The metalloids are arsenic, tellurium, polonium, and antimony. The staircase is a representation of the gradual transition from metals to nonmetals. The elements sometimes behave like transistors. "Semimetals" or "poor metals" are the terms used for metalloids.
Everything to the upper right of the staircase is a nonmetal. Carbon, nitrogen, phosphorus, oxygen, sulfur, and selenium are included.
The top four elements of Group 17 are fluorine, astatine, At, and F. Salt is produced by the combination of the halogens and alkali metals. The table salt in your kitchen is a marriage between the two elements.
Group 18 has noble gases, which are odorless and nonreactive. Oganesson, one of the four newly named elements, has a half-life measuring in the milliseconds, so no one has been able to test it directly. The seventh period of the periodic table is completed by Oganesson, so if anyone succeeds in synthesizing element 118, they will start row eight in the alkali metal column.
Some chemists prefer to see the table as a circle because of the periodicity that gives it its name.
There are additional resources.
There is a video about the periodic table and element groups.
There is an interactive periodic table at ptable.com.
There is a free online educational resource for understanding the elements.