A Russian chemist Dmitri Mendeleev, in the late 19th century, published his first attempt at grouping the chemical elements according to their atomic weights. There were only about 60 elements known during that the time, later Mendeleev observed that when the elements are organized according to their weight, certain types of elements occurred in regular periods or intervals.
Today, approximately 150 years later, chemists officially recognized 118 elements, including four newcomers in the year 2016, and still use Mendeleev’s periodic table of elements to organize them. The table starts with hydrogen, the simplest atom, and then organizes the rest of the elements by their atomic number, which is the number of protons each element contains. With a few exceptions, the order of the elements corresponds with the increasing mass of each atom.
The table has 18 columns and seven rows. Each row represents a period; the period number of an element indicates how many of its energy levels house electrons. Sodium, (Na) for instance, sits in the third period, which means a sodium atom has electrons in the first three energy levels. Moving down the table, periods are longer as it takes more electrons to fill the larger and more complex outer levels.
The columns of the table represent families, or groups, of elements. The elements in a group often behave and look similar, as they have the same number of electrons in their outermost shell. Group 18 elements, on the far-right side of the table, for example, have full outer shells and rarely participate in chemical reactions.
Elements are generally classified as metals and nonmetals, but the dividing line between the two is difficult. Metal elements are usually good conductors of heat and electricity. The subgroups within the metals are based on the similar characteristics and chemical properties of these collections.
According to the Los Alamos National Laboratory:
The alkali metals make up most of Group 1, that is, the table’s first column of the periodic table. They are shiny and soft enough to cut with a knife, these metals start with lithium (Li) and end with francium (Fr). They are also extremely reactive and will burst into flame or even explode when they come in contact with water, so chemists store them in oils or inert gases. Hydrogen, with its single electron, also placed in Group 1, but the gas is considered a nonmetal.
Alkaline-earth metals: The alkaline-earth metals make up Group 2, the second row of the periodic table and these elements start from beryllium (Be) through radium (Ra). Each of the elements in this row has two electrons in its outermost energy level, which makes the alkaline earth reactive enough that they’re rarely found alone in nature. But they’re not that reactive as the alkali metals. Their chemical reactions occur more slowly and produce less heat compared to that of alkali metals.
This is the third group, which is much too long to fit into the third column, so it is broken out and flipped sideways to become the top row of the island that floats at the bottom of the table. The lanthanides elements are lanthanum (La) to lutetium (Lu). The elements in this group have a silvery-white colour and lose lustre when coming in contact with air.
The bottom row of the island is the actinides line which comprises elements 89, actinium (Ac), through 103, lawrencium (Lr). Only thorium (Th) and uranium (U) occur naturally on Earth in substantial amounts. All these elements are radioactive. The lanthanides and actinides together form a group called the inner transition metals.
Coming to the main body of the table, the remainder of Groups 3 through 12 represent the rest rows are the transition metals. Malleable but hard, shiny, and possessing good conductivity, these elements are what you think of when you hear the word metal. Many of the greatest hits of the metal world including gold, silver, iron and platinum live here.
Jumping into the nonmetal world, shared characteristics aren’t neatly divided along vertical group lines. The post-transition metals are gallium (Ga), aluminium (Al), indium (In), thallium (Tl), lead (Pb), bismuth (Bi), tin (Sn) and they spread from Group 13 to Group 17. These elements have some of the characteristics of the transition metals, but they tend to be softer and conduct more poorly than transition metals. Periodic tables will feature a bolded staircase line below the diagonal connecting elements boron with astatine, the post-transition metals cluster to the lower left of this line.
The metalloids are silicon (Si), boron (B), arsenic (As), antimony (Sb), tellurium (Te), polonium (Po) and germanium (Ge). They form the staircase that represents the gradual transition from metals to the nonmetals. These elements sometimes behave as semiconductors (B, Si, Ge) rather than as conductors. Metalloids are also known as poor metals or semimetals.
Everything else to the upper right of the staircase, plus hydrogen (H), stranded way back in Group 1 is a nonmetal. These include elements like carbon (C), nitrogen (N), phosphorus (P), oxygen (O), selenium (Se) and sulfur (S).
The top four elements of the Group 17, from fluorine (F) through astatine (At), represent one of two subsets of nonmetals. The halogens are chemically reactive and tend to pair up with the alkali metals to produce various types of salts. The table salt in your kitchen, for example, is a marriage between halogen chlorine and the alkali metal sodium.
Colourless, odourless and almost entirely nonreactive, the inert, or noble gases round out the table in the Group 18. Many chemists expect oganesson, one of the newly named elements, to share these characteristics; however, as this element has a half-life measuring in the milliseconds, no one has been able to test it directly. Oganesson completes the seventh period of the periodic table, so if anyone manages to synthesize the element 119, it will loop around to start row eight in the alkali metal column.
Because of the cyclical nature created by the periodicity that gives the table its name, some chemists prefer to visualize Mendeleev’s table as a circle.
It’s Questions Time.
- What are alkali and alkaline earth metals?
- How many rows and colums are there in a periodic table?
- Who invented periodic table?
- What are noble gases?
- How many elements we found recently?