In the late 1800s, there was a Russian chemist named Dmitri Mendeleev. He was the first to group chemical elements in order according to their atomic weights. Mendeleev only had around 60 elements to work with at that time, but he understood that when he organized the elements by weight, certain ones occurred regularly in periods. Today, chemists officially recognize 118 elements, and they still use Mendeleev’s period table of elements to keep them straight. Starting with hydrogen, the simplest atom, the periodic table places the other elements in order according to their atomic number. The atomic number indicates how many protons each element contains.
The periodic table contains seven rows with 18 columns. Each row is one period with each period number of the elements showing the number of electrons housed in the energy levels. The periods get longer toward the bottom of the table, because more electrons are needed to fill bigger and more complicated outer levels. The columns group families of elements that tend to look and behave alike. These element families have the same number of electrons in their outer shells.
Elements are grouped together depending on whether they are metals or nonmetals. Metal elements are able to conduct electricity and heat well. Within the metals grouping, a number of subgroups exist. These subgroups are created with elements that have similar characteristics and chemical properties.
- Alkali Metals: Alkali metals make up most of the first column of the period table. Lithium is the first alkali metal, and francium is the last. These metals are very reactive, so chemists have to be very careful how they store them. If alkali metals contact water, they will burst into flames, so they are stored in either oils or inert gases. Because hydrogen has just one electron, it is categorized in group one, even though it’s not considered a metal.
- Alkaline-Earth Metals: Alkaline-earth metals are the second group of the periodic table. These metals have two electrons in their outermost energy level. These metals aren’t as reactive as the alkali metals, but they will have slower chemical reactions that produce less heat.
- Lanthanides: The third group of metals won’t fit nicely into the third column. To solve this, lanthanides are flipped sideways and broken out to sit at the bottom of the table. Elements in this group are silvery white in color. They will also tarnish when they’re exposed to air.
- Actinides: The bottom row of the island contains 89 elements, called the actinides. Just two actinides occur naturally on Earth, and these are thorium and uranium. Actinides are radioactive, and scientists combine them with lanthanides to create the inner transition metal group.
- Transition Metals: The transition metals make up groups three through 12 back in the main body of the periodic table. These metals are shiny, hard, malleable, and they have good conductivity. Some of the well-known transition metals include platinum, iron, silver, and gold.
- Post-transition Metals: Groups 13 through 17 make up the post-transition metals. Sharing some of the classic characteristics of transition metals, the post-transition metals don’t conduct as well, and they are softer.
The elements that sit in the upper right area from the staircase are the nonmetals. As a gas, hydrogen is technically a nonmetal, too, but it’s grouped in group one because it only has one electron. Nonmetals include carbon, nitrogen, phosphorus, oxygen, sulfur, and selenium.
- Halogens: In group 17, the top four elements are the halogen subset of nonmetals. These elements are chemically reactive, and they often pair with alkali metals to make different kinds of salt. For example, the salt you might put on your food is made out of sodium (an alkali) and chlorine (a halogen).
- Noble Gases: Noble gases are group 18, and these gases have no odor or color. Scientists call these gases inert, which means they are almost completely nonreactive.
Metalloids are the elements that make up the transition between (metals) and nonmetals. Some scientists call metalloids poor metals or semimetals, and these elements sometimes act as semiconductors instead of conductors. Metalloids are boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.
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