Dmitri Mendeleev was a Russian chemist who liked to play solitaire. In the 1860s when Mendeleev was working, scientists had identified only 60 different elements, and no one had figured out how to arrange them in any usable order. One day in 1869, Mendeleev decided to create a deck of cards. On each card he wrote one of these 60 elements and all of that element's known characteristics. On a long train ride, Mendeleev played solitaire with his cards. He arranged them every way he could think of, looking for a pattern among the properties of the elements.

After many hours of messing around with his element cards, Mendeleev finally discovered his pattern. He found that when elements were arranged by their atomic number (the number of protons in an atom), similar properties were repeated in every eighth element. When something has a repeating pattern like this, we say it is periodic.

Mendeleev not only was the first person to find a periodic pattern for the elements; he also discovered why no one before him had been able to do so: Several elements were missing! In putting the elements in order, he predicted that more elements would be discovered. He even predicted what characteristics these missing elements would have. Years later scientists found the missing elements that Mendeleev had predicted; today, more than 100 elements have been identified. Each element has been assigned an atomic number based on the number of protons in its nucleus. With that number, all of the elements fell neatly onto the chart we now call the periodic table of elements.

The smallest unit of matter is an atom. Atoms are made of sub-atomic particles, the three most important of which are protons, neutrons and electrons. Electrons are negatively charged particles with almost no mass. Protons and neutrons do have mass; protons are positively charged particles, while neutrons are, as their name suggests, neutral. Protons and neutrons lie at the center of the atom, making up the nucleus.

Surrounding the nucleus and moving around at tremendous speeds are the tiny electrons. In most atoms, the number of electrons and protons are the same, thus balancing the atom's electrical charge. Sometimes, however, a few of the electrons can become separated or shared with another atom. This creates a new molecule that has the atoms of two or more different elements and characteristics that may be different from either of the individual atoms.

Atoms are too small to be seen, even under a microscope. Molecules are groups of atoms that are bound together, arranged in regular geometric patterns that may also be too small to be seen. Molecules and atoms are constantly in motion, even in their patterns. The higher the temperature of a molecule, the greater the amount of motion it has. The simplest molecules are composed of just one element or substance; for example, an oxygen molecule has just two oxygen atoms.

An element is a pure substance: It cannot be broken down or separated into simpler substances either by physical or chemical means. Because an element is pure, every atom of that element is like every other atom of that element - with the exact same physical and chemical properties or characteristics, like color, odor, conductivity, malleability, density, boiling point, solubility, and reactivity to acid or oxygen (one of an element's chemical properties).

Elements are identified and grouped together into categories based on their physical and chemical properties. The two main categories of elements are metals and non-metals. Included in the non-metals are metalloids and noble gases.

Iron, gold, and copper are three examples of elements that are metals. They share some common properties: They are all malleable, which means they can be flattened or hammered into different shapes; they are all ductile, which means they can be drawn out into wire; they are all good conductors of electricity and heat; and they are all shiny. Some properties that may vary for each metal include boiling point, density, solubility, and color. Mercury, for instance, is a metal that is liquid at room temperature, while lead is an especially dense metal.

Non-metals are not shiny; they're not malleable or ductile; and most are poor conductors of heat and electricity. Some, like carbon, are brittle and will shatter and break if you smash them with a hammer.

Metalloids have some properties of metals and some of non-metals. They are also called semi-conductors. One semi-conductor, silicon, is the key element in making computer chips.

Today the periodic table of elements lists more than 100 known elements (see page 48). To read the table, note that each element has its own box with a large abbreviation of one or two letters. This is called the chemical symbol. For example, the chemical symbol of hydrogen is H, gold is Au, and iron is Fe. The name of the element is written under its chemical symbol. A little number at the top of the box is the element's atomic number, which is the number of protons in the nucleus of one atom of that element. A little number at the bottom is the atomic mass of this element, which is the combined number of protons and neutrons in the nucleus of one atom of that element.

On the periodic table, each horizontal row of boxes is called a period. This is because the properties of the elements in each row follow a repeating or periodic pattern as you move from left to right. In every row the first elements are metals, followed by non-metals, metalloids, and, finally, noble gases.

Each column of the periodic table is called a group. Elements in a group all share similar chemical and physical properties. For example, all the elements in group 1, like lithium and sodium, are very reactive metals - they're soft, shiny, and easily form compounds with other elements. Groups 3-12 are called transitional metals. Most of the elements in these groups are shiny, good conductors, and denser than the elements in groups 1 or 2. Examples are metals like copper, iron, nickel, and silver.

Groups 17 and 18 are non-metals. Elements in group 18 are called inert or noble gases. They are generally found in a gaseous state at room temperature. Some glow different colors when an electric current is passed through them, making them useful in neon lights. They are also called inert gases because they don't react easily with other elements.

Some transitional metals in periods 6 and 7 are placed separately at the bottom of the table to make the table easier to read, as most of those elements are not commonly found. Elements to the right of 94 (plutonium) can only be created in laboratories and may last for only one-millionth of a second.

Very few elements are actually found in their pure state. Most are combined with other elements to produce substances called compounds. These can include many different elements; there's an almost unlimited number of combinations possible. Some very simple compounds are familiar to everyone. Water, chemically called H2O, is really a compound containing two atoms of hydrogen and one of oxygen.

Substances can be combined or changed two different ways: physically or chemically. During physical changes a substance keeps its chemical composition and properties; Freezing, melting, boiling, evaporating, tearing, and crushing are all examples of physical changes. These changes can create such physical combinations as colloids, mixtures, or suspensions. In those cases, individual substances can be separated out by physical means such as filtering or precipitating.

Chemical changes occur when atoms or molecules share, lose, or gain electrons. These interactions are called chemical reactions. During a chemical reaction atoms interact to create a new substance with different physical and chemical properties than the two or more elements that are combining. Common examples of chemical reactions include rusting iron, souring milk, burning charcoal, or even baking cookies.

Chemical bonding is the joining of two or more atoms to form a new substance. A chemical bond is a force of attraction that holds two atoms together, a force that involves the electrons that whir around the nucleus of every atom. In some atoms different electrons are closer or further from the nucleus. The electrons in the outermost energy level of an atom are called its valence electrons. The number of valence electrons is one of the properties that elements in the same group on the periodic table share. The comparative number of valence electrons of two atoms determines how easy it is for them to bond because bonding involves gaining, losing, or sharing electrons in each atom's outermost energy level. Generally, atoms try to have an even number of valence electrons, usually eight.

There are three types of chemical bonds: ionic, covalent, and metallic. In ionic bonding an atom gives up or transfers one of its electrons to another atom. When an atom is missing an electron or has an extra electron, it is no longer neutrally charged. We call this positively or negatively charged atom an ion. When positive and negative ions interact, they form a new substance. For example, when sodium and chlorine interact in a solution, they combine to form sodium chloride, or salt.

Covalent bonds are formed between two atoms that are not ions. Instead of becoming a charged ion, in covalent bonds two atoms share an outer-shell electron. The resulting compound is a new particle called a molecule. Molecules are not crystals. H20 is a simple molecule, but some can be very complicated. For example, a carbohydrate or sugar molecule may contain dozens of different carbon, hydrogen, and oxygen atoms all joined by covalent bonds. Likewise, a protein molecule can have hundreds of carbon, nitrogen, oxygen, and hydrogen atoms. Think of all the different atoms you eat for breakfast every morning!

Metallic bonds are formed between metal atoms. Like ionic bonding, metal atoms share electrons, but instead of becoming charged, in metals the atoms' outermost energy levels overlap, allowing the electrons to move throughout the metal. This is why metals can conduct electricity, which is just the flow of electrons through a conductor. This sharing of electrons also explains why metals are malleable and not brittle like non-metals.

A chemical reaction is the process by which two or more substances interact to bond or form a new substance. We can tell when a chemical reaction has occurred because the new substance has new chemical and physical properties - it may change color, form, phase, or even temperature. The starting substances in a chemical reaction are called reactants; the resulting substance is called a product.

When chemical reactions occur, new products are formed. But the basic number of atoms always stays the same. Remember the law called the conservation of mass? It states that during a chemical reaction matter cannot be created or destroyed. The law is demonstrated when writing a chemical equation or formula. For example, when coal is burned, two reactants, carbon and oxygen, have a chemical reaction, during which heat is generated and the resulting product of carbon dioxide is formed. This reaction is thus carbon plus oxygen. Since oxygen molecules include two atoms, this is written using scientific abbreviations from the periodic table as C + O2 = CO2.

Conservation of mass is a really important law. Because of it, we know that during chemical reactions, the total mass of the reactants equals the total mass of the products. The elements involved may trade or share electrons or protons, and energy may be released or absorbed, but the number of atoms, protons, and electrons will stay the same on both sides of the equation.

Let's look at another example of the conservation of mass, this time using water. Hydrogen and oxygen molecules are both simple covalent molecules having two atoms in each. Individually, they are written as H2 and O2. When they combine to form water, however, two molecules of hydrogen are used for every one molecule of oxygen. The result is read 2 H2 + O2 = 2H2 O. This illustrates the conservation of mass because the number of atoms of oxygen and hydrogen remains the same on each side of the equal sign - four hydrogen atoms and two oxygen atoms.

All atoms of an element are generally identical, but sometimes a certain percentage of atoms contains one more or one less neutron in their nucleus. These slightly different variations of an element are called isotopes. One such isotope is called deuterium, or heavy water, and is found among the hydrogen atoms in water. Heavy water is used in nuclear reactors to slow down neutrons and thus control the rate of a nuclear reaction. During World War II, British and Norwegian commandos snuck up to a Nazi factory that was producing deuterium and blew it up, effectively preventing Adolf Hitler from building an atomic bomb and helping the Allies to win the war.

Think it would be crazy to try to fit the periodic table into a song? Try it. You won't be the only one who has ever tried. The singer Tom Lehrer famously put the elements to the tune of a song from the musical Pirates of Penzance. More recently, Pittsburgh-based rapper NOVA dropped a periodic table-inspired rap. Here, he slips some elements into his verse: "Up in the laboratory, Melting lithium in your iPod, I'm serving suckers up like IHOP, Yeah, it's pancakes, You rappers can't hate, I'm emceeing the raw iridium, You suck beryllium, My rhymes make you smarter Just hearing 'em."

Today the periodic table contains more than 100 elements. Some of the new elements are named for famous scientists or places where they were discovered. Element No. 101, for instance, is mendelevium, named after the same Mendeleev who discovered the periodic table. Here are some other elements named for famous people or places:
95 americium (America)
96 curium (Marie Curie)
97 berkelium (Berkeley, California)
98 californium (California)
99 einsteinium (Albert Einstein)
100 fermium (Enrico Fermi)
102 nobelium (Alfred Nobel)