However, all compounds are made up of two or more different types of atoms. Elements are rarely found in their pure state; compounds are much more common. There are just over different kinds of atoms, but there are millions of different kinds of substances made up of different types of molecules. Probably everything you see around you is some type of compound. When atoms of different kinds combine to form a compound, a new substance is created. New compounds do not have the same physical or chemical traits of the original elements.
They have a new life of their own. Compounds are written with formulas showing which elements from the periodic table are combined.
One very familiar compound is water. When two hydrogen atoms H 2 combine with one oxygen atom O , it makes the compound H 2 O, which we know as water. All water molecules have this same combination of atoms. Water is not hydrogen or oxygen. You couldn't pour oxygen and hydrogen atoms on a fire and expect to put it out. But when they are bonded together as water molecules, they behave like water.
A compound is a brand new substance with its own properties. The same elements can build very different compounds. If you took those two hydrogen atoms and joined them to two oxygen atoms instead of one , you would wind up not with water but with H 2 O 2 , a very different compound called hydrogen peroxide - you wouldn't want to drink it! How do these compounds form? What holds the atoms in a molecule together?
The answer is that compounds are formed when elements are joined and held together by strong forces called chemical bonds. These bonds involve the electrons that orbit the nucleus of the atom. Electrons are located in energy levels that occur at certain distances from the nucleus, called shells.
These shells can each carry a certain number of electrons for example, 2 in the first shell, 8 in the second, and so on. Atoms want to have their shells full with as many electrons as they can carry, and when their outermost shell isn't full, atoms try to bond with other atoms by giving up or gaining electrons. Atoms with an almost-empty outer shell will want to give up electrons, while atoms with an almost-full outer shell will want to gain electrons in order to fill it up.
There are two main types of bonds that hold most compounds together. Ionic bonds form when one atom gives up, or donates, an electron to another in order that both will have a full outer shell. In doing so, the atoms are bonded and create a compound. Covalent bonds share electrons between atoms in order to fill their electron shells.
In the compound, molecules are held together by the attraction between the nucleus and the shared electrons. Often substances may combine without forming a compound. To make a compound, there must be a chemical reaction where bonds are formed and an entirely new substance is created. Without that chemical reaction, combined substances may instead form a mixture.
The components of a mixture keep their original properties and can easily be separated. For example, a mixture of fruits in a salad can be separated back into groups of different kinds of fruit. Salt and water can be combined in a mixture, but water is still water, and salt is still salt. To separate the two components, the water can be evaporated so that the salt can be collected.
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Sand and water can be separated by using a filter. The ocean, rocks, blood, and even the air we breathe are mixtures rather than compounds. On the other hand, the components in a compound cannot be separated by physical means.
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Learn more about compounds and mixtures. Physical changes do not break down compounds. Physical changes affect the size, shape, or state of the substance, but not the chemical properties. You can change the state of matter , but the compound does not change. If you leave an ice cube out in the sun it will melt into liquid water, but in either state it is still made of water molecules.
You can apply a physical force to a solid glass and break it, but the molecules that make up glass will remain.
Chemical changes in compounds happen when chemical bonds are created or destroyed. Then the molecular structure changes; new molecules form and a new substance is created. Often heat is used to begin a chemical change, as when baking a cake. Another example of a chemical reaction is the rusting of a metal trash can. The rusting happens because the iron Fe in the metal combines with oxygen O 2 in the air. As such, the handbook continues to provide detailed information on the environmental metals that influence the health of plants, animals and humans, with particular attention given to environmental and analytical chemistry, bioavailability, metabolic pathways and biological effects.
It also delineates the problems related to waste, soils and wildlife as well as the risks caused by the increasing output of metals from industry and households. It retains the proven features of the first edition, such as the extensive bibliography, numerous tables with useful data and a glossary of terms, and once again all the contributions were not only written but also reviewed by acknowledged and experienced experts.
In total, international experts from 15 countries have pooled their knowledge and experience to create this ultimate resource. Essential information for chemists, biologists, geologists, food scientists, toxicologists and physiologists involved in environmental research and remediation, risk assessment, food research and industrial hygiene.
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This masterwork is indispensable for all those involved in the different aspects of the occurrence of trace elements in the environment, either analytical or environmental chemists, toxicologists or policy-makers. Scientists will find in these pages much relevant information for their purposes.
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