- CHEMISTRY
is the study of matter.
Matter consists of anything that occupies space and has mass.
A compound is any form of matter that can be broken down (decomposed) into simpler forms.
An element is any form of matter that cannot be broken down into a simpler form.
Metallic elements have a shiny, metallic luster and are malleable and ductile (they can be hammered into shape or drawn out into a wire).
Most metallic elements are solid at room temperature (exception: mercury is a liquid).
Nonmetallic elements can be solid, liquid, or gas, and do not have metallic luster. The solids are generally brittle.
- Ionic compounds:
An ion is an atom that has lost of gained an electron and therefore has a positive or negative charge.
In general, metallic elements (and hydrogen) can lose electrons and become positive ions.
Nonmetallic elements tend to gain electrons and become negative ions.
Some "polyatomic ions" contain several elements combined together. Some common polyatomic ions are:
carbonate CO3-2
nitrate NO3-1
sulfate SO4-2
Metallic elements generally combine with nonmetallic elements, or with negatively charged polyatomic ions, to make ionic compounds.
Many ionic compounds dissolve in water, and the ions separate. Such soluble compounds do not usually occur naturally in solid form,
except where water has evaporated.
Rock-forming compounds are generally insoluble. Many of them are silicate minerals, containing silicon and oxygen bonded strongly together.
- Covalent compounds:
Covalent compounds are those in which electrons are shared. Most covalent compounds are complex and hard to break apart.
Carbon forms many strongly bonded covalent compounds by sharing electrons with other elements or with other carbon atoms.
Most of these compounds are organic (formed by organisms); some are very complex.
Silicon also forms many strongly bonded covalent compounds, usually by bonding strongly to oxygen.
Nearly all silicates are hard, insoluble rock-forming solids.
- Water is special:
Water is called a "polar covalent" compound because its electrons are shared, but they are shared unequally, so that part of every water molecule
has a positive charge and part has a negative charge. Because it is covalent, it is hard to break apart.
Because water molecules are polar (with positive and negative poles on each molecule), it can dissolve most ionic compounds.
Any compound that dissolves in water to release hydrogen ions is an acid. Any compound that combines with hydrogen ions is called a base.
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