端木·宇 2008-6-19 22:24
The Building Blocks of Matter
All matter, from a rock to an animal to the magma at the center of theEarth, is made from different combinations of 92 naturally occurringsubstances known as [b]elements[/b]. The smallest quantity of an element that still exhibits the characteristics of that element is known as an [b]atom[/b].One atom of carbon, for example, is the smallest piece of matter thatstill retains the chemical and physical characteristics of carbon.
Atoms are made up of even smaller particles called [b]electrons[/b], [b]protons[/b], and [b]neutrons[/b].Each of these particles has a different electrical charge. Protons arepositively charged, neutrons have no charge, and electrons arenegatively charged. The protons and neutrons of an atom reside in acentral body called a nucleus. Electrons appear around the nucleuswithin orbitals of varying energy. Overall, the atom is neutrallycharged with equal numbers of positively charged protons and negativelycharged electrons.
Elements are distinguished by the numberof protons in their nuclei. All atoms containing six protons are calledcarbon. Any element with one proton is called hydrogen. Only the numberof protons—and not the number of neutrons or electrons—distinguisheselements from each other.
[b] Isotopes and Ions[/b]
Though the number of neutrons andelectrons in an atom won’t change the atom’s status as a particularelement, it can affect the properties of an element in subtle ways. Anatom that contains a larger or smaller number of neutrons than usual iscalled an isotope. Carbon usually has six protons and six neutrons andcan be called carbon-12 because the number of its protons and neutronsadd up to 12. But some carbon atoms have seven or even eight neutrons.These two isotopes are called carbon-13 and carbon-14. Isotopes do nothave charge, because the numbers of positive and negative particlesremain balanced. Even though they have different masses, isotopes ofthe same element all have similar chemical properties, because thenumber of electrons (not the number of neutrons or protons) determinesthe way an atom will interact with other atoms.
Ions are atoms that either lack or haveextra electrons. Because these atoms have unequal numbers of electronsand protons, they are charged particles and are often quite chemicallyinteractive with other atoms. Though the SAT II Biology Test rarelyasks direct questions about ions, ions do play an important role inmany biological processes and phenomena, so understanding the basics ofions can help you understand the processes that the test covers.
[b] Molecules and Compounds[/b]
Atoms combine with each other in chemicalreactions to create molecules, unique substances with physical andchemical properties distinct from those of their constituent elements.Combining two hydrogen atoms with one oxygen atom creates water, whichhas very different characteristics than hydrogen or oxygen do alone.Molecules such as water containing more than one type of element canalso be called compounds. A water molecule made up of oxygen andhydrogen can be called a compound; a hydrogen molecule, which containsonly two hydrogen atoms, cannot be called a compound.
You may have heard water referred to as H2O.This notation is the standard way of representing molecules andcompounds by shorthand. The “H” and “O” stand for the elements hydrogenand oxygen, and the subscript indicates that water contains two partshydrogen for every one part oxygen. You can create the formula for anycompound by writing down the letter symbol of each of its constituentelements and using subscripted numbers to indicate how many atoms ofeach element are present.
