All matter, from a rock to an animal to the magma at the center of the Earth, is made from different combinations of 92 naturally occurring substances known as elements. The smallest quantity of an element that still exhibits the of that element is known as an atom. One atom of carbon, for example, is the smallest piece of matter that still retains the chemical and physical of carbon.
Atoms are made up of even smaller particles called electrons, protons, and neutrons. Each of these particles has a different electrical charge. Protons are positively charged, neutrons have no charge, and electrons are negatively charged. The protons and neutrons of an atom reside in a central body called a nucleus. Electrons appear around the nucleus within orbitals of varying energy. Overall, the atom is neutrally charged with equal numbers of positively charged protons and negatively charged electrons.
Elements are distinguished by the number of protons in their nuclei. All atoms containing six protons are called carbon. Any element with one proton is called hydrogen. Only the number of protons—and not the number of neutrons or electrons—distinguishes elements from each other.
Isotopes and Ions
Though the number of neutrons and electrons in an atom won’t change the atom’s status as a particular element, it can affect the properties of an element in subtle ways. An atom that contains a larger or smaller number of neutrons than usual is called an isotope. Carbon usually has six protons and six neutrons and can be called carbon-12 because the number of its protons and neutrons add 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 not have charge, because the numbers of positive and negative particles remain balanced. Even though they have different masses, isotopes of the same element all have similar chemical properties, because the number of electrons (not the number of neutrons or protons) determines the way an atom will interact with other atoms.
Ions are atoms that either lack or have extra electrons. Because these atoms have unequal numbers of electrons and protons, they are charged particles and are often quite chemically interactive with other atoms. Though the SAT II Biology Test rarely asks direct questions about ions, ions do play an important role in many biological processes and phenomena, so understanding the basics of ions can help you understand the processes that the test covers.
Molecules and Compounds
Atoms combine with each other in chemical reactions to create molecules, unique substances with physical and chemical properties distinct from those of their constituent elements. Combining two hydrogen atoms with one oxygen atom creates water, which has very different than hydrogen or oxygen do alone. Molecules such as water containing more than one type of element can also be called compounds. A water molecule made up of oxygen and hydrogen can be called a compound; a hydrogen molecule, which contains only two hydrogen atoms, cannot be called a compound.
