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atom consists of electrons, protons, and neutrons nucleus small, dense, positively charged region in the center of the atom proton positively charged particle neutron no charge electron negatively charged particle; located outside the atomic nucleus atomic number number of protons in the atom mass number sum of the number of protons and neutrons protons+neutrons=mass number number of neutrons mass number-number of protons=number of neutrons number of protons number of electrons IF positive and negative charges cancel, the atom charge=0 isotopes atoms of the same element having different masses contains same number of protons and different number of neutrons atomic mass weighted average of the masses of all the isotopes that make up chlorine weighted average average corrected by the relative amounts of each isotope present in nature Dalton's Atomic Theory first experimentally based theory of atomic structure of the atom Periodic Law physical and chemical properties of the elements are periodic functions of their atomic numbers period horizontal row of elements in the periodic table group/families columns of elements in the periodic table metals elements that tend to lose electrons during chemical change, forming positive ions nonmetals a substance whose atoms tend to gain electrons during chemical change, forming negative ions metalloids have properties intermediate between metals and nonmetals electron configuration arrangement of electrons in atomic orbitals valence electrons outermost electrons (involved in chemical bonding) Aufbau Principle building up principle helps determine the electron configuration octet rule elements usually react in such a way as to attain the electron configuration of the noble gas closest to them in the periodic table ions electrically charged particles that result from a gain or loss of one or more electrons by the parent atom cation positively charged (result from the loss of electrons) anion negatively charged (result from the gain of electrons)