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Chapter 2 The Atomic Nature of Matter Department of Chemistry and Biochemistry Seton Hall University Atomic Theory • Bulk of atomic theory was described by Dalton • Basic key is the definition of a compound (anybody remember?) • We can “see” individual atoms via STM, and it confirms our expectations (consider: what if it didn’t?) 2 Dalton’s Theory • All matter is composed of tiny particles called atoms • All atoms of a given element have identical chemical properties that are characteristic of that element • Atoms form chemical compounds by combining in whole-number ratios • Atoms can change how they are combined, but they are neither created nor destroyed in chemical reactions (the Law of the Conservation of Mass) 3 Several Related Points • Matter and energy are interconverted, the amount of mass involved is unmeasureable • Atoms and molecules are continually in motion • Diffusion - the passage of a particle • Dynamic equilibrium - balanced motion of particles (consider evaporation, diffusion) 4 Atomic Architecture • Matter is held together by forces – gravitational – electrical – magnetic • Atoms consist of: – nucleus - contains protons and neutrons – electrons 5 Atomic Architecture • Nucleus contains essentially all of the mass of the atom, but essentially none of the volume • Electrons have virtually none of the mass, but occupy basically all of the volume • An atom the size of a baseball stadium will have a nucleus the size of a pea 6 Atomic Diversity • An element is identified by the charge of its nucleus (the number of protons) • Isotopes have the same number of protons, but different number of neutrons • X - element symbol • A - atomic mass (number of protons and neutrons) • Z - atomic number (number of protons) A Z X 7 Isotopes • Most elements occur naturally as two or more isotopes, hence the fractional atomic mass • Isotopes of hydrogen – protium - one proton, one electron – deuterium - one proton, one neutron, one electron – tritium - one proton, two neutrons, one electron • Mass spectrometry - see fig 53; measures mass very precisely 8 Charged species • Charged chemical species are called ions – cations - positively charged – anions - negatively charged • Ions can be atoms or molecules • Compounds formed from two or more ions are called ionic compounds or salts • Salts often dissolve in liquids to give separated ions 9 Forms of energy • • • • • • Kinetic - E=0.5mv2 Potential Thermal Chemical Radiant Electrical q1q2 Ek r 2 kg m 1J 1 2 s 10 Conservation of Energy • Energy is neither created nor destroyed in a chemical process, but may be transferred from one body to another or changed from one form to another • End of Chapter 2 11