Chapter 2 The Atomic Nature of Matter Department of Chemistry and Biochemistry Seton Hall University

2 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?)

3 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)

4 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)

5 Atomic Architecture Matter is held together by forces –gravitational –electrical –magnetic Atoms consist of: –nucleus - contains protons and neutrons –electrons

6 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

7 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)

8 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

9 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

10 Forms of energy Kinetic - E=0.5mv 2 Potential Thermal Chemical Radiant Electrical

11 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