Ch.2 Chem 111 STUDENT

Ch.2 Chem 111 STUDENT - C111 Eric C Brown Ch 2 Lectures I...

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C111 Eric C. Brown Ch. 2 Lectures I. EVERYTHING IS MADE OF ATOMS - Atoms are the basic building blocks of matter; they are the smallest units of an element. - Need to understand the structure of the atom to explain chemical properties/reactivity. II. ATOMIC THEORY OF MATTER John Dalton, English schoolteacher (proposed 1803-1807) Developed theory to explain prior experimental observations - law of conservation of mass (Lavoisier) - law of constant composition (Proust) DALTON’S ATOMIC THEORY: (A) Matter is composed of small, indivisible particles called atoms (atoms are basic building blocks) (B) Atoms of a given element are identical (have same mass, properties) (atoms of different elements have diff. mass, properties) (C) Compounds are formed when 2 or more atoms combine (in a molecule) in definite arrangements (given compound always has same # & kinds of atoms) 1
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C111 Eric C. Brown Ch. 2 Lectures (D) Atoms of an element are not changed by chemical reactions (not created nor destroyed), but are rearranged to form new compounds. III. STRUCTURE OF THE ATOM 1850s - New experiments questioned Dalton s theory. - Experiments showed that the atom is made up of subatomic particles Æ atom is divisible - Subatomic particles electrons, protons, neutrons ELECTRON 1897 J.J. Thomson - Studied electrical discharge (high voltage generated cathode ray ) - Discovered that the ray was deflected by the negative pole of an electric/magnetic field CONCLUSIONS OF EXPERIMENT - Ray consists of negatively charged particles Æ electrons. - Charge-to-mass ratio of electron determined = -1.76 x 10 8 C/g - Ratio is the same for every element! 1909 Robert Millikan - Examined the motion of charged oil droplets in an electric field. CONCLUSIONS OF EXPERIMENT - Charge of an electron = -1.6022 x 10 -19 C 2
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C111 Eric C. Brown Ch. 2 Lectures - Using the charge-to-mass ratio from Thomson, the mass of the electron was determined. e- = 9.10 x 10 -28 g - Electron is extremely small; ~1/1840 of a H atom PROTONS Atoms are electronically neutral , so an atom must contain an equal number of positive and negative charges. 1900 J.J. Thomson - Proposed atom as a uniform, positive sphere of matter in which electrons are embedded like raisins.
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