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Atomic theory Matters are built from atoms. John Dalton (1768 -1828) • Atoms are small, cannot be separated (by chemical means). • All atoms of the same element are the same. • Atoms cannot be destroyed, so the mass is conserved. • Atoms can combine differently to make different compounds. • Compounds of the same kind always have the same number of each type of atom. Do you see the atom? Thompson’s cathode ray experiment - How electron was found electron: charge mass = - 1.76 x 10 11 C/kg Millikan Oil Drop Experiment - How electron charge was measured Charge for each electron: 1.602 x 10-19 Coulomb Mass of electron: 9.10 x 10-28 g Picture of an atom (~1900) Thompson model Where to find the α particles? - Rutherford’s experiment http://www.metacafe.com/fplayer/109587/bullets_path.swf Where to find the a particles? - Rutherford’s experiment Where to find the a particles? - Rutherford’s experiment Anatomy of atom nucleus Anatomy of atom Atomic theory Matters are built from atoms. • • • • • Atoms are small, cannot be separated (by chemical means). All atoms of the same element are the same. Atoms cannot be destroyed, so the mass is conserved. Atoms can combine differently to make different compounds. Compounds of the same kind always have the same number of each type of atom. Atomic theory H 2 protons → He 100 ___ protons → Fm Au 79 protons → ____ 1 proton → How to denote an atom? mass number (protons + neutrons) number of protons (atomic number) 23 Na 11 symbol of the element How to denote this atom? A X Z (1) How many protons? (2) What is the symbol for this element? (3) How many neutrons? (4) What is its mass number? What atom is it? A X 5 (1) How many protons? (2) What is the symbol for this element? (3) How many neutrons? (4) What is its mass number? What atom is it? A B 5 (1) How many protons? (2) What is the symbol for this element? (3) How many neutrons? (4) What is its mass number? What atom is it? 10 B 5 (1) How many protons? (2) What is the symbol for this element? (3) How many neutrons? (4) What is its mass number? What atom is this one? What atom is it? 10 B (1) How many protons? (2) What is the symbol for this element? (3) How many neutrons? (4) What is its mass number? How to denote these atoms? 1 1H 2 1H (D) 3 1H (T) Isotopes: • Same number of protons • Different number of neutrons Different mass! Isotopes Ions Electrons are loosely bond to the nucleus. An atom can lose or gain electrons. A neutral atom: number of protons (+) = number of electrons (-) An ion: a neutral atom loses or gain electrons number of protons (+) ≠ number of electrons (-) cation: more protons, thus positively charged anion: more electrons, thus negatively charged Ions 12 C (6 p+, 6e- ) neutral atom take1 extra electron: - 12 C (6 p+, 7e- ) anion Ions 12 C (6 p+, 6e- ) neutral atom lose 1 electron: - 12 + C (6 p+, 5e- ) cation Ions Ca lose or gain ? electrons Ca 2+ Fill in the blanks Symbol O2- 16 Number of protons 1 6 Number of neutrons 0 7 Number of electrons 0 3 Mass number Atomic number Fill in the blanks Symbol Number of protons O2- 16 1 6 Number of neutrons 0 7 Number of electrons 0 3 Mass number Atomic number 8 Fill in the blanks Symbol Number of protons O2- 16 1 6 Number of neutrons =16-8=8 0 7 Number of electrons 0 3 Mass number Atomic number 8 Fill in the blanks Symbol Number of protons O2- 16 1 6 Number of neutrons =16-8=8 0 7 Number of electrons =8+2=10 0 3 Mass number Atomic number 8 Fill in the blanks Symbol Number of protons O2- 16 1 6 Number of neutrons =16-8=8 0 7 Number of electrons =8+2=10 0 3 Mass number Atomic number 8 16 Fill in the blanks Symbol Number of protons O2- 16 8 1 6 Number of neutrons =16-8=8 0 7 Number of electrons =8+2=10 0 3 Mass number 16 Atomic number 8 Atomic theory H2O CO2 Atomic theory ? ? Molecular compounds • No ions! • Mostly compounds by non-metal atoms. • Examples: Empirical formulas Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound. Molecular formula Empirical formula H2O2 HO C2H4 CH2 C3H6 ? H2O ? Empirical formulas Empirical formulas give the lowest whole-number ratio of atoms of each element in a compound. Molecular formula Empirical formula H2O2 HO C2H4 CH2 C3H6 CH2 H2O H2O Ionic compounds molecular formula for magnesium nitride? Mg2+ N3- What is the molecular formula for sodium oxide Chromic bicarbonate Iron(III) phosphate What does each number mean? H 2SO 4 2 SO 4 2− 3 Mg 14 2 2+ N H3