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IB Chemistry 1: Topic 2 Atomic Structure: atomic particles, atomic number, mass #, isotopes and atomic mass.

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Presentation on theme: "IB Chemistry 1: Topic 2 Atomic Structure: atomic particles, atomic number, mass #, isotopes and atomic mass."— Presentation transcript:

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2 IB Chemistry 1: Topic 2 Atomic Structure: atomic particles, atomic number, mass #, isotopes and atomic mass

3 Defining Atoms & Electrons in Atoms

4 Democritus ( BC) Originated idea of the atom

5 For the next 2000 years… Alchemy Good or bad? Contributions?

6 Isaac Newton – 1704 AD

7 John Dalton ( ) 1803 Dalton’s Atomic Theory

8 Proust

9 At the time, current thinking…

10 Several changes have been made to Dalton’s theory. Modern Atomic Theory Dalton said: Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties Modern theory states: Atoms of an element have a characteristic average mass which is unique to that element.

11 Modern Atomic Theory Dalton said: Atoms cannot be subdivided, created, or destroyed Modern theory states: Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions!

12 J.J. Thomson ( ) 1897 Discovered the electron (“plum pudding” model)

13 Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle.

14 Cathode Ray Tube Cathode Tube Anode Stand Alligator Clip

15 Cathode Ray Tube in Action

16 Mass of the Electron 1909 – Robert Millikan determines the mass of the electron. The oil drop apparatus Mass of the electron is x kg

17 Conclusions from the Study of Electrons Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons. Electrons have so little mass that atoms must contain other particles that account for most of the mass

18 Thompson’s Plum Pudding Model

19 Rutherford ( ) 1911 Discovered the nucleus (gold foil experiment)

20 Rutherford’s Gold Foil Experiment Alpha particles are helium nuclei Particles were fired at a thin sheet of gold foil Particle hits on the detecting screen (film) are recorded

21 Try it Yourself In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

22 The Answers Target #1Target #2

23 Rutherford’s Findings Most of the particles passed right through A few particles were deflected VERY FEW were greatly deflected  The nucleus is small  The nucleus is dense  The nucleus is positively charged “ Like howitzer shells bouncing off of tissue paper!” Conclusions:

24 Since opposite charges attract each other, why don’t the electrons fall into the nucleus?

25 Niels Bohr ( ) 1913 proposed Planetary model

26 Neils Bohr I pictured electrons orbiting the nucleus much like planets orbiting the sun. But I was wrong! They’re more like bees around a hive. WRONG!!! The Bohr Model of the Atom

27 Based upon the work of several men, a new mathematical model was developed to describe the structure of the atom The Quantum-Mechanical Model Louis de Broglie ( ) Werner Heisenberg ( ) Erwin Schrodinger ( )

28 Previous models were physical models based upon the motion of objects. This model is primarily a mathematical model and had never been proven....

29 …until recently! The New York Times, 9/7/99

30 Now we know for sure!

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33 Atomic Number Atomic number of an element is the number of protons in the nucleus of each atom of that element. Element Atomic ## of protons # of electrons Carbon6 Phosphorus15 Gold79 In a neutral atom: # electrons = # protons

34 The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is the electron cloud.

35 ParticleChargeMass (g)Location Electron e x (1/1840 amu) Electron cloud Proton p x (1 amu) Nucleus Neutron n x (1 amu) Nucleus Atomic Particles

36 The Atomic Scale Most of the mass of the atom is in the nucleus (protons and neutrons) Electrons are found outside of the nucleus (the electron cloud) Most of the volume of the atom is empty space “q” is a particle called a “quark”

37 About Quarks… Protons and neutrons are NOT fundamental particles. Protons are made of two “up” quarks and one “down” quark. Neutrons are made of one “up” quark and two “down” quarks. Quarks are held together by “gluons”

38 Reading the Periodic Table Atomic Number 3 Li Lithium Element Symbol Element Name Atomic Mass # p + # e - (in a neutral atom) # p + + # n 0 # n 0 = Atomic Mass - #p +

39 Isotopes Elements occur in nature as mixtures of isotopes. Isotopes are atoms of the same element that differ in the number of neutrons

40 Mass Number Mass number is the number of protons and neutrons in the nucleus of an isotope. Mass # = p + + n 0

41 Symbols of Isotopes Carbon-12 Mass # C 12 Atomic Number 14 6 Carbon-14 Mass # C Atomic Number Atomic Number Chlorine-35 Mass # Cl 6

42 Mass Number Mass # = p + + n 0 Isotope p+p+ n0n0 e-e- Mass # Oxygen Arsenic Phosphorus O188 As7533 P3115

43 IsotopeProtonsElectronsNeutronsNucleus Hydrogen–1 (protium) 110 Hydrogen-2 (deuterium) 111 Hydrogen-3 (tritium) 112 Isotopes…Again (must be on the test) Isotopes are atoms of the same element having different masses due to varying numbers of neutrons.

44 IsotopeSymbol Composition of the nucleus % in nature Carbon C 6 protons 6 neutrons 98.89% Carbon C 6 protons 7 neutrons 1.11% Carbon C 6 protons 8 neutrons <0.01% Atomic mass is the weighted average of all the naturally occuring isotopes of that element. Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. Carbon = Atomic Masses

45 Isotope nameIsotope mass (amu)percentage Silver Silver remainder Silver has two naturally occurring isotopes: Find the missing percentage. Find the average atomic mass of an atom of silver.

46 Silicon has three naturally occurring isotopes: Look over the data before you begin the problem. Estimate the value of the answer before you begin the calculation. Will the weighted average be closer to 28, 29, or 30? Find the average atomic mass of silicon. Isotope nameIsotope mass (amu)Relative Abundance Silicon Silicon Silicon

47 Iron has four naturally occurring isotopes: Estimate the average mass. Find the average atomic mass of iron. Isotope nameIsotope abundanceIsotope mass (amu) Iron %53.94 Iron %55.93 Iron %56.94 Iron %57.93


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