History and Structure of the Atom From Democritus to...

History of Atomic Theory 2 Democritus (from about 440 BC) coined the term atom which means uncuttable He felt that if you kept cutting matter smaller and smaller eventually you will no longer be able to cut any further.

John Dalton (1766-1844) Felt that an atom was indivisible 3 Felt that an atom was indivisible spherical in shape Model: Sphere Analogy: Billiard ball

Dalton’s Postulates Atoms are tiny, indivisible particles. Atoms of the same element are identical. Atoms of different elements can chemically combine. Chemical reactions occur when atoms are separated, joined or rearranged. Atoms of one element can never be changed to atoms of another element.

Size of an atom Pure copper coin the size of a penny = 2.4X1022 atoms Earth’s population is about 6 X 109 people

JJ Thomson (1856-1940) Discovered electrons Felt that an atom was negatively charged particles floating in a positive soup Model: Charges floating around Analogy: Raisin bun or Plum Pudding

Ernest Rutherford (1871-1937) 7 Discovered the nucleus, the proton and first split an atom Model: An atom was a small positively charged nucleus surrounded by electrons orbiting around it. Analogy: A beehive

Rutherfords Famous Experiment 8 Called the Gold Foil Experiment Alpha particles were fired at thin gold foil A detector encircled the foil and lit up when hit with alpha particles. If the plum pudding model were true it was expected most particles would go straight through the foil with only slight deflection. However, in the experiment, some particles were deflected back at a sharp angle proving the existence of a small, dense, and positively charged nucleus.

Plum pudding expected results. Particles go through. Rutherfords results Plum pudding expected results. Particles go through. Text Actual results. Some are deflected back due to a nucleus

Another view of the experiment 10

Niels Bohr 11 Worked out details of atomic structure. Notably orbital layers. Solved problems related to Rutherfords model. Model: Electrons orbit in rings at different distances from the nucleus. Analogy: Planets orbiting the sun

Erwin Schroedinger (1887-1961) 12 Major discoveries in quantum mechanics. Nature of electrons in atoms. Model: Electrons exist in a probability distribution around the atom. Kind of like a cloud. Analogy: A spinning fan blade.

Quick Quiz: Historical Atom 13 1. What did Dalton say an atom was like? 2. What did JJ Thomson say an atom was like? 3. What did Rutherford say an atom was like? 4. What are the 3 parts of an atom? 1. A billiard ball, or a pool table ball 2. Raisin bun 3. A beehive 4. Proton, Neutron, Electron

Parts of the Atom 14 Electrons The Nucleus

Parts of the Atom Electrons Mass=0 Charge=-1 Protons Mass=1 Charge=+1 15 Electrons Mass=0 Charge=-1 Protons Mass=1 Charge=+1 Neutrons Mass=1 Charge=0 The Nucleus

Atomic Structure An atom is considered electrically neutral. Electrically neutral means the number of protons (+) = the number of electrons (-) 4 red protons = 4 blue electrons

Atoms on the Periodic Table 17 Atomic # = Number of protons 3 Be 7.0 neutrons + protons Atomic mass

You must know how to find: Nuclear Symbol You must know how to find: # of protons = atomic number mass # = # of n0 + # of p+ (atomic #) What’s in the nucleus of the atom # of electrons = # of protons (in a neutral atom) Boron 5 B 10.811 atomic number (Not the same as the mass #) atomic mass

16. What is the mass number? The mass of the entire atom! The sum of the protons and neutrons in an atom. Electrons are not included (too small)

17. What is the atomic mass? The weighted average of the masses of all naturally occurring isotopes.

How to write a Nuclear Symbol Charge if ion Element Symbol B Mass Number = p+ + n0 -3 11 5 Atomic Number = p+

To Determine Other Numbers 22 To find Do this Proton # same as atomic number Electron # same as atomic number for neutral atoms Neutron # =atomic mass-atomic number Mass =Prot # + Neut#

Cl Mg Nuclear Symbol Examples 35 17 27 +2 12 17 35 17 18 17 12 27 12 Number of Protons Number of Neutrons Number of Electrons Atomic Number Mass Number 17 35 17 18 17 Mg 27 +2 12 Number of Protons Number of Neutrons Number of Electrons Atomic Number Mass Number 12 27 12 15 10

Ca O Nuclear Symbol Examples 40 +2 20 17 -2 8 20 40 20 20 18 8 17 8 9 Number of Protons Number of Neutrons Number of Electrons Atomic Number Mass Number 20 40 20 20 18 O 17 -2 8 Number of Protons Number of Neutrons Number of Electrons Atomic Number Mass Number 8 17 8 9 10

Try This 25 Atomic # Element Symbol Atomic Mass Be Na Fe I

Fill in the blanks 26 Element Atomic # e- # Mass # Neut # 12 P 15 16 8

Foldable

Ions - an atom that has lost or gained an electron(s) The charge on an ion indicates an imbalance between protons and electrons. If the atom GAINS electrons it will have a negative charge. (more e- than p+) If the atom LOSES electrons it will have a positive charge. (more p+ than e-) Protons = Electrons = Protons = B 11 B Electrons = 11 +2 5 5

Isotopes Isotopes are atoms of the same element that have the same # p+, but different mass numbers. Isotopes have different masses because they have a different number of neutrons. Boron 5 B 10.811 atomic number (average) atomic mass

19. How are isotopes written? The element name with its mass number after it: Carbon-12 Carbon-13 Carbon-14

Learning Check Naturally occurring carbon consists of three isotopes, C-12, C-13, and C-14. State the number of protons, neutrons, and electrons in each of these carbon atoms. 12C 13C 14C 6 6 6 # p _______ _______ _______ #n _______ _______ _______ #e _______ _______ _______ #

The average atomic mass - weighted average of all the naturally occurring isotopes of an element. Example: A sample of cesium is 75% 133Cs, 20% 132Cs and 5% 134Cs. What is the average atomic mass? Answer: .75 x 133 = 99.75 .20 x 132 = 26.4 .05 x 134 = 6.7 132.85 = average atomic mass