Chapter 4 The structure of the atom AL-COS Objectives 1, 2,3,4,7, 10, 15, 20, 21, 22, 27and 28
You ll learn to Identify the experiments that led to the development of the nuclear model of atomic structure Describe the structure of the atom and the subatomic particles that comprise it and
You ll also learn Explain the relationship between nuclear stability and radioactivity Write equations representing nuclear decay Discuss the atomic models of Democritus, Thomson, Rutherford, and Bohr
Vocabulary *atom *atomic mass *atomic mass unit (amu) *atomic number *electron *isotope *mass number *neutron *nucleus *proton
Early Theories of Matter Democritus ( 460-370 BC)
A man ahead of his time First to believe matter was not infintely divisible Believed matter to be made of small particles he called atomos Believed these particles could not be created or destroyed
Democritus Ideas Matter is composed of empty space through which atoms move Atoms are solid, homogeneous, indestructible and indivisible Different kinds of atoms have different sizes and shapes (give matter its properties) Changes in matter are due to changes in grouping of atoms
Aristotle One of most influential Greek philosophers
Aristotle Believed matter was formed of air, water, earth and fire Did not believe in the nothingness of space All of his ideas were NOT based on empirical (experimental) science just ideas
Aristotle Given credit for setting the field of science back nearly 2000 years!
John Dalton (1766-1844) Revived the ideas of Democritus First atomic model A teacher in England
Dalton s atomic theory Matter is made of small particles called atoms All atoms of an element are identical **** (isotopes?) Atoms of different elements are different
As well as Atoms cannot be created or destroyed or divided into smaller particles ****(nuclear fission?) Different atoms combine in simple whole number ratios to form compounds In chemical reactions, atoms are separated, combined or rearranged.
The atom Smallest particle of an element that retains the properties of that element
Relative size of an atom
How many atoms? World population = 6 000 000 000 Number of atoms in a penny= 29 000 000 000 000 000 000 000
Subatomic particles and the nuclear atom The electron - Sir William Crookes noticed the flash of light in a tube Tube filled with a gas, + and a end,
The cathode ray Scientists were convinced this cathode ray was really a stream of charged particles -> electrons!
Further cathode tube experiments were conducted by J.J. Thomson (1856-1940) Was trying to determine the charge/mass ratio of these electrons Found the mass of this particles was less than hydrogen s
J.J. Thomson Identified the first subatomic particle the electron Atoms were divisible into smaller particles
J.J. Thomson s atomic model A spherical atom composed of evenly distributed positive charges within which the negatively charged electrons were also found
Robert Millikan (1868-1953) American physicist Determined the charge of an electron Calculated the mass of a single electron (e - )
Sir Ernest Rutherford Originally from New Zealand Won a scholarship competition to study at Cambridge Univ. in England Won the Nobel prize in 1908 for discovering radioactivity
3 types Alpha particles Radioactivity Actually the nucleus of a helium atom Beta particle A high speed electron Gamma particle A photon of electromagnetic radiation
Radiation can be shielded by..
Designed an experiment to study the atom Experiment was actually conducted by his two graduate students, Geiger (later developed the Geiger counter) and Marsden Rutherford was their supervising professor
The gold foil experiment..
Used alpha particles for bullets Alpha particles = Helium nucleus 2 protons + 2 neutrons
What happened in the experiment
Observed that.. Most of the alpha particles went straight through Some were deflected away from the foil Some hit something solid and bounced back
What was learned? Most of an atom is empty space The nucleus has a positive charge (He nucleus was also positive, which caused the deflection) The nucleus, when hit straight on, is dense and solid not a plum pudding!
Other subatomic particles Proton Discovered by Goldstein using a cathode ray tube (1920 s) Neutron Discovered by James Chadwick in 1932
Atomic number Henry Moseley discovered that atoms of different elements each have a unique positive charge in their nucleus
Henry Moseley (1887-1915) Discovered atomic numbers Died at a young age in WWI
The atomic number Mass number= Protons + neutrons 12 C Symbol for element 6 Atomic number = number of protons in the nucleus
Practice writing atomic notation 23 Na What does this tell us? 11
Write atomic notation For an atom of Fe For an atom of U And
56 238 Fe U 26 92
Isotopes Same atomic number but a different atomic mass The number of protons remains the same, but the number of neutrons vary.
Isotopes of hydrogen 1 2 3 H H H 1 1 1 Protium deuterium tritium
Calculating Atomic Mass (Turn to pg. 102) 1. mass x %abundance (for each isotope) 2. Add them together to get the atomic mass 3. find that atomic mass on the periodic table and that is the element VIOLA!
Example Problem: pg. 103 1.) 6 X =(6.015)(0.075)= 0.451 amu 7 X =(7.016)(0.925)= 6.490 amu 2.) 0.451 + 6.490 = 6.941 amu 3.) The element with an atomic mass of 6.941 amu is Lithium, Li. COOL!!!!!!!!!!!!
Look on pg. 104 Let s do # 15 and #16 together. Now, you do #17 on your own.
Pg. 104, #15 (10.013)(0.198) = 1.982 amu (11.009)(0.802) = 8.829 amu 10.811 amu
Pg. 104, #16 Helium-4 is more abundant in nature because the atomic mass of naturally occurring helium is closer to the mass of helium-4 than to the mass of helium 3.
Pg. 104, #17 (23.985)(0.7899) = 18.946 amu (24.986)(0.1000) = 2.499 amu (25.982)(0.1101) = 2.861 amu 24.306 amu The element with that atomic mass is magnesium, Mg.
Nuclear reactions The nucleus of the atoms actually change Nuclear stability is based on neutron/proton ratio Experience nuclear decay to become more stable
Alpha decay
Nuclear equation becomes 238 234 4 U Th + He 92 90 2
Beta decay
Nuclear equation 14 0 15 N e + O 7-1 8
Another example of Beta decay
Gamma radiation γ Electromagentic radiation Has no charge Often found along with Beta or alpha radiation
Gamma radiation
Fission of atoms
The end for now!