Atomic Structure
Chapter 4
Dalton’s Atomic Theory 4.1
John Dalton – 18th century• All matter is composed of atoms• All atoms of a given element are identical• Atoms cannot be created, divided or
destroyed• Different atoms combine to form
compounds• In chemical reactions atoms are
rearranged.
Subatomic Particles 4-2
Discovering the Electron-
• Thomson identified the electron
through use of a cathode ray tube.
• Electrons carry a negative charge
• Electrons have an extremely small mass
• Thomson created the plum pudding model (chocolate chip cookie dough model)
Electrons --- charge = -1 e-
Plum Pudding Model
Negatively charged electrons were distributed throughout a uniform positive charge
The Nuclear Atom 4.2
Rutherford’s Gold Foil Experiment
• Discovered the nucleus
• Atoms consist of mostly empty space through which the electrons move
• There is a tiny dense region (nucleus) which contains all of the atom’s positive charge and almost all its mass
Gold Foil Experiment
Plum Pudding Model
Nuclear Model
Because some of the alpha particles bounced back – they must have hit something with a positive charge
What’s in the nucleus
Protons – subatomic particle with a charge equal and opposite to an electron
Proton --- charge = +1 p+
Neutrons – subatomic particle with a mass almost equal to that of a proton but it has no charge – neutral
Neutron --- charge = 0 n0
How atoms differ 4.3
Atomic Number:
• The number of protons in an atom is the elements atomic number
• The number of protons in an atom identifies it as an atom of a particular element
• Atoms are neutral so # protons = # electrons
Example: Helium - He
Atomic # = 2 so it has 2 p+ and 2 e-
How atoms differ 4.3
Isotopes and Mass Number
• Not all atoms of a particular element are identical. They all have the same atomic number (protons), but they can have a different number of neutrons
• Isotopes - same # of protons
different # of neutrons
How atoms differ 4.3
Example:
• 3 types of potassium - all have 19 protons (that’s what makes them K)
- 20 neutrons; 21 neutrons; 22 neutrons
• Differ in mass, but they have the same chemical properties
Mass # = protons + neutrons
How atoms differ 4.3
Example cont.
3 isotopes of Potassium
(same protons, different neutrons)
Potassium – 39 (19 + 20)
Potassium – 40 (19 + 21)
Potassium – 41 (19 + 22)
Mass of Individual Atoms 4.3
Atomic Mass Units (AMU) Table 4-2
1 amu is almost equal to the mass of one
proton or one neutron
The atomic mass of an element is the average mass of the isotopes of that element
Mass of Individual Atoms 4.3
Example: Chlorine - two isotopes
Chlorine – 35 and Chlorine – 37How many protons and neutrons does each have?
Isotopes Mass % abundance
Chlorine - 35 35.00 amu 75.0%
Chlorine - 37 37.00 amu 25.0%
Atomic Mass = 35.00 x 0.75 = 26.25
37.00 x 0.25 = 9.25
26.25 + 9.25 = 35.5 amu
Mass of Individual Atoms 4.3
Practice Problem:
Isotopes Mass % abundance
Boron – 10 10.013 amu 19.8%
Boron – 11 11.009 amu 80.2%
10.013 x 0.198 = 1.982
11.009 x 0.802 = 8.829
1.982 + 8.829 = 10.811 amu
Mass of Individual Atoms 4.3
Isotopes Mass % abundance
X – 6 6.015 amu 7.50 %
X - 7 7.016 amu 92.5 %
You Try:
6.015 x 0.0750 = 0.451 0.451
7.016 x 0.925 = 6.489 + 6.489 = 6.940 amu
What element has a average atomic mass of 6.94 amu?
Lithium (Li)
Radioactive Decay 4.4
Nuclear Reactions – a rxn that involves a change in the atoms nucleus
• change in the # of protons
• atoms of one element turn into atoms of another element
Radioactive Decay 4.4
Radioactivity – the process by which some substances spontaneously emit radiation
• They do this because they are unstable – they have a unstable ratio of protons to neutrons in their nucleus.
• They lose energy and undergo radioactive decay to become stable, non-radioactive atoms
Radioactive Decay 4.43 types of radiation: Alpha, Beta, & Gamma
Alpha Radiation-
Alpha Particle - 2 p+ and 2 n0
- has a charge of 2+
- equivalent to a Helium -4 nucleus
- symbol = α or 42 He
Ex:
Radioactive Decay 4.4
Beta Radiation-
Beta Particle – fast moving electrons
- negative charge ( -1)
- symbol = 0-1β
Ex:
Radioactive Decay 4.4Gamma Radiation
Gamma Rays - 00γ
- high energy
- have no mass, no charge
- usually accompany alpha or beta
- don’t make a new atom by themselves
Ex:
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