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Chapter 4Atomic Theory
Matter
• All matter is made of atoms
o Alone as elements• Au, Na, O, He
o In combination of elements as compounds
• H2O, NaCl, LiO2
• Democritus (460-370 B.C.) proposed & believed thato Matter was not infinitely
divisibleo Made up of tiny particles
called atomos or “indivisible”
o Atoms could not be created, destroyed, or further divided
o He did not do any experiments that backed up his theory.
Early Theories of Matter
John Dalton, 19th Century
• School teacher
• Dalton revised Democritus's ideas based upon the results of scientific research he conducted
• Dalton’s atomic theoryo Not totally correct
Dalton’s First Atomic Theory:
o Elements are made up of small indivisible particles called atoms
o Atoms of the same element are identical, different elements are different- (Same size, mass and chemical properties)
o Atoms are not created or destroyed in a chemical reaction
o A compound always has the same relative numbers and kinds of atoms
What is an atom?• An atom is the smallest particle of an element
that retains the properties of the element
Atomic Models
• Thomson: Plum Pudding Model
• Based off experiments using cathode rays
Atomic Models• Based off of his Gold
foil experiments• Rutherford:
Electron Cloud Model
Rutherford: The Nuclear Atom
• His model consisted of the following ideas:
o an atom consists mostly of empty space through which electrons move
o electrons are held within the atom by their attraction to the positively charged nucleus
o small, dense, positive charged nucleus
Bohr Model
• Based on Rutherford’s model
• “Planetary” model
• Adds idea of “quantized” energy levels
Quantum Model• Schrodinger (1920’s) came up with the modern
atomic model: proposing that electrons travelled in wave-like patterns
Inside an atom: subatomic particles
• These particles have mass and charge
• Nucleus holds protons & neutronso Protons: + chargeo Neutrons: Ø charge
• Outside of nucleus is the electron cloudo Electron: - charge
Parts of the AtomName Symbol Charge Mass Location
proton p +1 1 amu nucleus
neutron n 0 1 amu nucleus
electron e- -1 ~ 0 amu outside nucleus
Atomic Mass Unit (amu)
• Small mass #’s are not easy to work with, so the atomic mass unit (amu) was developed
• The mass of 1 amu is nearly equal to the mass of one proton or neutron
Reading the Periodic Table
Atomic Number• The number of protons determine the
element (ALWAYS!!!)
• Number of protons are unique to each element
• Examples:o Carbon (C) has 6 protons
• Atomic number is 6
o Copper has 29 protons• Atomic number is 29
Atomic Number
• In uncharged atoms, atomic number is also the number of electronsoWhy? o If an atom is charged, then it is an ion
• Uncharged atom:
Atomic number = # of protons = # of electrons
Charge = # protons - # electrons
Mass Number
• To find the Mass number # protons + # neutrons = mass number
• To find # neutrons mass number – proton (or atomic number) =
neutrons
• Mass numbers are always WHOLE #’s!!
Symbols for Atoms
X= symbol of elementA= mass numberZ= number of protons
X or X
- Can also be written “element - A”- i.e. carbon - 12
AZ
A
Mg-25 Zn
atomic #
proton
neutron
electron
mass #
12
12
13
12
25
30
30
40
30
70
7030
Turn to your partner and summarize…
• How does the number of protons affect the identity of the atom?
• How do the number of protons and electrons relate to each other in a neutral atom?
• What is an ion?
Isotopes and Mass Number
C C carbon-12 carbon-13
Isotopes are atoms with the same number of protons but different number of neutrons
13
6126
Isotopes and Mass Number
• Example: 3 types of Potassium
All 3 types contain 19 protons and __ electrons
# of Protons # of Neutrons Mass Number
19 20 19 21 19 22
19
39
40
41
What’s the difference between mass
number and average atomic
mass(weight)?
C carbon-12
But if you look on the periodic table, the number states 12.01…
Atomic Mass (atomic weight) – a weighted average of the masses of all of the isotopes of that element. It is not the same as the mass number.
12 6
What’s the difference between mass
number and average atomic mass?
• Mass number- specifically about one isotope; the number of protons + neutrons
• Average atomic mass- includes the masses of all the different isotopes for that element
Mass of Individual Atoms
• Average atomic mass:o The weighted average mass of the isotopes of
an element
o Example: Chlorine• Mixture of 75% chlorine-35 and 25%
chlorine-37
Atomic mass = (0.75)*35.0 + (0.25)*37.0 = 35.5 amu
Try this one• 3 isotopes of neon:
o Ne-20 (90.92%)o Ne-21 (0.25%)o Ne-22 (8.83%)
What is the average atomic mass of Ne?
(20)*(0.9092) + (21)*(0.0025) + (22)*(0.0883) =
20.18 amu
Radioactivity – when the nucleus of an atom is unstable causing it to decompose into another nucleus
There are three types of radioactive decay:
1.Alpha Decay
2.Beta Decay
3.Gamma Decay
Shielding: Alphas, Betas, Gammas and Neutrons
Alpha Decay• An alpha particle (α ) is produced • An alpha particle is just a helium nucleus,
He+242
Beta Decay• An e- is kicked out of the nucleus (a neutron
breaks up into a proton and e-), e 0-1
Gamma Decay• Only ENERGY is released from the nucleus• the nucleus itself does not change, but almost
always accompanies alpha and beta decay
•Alpha Decay230
90Th 42He + 226
88Ra
22288Ra
•Beta Decay234
90Th 0-1e + 234
91Pa
13153I
•Gamma Decay238
92U 42He + 234
90Th + energy
42He + 218
86Rn
0-1e + 131
54Xe
What are the products of Po-218 after it undergoes alpha decay followed by a beta decay, followed by beta decay followed by alpha decay?
21884Po 4
2He + 21482Pb
21482Pb 0
-1e + 21483Bi
21483Bi 0
-1e + 21484Po
21484Po 4
2He + 21082Pb
Where Does the Radiation Come From?• The radiation you receive can be either Natural or Man-
made
Turn to your partner and summarize…
• List the 3 types of radioactive decay and the particle that is released in each
• Which type of radioactive decay is most penetrating?
Half-Life (t1/2) - time required for one half of the original sample of nuclei to decay.
•The half-life of Ra-223 is 12 days. If you start with 100.0 grams of Ra-223, how much will be left after 36 days?
100.0 g 50.00 g 25.00 g 12.50 g
•The half life of Ra-225 is 15 minutes. If you have 10.0 grams now, how much did you start with 60 minutes ago?
•10.0 g 20.0 g 40.0 g 80.0 g 160.g
Fission – splitting a nucleus into two or more smaller nuclei
1 n + U Kr + Ba + 3 n + energy
• This is what takes place in a nuclear reactor or an atomic bomb.
235 92
9236
141 56
F ission• chain reaction - self-propagating reaction
Fusion – combining of two nuclei to form one nucleus of larger mass
H + H He + 1n + energy
• This is how all of the elements were created in nature and occurs naturally in stars.
21
31
42