Download `atoms`. - MrsCoxsChemistryCorner

Chapter 4
History and Atomic Structure
IDEAS:
• He believed that matter was not divisible
• He believed that matter was made up of tiny particles –
atomos.
• Properties of matter were due to the size and shape of
the atoms.
• Changes in matter were due to the grouping of atoms
(not from changes within the atom)
PROBLEMS:
• He could not experiment, therefore he could not validate his
ideas.
• (For example, he could not explain what held the atoms
together)
IDEAS
• Questioned the idea that atoms moved through empty
space.
• He rejected the idea of atoms.
PROBLEMS
• Many people respected his ideas, therefore the atomic
theory that was proposed 100 years earlier was rejected for
the next 2000 years.
Obj. 1…Dalton’s Atomic Theory
• Four postulates (1808)...
*
*
proton, neutron and electron
1. All elements are composed of tiny, indivisible particles called ‘atoms’.
isotopes exist
2. Atoms of the same element are identical... each element is unique
3. Atoms of different elements can physically mix or
chemically combine (compounds).
4. Chemical rxns. occur when atoms are
separated, joined, or rearranged.
• Much of this theory is still accepted,
with 2 exceptions
Obj. 2…Atomic Scientists
• J. J. Thomson discovered electrons (e-) in 1897.
• He passed an electric current through a glass
tube filled with gas.
• He discovered that a beam of negative charges
traveled from the cathode (+) to the anode (-).
Obj. 2 cont...
• E. Goldstein discovered positive particles in atoms
• He observed rays traveling in the opposite direction of
cathode rays.
• These particles were termed ‘ protons’ (p+) by Ernest
Rutherford in 1920.
• James Chaddwick discovered neutrons (n0)
in 1932.
• His discovery was based on the fact that
different atoms’ atomic mass and atomic #
(# of protons) were not adding up.
in 1886.
Obj. 2 cont...
• Robert Millikan furthered Thomson’s work by describing
the electron more in depth.
• all e- carry exactly one unit of negative charge.
• mass of an e- is 1/1840 the mass of a hydrogen atom (p+).
Obj. 3 and 4…The Bohr-Rutherford Model
• Ernest Rutherford conducted the famous ‘gold foil
experiment’ (1911) which concluded that:
1. atoms are mostly space. (football arena)
2. atoms have a solid nucleus at the
center which contains most of the mass.
• This overturned the accepted ‘plum pudding model’ of the
time.
Obj. 3 and 4 cont...
• The Gold Foil Experiment...
(BBs and cream cheese!)
• Stats...
• 98% of particles went straight through (expected)
• 2% of particles deflected at large angles
• 0.01% of particles deflected straight back (canon balls & tissue!)
Obj. 3 and 4 cont...
• After Rutherford’s findings, Niels Bohr further explained
the atom by concentrating on the e- (1913).
• Why don’t the (-) e- fall into the (+) nucleus?
• Bohr proposed that the e- travel on concentric
orbits around the nucleus.
• Each orbit has a fixed energy (energy level) and e- do not
lose energy.
• The progression of the atom...
Plum Pudding
Rutherford
Rutherford-Bohr
Quantum
Obj. 5…Subatomic Particles
PROPERTIES OF SUBATOMIC PARTICLES
SYMBOL
ELECTRICAL CHARGE
RELATIVE
MASS
ACTUAL MASS
(g)
Electron
e-
-1
1/1840
9.11 x 10-28
Proton
p+
+1
1
1.67 x 10-24
Neutron
n
0
1
1.67 x 10-24
PARTICLE
0
Obj. 6…Characteristics of Elements
19
K
atomic #
element symbol
Potassium
element name
39.098
atomic mass
Obj. 6 cont...
• Atomic #:
 # of p+ in an atom
 identifies element
(change atomic # = change of element).
 # p+ = # e- in neutral atom
** (+) charge = less e- than p+
** (-) charge = more e- than p+
• Atomic mass:
(a.k.a. mass #)
 mass of the nucleus
 p+ + n0
 units are a.m.u.
(atomic mass unit)
Obj. 7…Atomic Symbols
• There are two ways to represent elements:
• Symbol Form:
atomic mass
atomic #
(# of p+)
#
#
X
element symbol
OR
atomic mass
#
• Shorthand Form:
name of element followed by atomic mass.
• Ex...
Aluminum - 27
Nitrogen - 14
Carbon - 14
• You can find mass #, atomic #, # of n0, and # of e- with
either notation!
X
element symbol
Obj. 8…Atomic Calculations
• All mass of the atom is in the nucleus.
- only p+ and n0 are in the nucleus.
- if you know the mass of any atom, you can find the # of n0.
- if you know the # of n0 and the # of p+, you can find the
mass.
• Ex... electrical charge on atom
31
15
+1
P
Manganese - 55
element = _______________ Phosphorus
atomic mass = ___________
atomic # = ______________
# of p+ = _______________
# of n0 = _______________
# of e- = _______________
31 a.m.u.s
15
15
16
14
symbol = ______________
atomic mass = __________
atomic # = ____________
# of p+ = ______________
# of n0 = ______________
# of e- = ______________
Mn
55 a.m.u.s
25
25
30
25
Obj. 9…Isotopes
• Isotopes: atoms of the same element with different atomic
masses.
- different # of n0 !!!
• Three isotopes of Carbon:
element of life
(6p+, 6n0)
Carbon - 12
extremely rare
(6p+, 7n0)
Carbon - 13
radioactive…carbon dating
(6p+, 8n0)
Carbon - 14
• Note: atomic # will NEVER change in isotopes…
only mass and # of n0 do!
• Which of the following are isotopes of the same element?
22
12
+3
X
22
10
X
25
10
-1
X
Neon - 20
Fluorine - 20
Neon - 22
Obj. 10…Average Mass of Isotopes
• Isotopes are naturally occurring.
• The mass # of an element (periodic table) is the weighted
avg. of all isotopes that exist in nature.
- abundance of isotope is just as important as mass!
• Ex...
Natural copper (Cu) consists of 2 isotopes ...
Copper - 63 (mass = 62 .930 g/mole)
69%
Copper - 65 (mass = 64 .930 g/mole)
31%
• To calculate avg. mass...
mass x abundance for each isotope
Step 1 :
add the two values from step 1 together
Step 2 :
43.42
62 .93 x .69 =
43.42
+ 20.13
64 .93 x .31 =
20.13
63.55 g/mole
Obj. 10 cont...
• The average mass of an element is closest to the isotope
that is most plentiful in nature.
• Ex...
Three isotopes of Oxygen:
Oxygen - 16
Oxygen - 17
Oxygen - 18
99 . 759%
0.037%
0.204%
• The avg. mass (from P.T.) is closest to 16, therefore,
Oxygen-16 is the isotope that is most abundant in nature.