Download Atoms, Molecules and Ions

Atoms, Molecules and Ions
Chapter 2
Foundations of Atomic Theory
• Law of conservation of mass: Antoine Lavoisier
– Mass is neither created nor destroyed. The total mass of
a compound must be the same as the total mass of
individual elements.
HgO  Hg + O
433.2 g
401.2g +
32g
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small
particles called atoms. All atoms of a given
element are identical, having the same size,
mass and chemical properties. The atoms of
one element are different from the atoms of all
other elements.
2. Compounds are composed of atoms of more
than one element. The relative number of atoms
of each element in a given compound is always
the same.
3. Chemical reactions only involve the rearrangement
of atoms. Atoms are not created or destroyed in
chemical reactions.
JJ Thomson’s Cathode Ray Tube
Negatively Charged
Electrode
Positively Charged
Electrode
Cathode Ray Tube
• Scientists studied the flow of electric current in a
glass vacuum tube with electrodes at each end.
• The electrode is named by what type of particle it
attracts
– Cathode: Negative (-)
– Anode: Positive (+)
• When connected to electric current the remaining gas
glowed forming a BEAM OF LIGHT.
• The beam always originated at the NEGATIVE
electrode and toward the POSITIVE electrode.
• 1897 JJ Thomson used magnets to deflect
the beam proving that particles had a
NEGATIVE CHARGE.
JJ THOMSON DISCOVERED
A NEGATIVE PARTICLE
CALLED THE
ELECTRON!
• Now with the knowledge of electrons,
and knowing the atom is neutral, there
must be a particle that is positive to
balance the negative charge.
• A small paddle wheel was placed inside
and it rolled toward the anode, providing
evidence that some PARTICLE MUST
BE STRIKING THE WHEEL to make it
move.
CRT Video
Plum Pudding Model
Plum Pudding
OR
Chocolate Chip
Cookie
Robert A. Millikan
• Performed the Oil Drop Experiment
• Determined the exact charge of an
electron
Measured Charge of e(1923 Nobel Prize in Physics)
e- charge = -1.60 x 10-19 C
Thomson’s charge/mass of e- = -1.76 x 108 C/g
e- mass = 9.10 x 10-28 g
Oil Drop Experiment Video
Radioactivity
• Becquerel
– discovered RADIATION
• uranium would expose photographic plates in the dark
– The properties of an element changed as it gave off
radiation
• Curie
– Discovered radium and polonium
The radioactive emissions of alpha, beta and
gamma rays were identified.
(Uranium compound)
Ernest Rutherford’s Gold Foil
Experiment
• Set up Gold Foil with a detection sheet
around it.
• Set up radioactive source emitting alpha
particles.
• ALPHA PARTICLES shot at gold foil.
Rutherford’s Gold Foil Experiment
Rutherford: It’s like shooting a cannon at a piece of tissue
paper and having it bounce back at you!
MOST particles went through the gold foil
SOME particles BOUNCED back
Gold Foil Conclusions
1. The atom is made up of mostly
EMPTY SPACE
2. The center of the atom contains a
POSITIVE CHARGE
3. Rutherford called this positive
bundle of matter the NUCLEUS
Gold Foil Experiment Video
Rutherford’s Model of
the Atom
Goldstein and Wien - 1886
• Cathode Ray Tube with perforated cathode
• Discovered collection of positively charged
particles
**DISCOVERED THE PROTON!!
Chadwick’s Experiment - 1932
• Found that alpha particles shot at
beryllium made a beam form
• The beam had the same mass of a proton
but was electrically neutral
**DISCOVERED THE NEUTRON!!
Niels Bohr – 1913
• Developed a new diagram of the atom
• Electrons can only be at certain energies
• Electrons must gain a specific amount of
energy to move to a higher level, called a
quantum
**DISCOVERED ENERGY LEVELS!!
Bohr’s Model of the Atom
SUBATOMIC PARTICLES
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Mass Number
AX
Z
Atomic Number
Element Symbol
Isotopes: atoms of the same element (X) with
different numbers of neutrons in their nuclei
1
1H
235
92
2
1H
U
(D)
238
92
3
1H
U
(T)
Do You Understand Isotopes?
How many protons, neutrons, and electrons are in 146
C?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in 116
6 protons, 5 (11 - 6) neutrons, 6 electrons
C?
Group
Period
Molecule: an aggregate of two or more atoms in a
definite arrangement held together by chemical bonds
H2
H2O
NH3
CH4
diatomic molecule: contains only two atoms
H2, N2, O2, Br2, HCl, CO
polyatomic molecule: contains more than two atoms
O3, H2O, NH3, CH4
An ion is an atom, or group of atoms, that has a net
positive or negative charge.
cation – ion with a positive charge
If a neutral atom loses one or more electrons
it becomes a cation.
Na
11 protons
11 electrons
Na+
11 protons
10 electrons
anion – ion with a negative charge
If a neutral atom gains one or more electrons
it becomes an anion.
Cl
17 protons
17 electrons
Cl-
17 protons
18 electrons
monatomic ion: contains only one atom
Na+, Cl-, Ca2+, O2-, Al3+, N3-
polyatomic ion: contains more than one atom
OH-, CN-, NH4+, NO3-
Do You Understand Ions?
+
27
3
How many protons and electrons are in 13 Al ?
13 protons, 14 neutrons, 10 (13 – 3) electrons
2- ?
Se
How many protons and electrons are in 78
34
34 protons, 44 neutrons, 36 (34 + 2) electrons
Relative Atomic Mass
• One atom is the standard – Carbon
• Mass of other elements are based off of the
standard
• Carbon: 6 p and 6 n = 12 amu
Atomic Mass Unit
• 1/12 mass of Carbon atom
• Periodic table lists weighted average atomic
masses of elements (like a GPA calculation)
Calculation AVERAGE Atomic Mass
75%
20%
??%
133Cs
132Cs
134Cs
Steps:
1. Percent to decimal
2. Multiply by mass
3. Add it up!