Post on 24-Dec-2015
Chapter 2
Atoms, molecules, and ions
1
The Atomic TheoryHistory
The Structure of the AtomProton, Neutron, Electron
Atomic Number, Mass NumberIsotopesThe Periodic Table
DAB/CAS/CSU/2010
Matter
2
Brownian Motion
3
Atomic theory
4
Atoms are incredibly small.What we know about them is based on
indirect evidence.
History of atomic theory
5
500 BC Democritus proposed the idea of atom but had no experimental data to supportBelieved that all matter consists of very
small, indivisible particles, which he named atomos (meaning uncuttable or indivisible).
1800 Dalton proposed an atomic theory supported by an experimental data
Dalton’s atomic theory: Support
6
Support for his theory came when he began looking at the masses of elements that combined with each other
He found that elements combine by simple ratio by mass
Dalton’s atomic theory:
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1. All matter is composed of tiny indivisible particles called atoms.
2. All atoms of the same element are identical (having same size, mass, and chemical properties). The atoms of one element are different from the atoms of all other elements.
3. Atoms of two or more elements can combine in small whole number ratios to form compounds.
4. A chemical reaction involves only the separation, combination, or rearrangement of atoms; it does not result in their creation or destruction.
Dalton’s atomic theory: Support
8
Law of Multiple Proportions• If two elements can combine to form more than
one compound, the masses of one element that combine with a fixed mass of the other element are in ratios of small whole numbers.
Example: CO & CO2
Law of Definite Proportion• Different samples of the same compound
always contain its constituent elements in the same proportion
Dalton’s atomic theory: Support
9
Law of Conservation of MassMatter can neither be created nor destroyedThe mass of atoms before and after reaction
is conserved
Dalton’s atomic theory: Support
10
Law of Conservation of Mass
Dalton’s atomic theory:
11
1. Atoms of the same element have different masses
2. Atoms have subatomic particles (protons, electrons, neutrons.)
Problems with Daltons Theory
So, Dalton’s theory is not completely correct!
The structure of the atom
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Subatomic Particles
13
Particle Symbol Charge RelativeMass
(amu)
Electron e- 1- 0
Proton p+ + 1
Neutron n 0 1
Subatomic Particles: Discovery
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Electron was discovered by JJ Thomson in ～ 1900 using his cathode ray tube.
Electron charge was found by American Robert Millikan ～ 1910 by an oil-drop experiment.
Proton was found by the adaptation of cathode ray tube ～ 1900.
Neutron was discovered by Chadwick in 1932.
The Electron Discovery
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1890’s discovery of radiation lead to discovery of electron
Radiation – the emission and transmission of energy through space in the form of waves
Used cathode ray tubeCathode – negatively charged plated, emits
an invisible rayAnode – positively charged tube
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Thomson’s plum pudding model of the atom
Subatomic Particles: Discovery
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Plum pudding model: atom has a weak positive charge.
Rutherford tested the idea by positively charged particles at gold foil.
He expected particles to pass through.His results surprised him.
Rutherford’s Gold Foil Experiment:
Movie on Gold Foil Experiment
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Subatomic Particles: Discovery
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Atoms is mostly empty space.Protons are found in the dense nucleus in
the center of the atom.
Rutherford’s Gold Foil Experiment: Results
Location of Subatomic Particles
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10-13 cm electrons
protons
neutrons
10-8 cm
nucleus
Rutherford’s Model:
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Problem:Could not explain why electrons would not
spiral into the nucleus.
Implications of Rutherford’s Exp’t.
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Nucleus – at the center of the atomDense central core within the atom
Contains positively charged particle (protons)
Protons carry the same charge as that of electron
Mass of protons is 1840 times that of electron
Neutrons – neutral particles within the nucleus having a mass slightly greater than that of protons
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Particle Mass (g) Coulumb Charge unit
Electron 9.10939 x 10 -
28
-1.6022 x 10-
19
-1
Proton 1.67262 x 10 -
24
+1.6022 x 10-
19
+1
Neutron 1.67493 x 10 -
24
0 0
24
ATOMS TO ELEMENTS
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The number of protons, electrons, and neutrons in them.
Example:Gold and Mercury
What makes elements different from one another?
Atomic Number, Z
26
Counts the number of
protonsin an atom
Atomic Number on the Periodic Table
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11Na
Atomic Number
Symbol
All atoms of an element have the same number of protons
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11Na
11 protons
Sodium
Learning Check
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State the number of protons for atoms of each of the following:A. Nitrogen 1) 5 protons 2) 7 protons 3) 14 protons
B. Sulfur 1) 32 protons 2) 16 protons 3) 6 protons
C. Barium1) 137 protons 2) 81 protons 3) 56 protons
Mass Number, A
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Counts the number of
protons and neutronsin an atom
Mass Number, A = p + n
n = A - p
Atomic Symbols
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Show the mass number and atomic
number
Give the symbol of the element
mass number
23 Na sodium-23
atomic number 11
Number of Electrons
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An atom is neutral The net charge is zero
Number of protons = Number of electrons
Atomic number = Number of electrons
Atomic Number =Number of protons
+ 2 + (-2) = 0
Subatomic Particles in Some Atoms
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16 31 65 O P Zn 8 15 30
8 p+
8 n8 e-
15 p+
16 n15 e-
30 p+
35 n30 e-
Subatomic Particles in Some Atoms
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16 31 65 O P Zn 8 15 30
8 p+
8 n8 e-
15 p+
16 n15 e-
30 p+
35 n30 e-
What happens when atoms losses electrons?
35
2311Na
When Na losses 1 electron?
11 p+
11 e-
12 n
2311Na+
11 p+
10 e-
12 n
11 p+
11 e-
+11
-10 1
What happens when atoms gains electrons?
36
3517Cl
When Cl gains 1 electron?
17 p+
17 e-
18 n
17 p+
18 e-
18 n
17 p+
17 e-
+17
- 18 - 1
3517Cl-
What happens when atoms gains electrons?
37
168O
When O gains 2 electron?
8 p+
8 e-
8 n
8 p+
10 e-
8 n
8 p+
8 e-
+ 8
- 10 - 2
168O
Isotopes
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Atoms with the same number of
protons, but different numbers of
neutrons. Atoms of the same element (same
atomic number) with different mass numbers
Isotopes of chlorine
35Cl 37Cl17 17
chlorine - 35 chlorine - 37
17 p+
18 n17 e-
17 p+
20 n17 e-
686
676
666
Learning Check
39
Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.
12C 13C 14C 6 6 6
#p _______ _______ _______
#n _______ _______ _______
#e _______ _______ _______
Learning Check
40
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom 1) 30 2) 35 3) 65
B. Number of neutrons in the zinc atom 1) 30 2) 35 3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons? 1) 37 2) 65 3) 67
Learning Check
41
Write the atomic symbols for atoms with the following:
A. 8 p+, 8 n, 8 e- ___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e- ___________
16O8
37Cl17
107Ag 47
Atomic Mass on the Periodic Table
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11Na
22.99
Atomic Number
Symbol
Atomic Mass
Atomic Mass
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Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom.
Example of an Average Atomic Mass
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Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine. 35 x 75.5 = 26.4 100
35.5 37 x 24.5 = 9.07
100