Lecture 4. Chapter 2. Structure of the Atom (Contd.)
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Transcript of Lecture 4. Chapter 2. Structure of the Atom (Contd.)
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Lecture 4. Chapter 2. Structure of the Atom (Contd.)
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Lecture 4.
Exercise 2.1. A US penny is 19 mm across. How many Ag atoms could span this if diameter Ag atom is 2.88 Å? Note 1 Ag atom per 2.88 Å.
Conversion factors: 1 = 10-3 m/1 mm and
1 = 10-10 m/ 1 Å.
Number = 19 mm x 10-3 m x 1 Ag atom x 1 Å
1 mm 2.88 Å 10-10 m
= 6.6 x 107 Ag atoms
which is sixty six million Ag atoms.
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Diameters of Atomic Nuclei.
These are on the order of 10-4 Å. So if we draw an atom on the board with a diameter of 1m, then the nucleus will be 0.1 mm across!
.
Diameter= 1 meter
diameter of nucleus = 0.1 mm
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Atomic numbers, mass numbers, and Isotopes. (p. 45).
The chemical properties of the elements are controlled by the numbers of electrons and protons. The number of protons (= no. of electrons) is the Atomic Number (Z) of the
element. Z = 1 6 8 13
H C O Al
protons
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Mass NumberThe Mass Number is the sum of the number of protons
plus neutrons. Atoms of a given element can have different numbers of neutrons. Thus, C (carbon) always has Z = 6, i.e. it has 6 protons, since C determines its chemistry, but can have different numbers of neutrons.
carbon-12 carbon-13 carbon-14
all have six protons (red)
six neutrons (blue)
eightneutrons
Nuclei ofCarbon atoms:
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Isotopes• Different isotopes of the same element
have different mass numbers. Thus, 12C and 14C both have 6 protons (Atomic Number 6), but 12C has 6 neutrons, while 14C has 8.
Mass number: 14Atomic number: 6C
‘carbon-14’
Usually, only the mass number is indicated, and referred to as ‘carbon-14’.
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Isotopes of Hydrogen (Z = 1)
Hydrogen Deuterium Tritium 99.9844% 0.0156% radioactive, unstable
one proton one proton, one proton in nucleus one neutron two neutrons
H H H1 2 31 1 1
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The 12C atom:
nucleus –contains 6protons (Z =6) and 6 neutrons
6 electrons
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2.4. Atomic weights.
These are expressed in amu. The mass of the 12C isotope has been defined as exactly 12 amu. 1 amu = 1.66054 x 10-24 g, or 1/6.022 x 1023 g. In these units, the 1H isotope has a mass of 1.0078 amu.
The mass of the12C isotope is exactly 12 amu.
Carbon-12
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Atomic weights of the elements:
Elements usually consist of mixtures of isotopes. The Atomic Weights for the elements are thus the average masses of these mixtures, expressed in amu. Thus, naturally occurring C consists of 98.93% 12C and 1.07% 13C. The Atomic Weight of the mixture is thus given by:
Atomic weight = 98.93 x 12 + 1.07 x 13.0335 = 100 100
12.01 amu.Divide by 100because ‘%’= ‘per one hundred’
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Average Atomic Mass = Atomic Weight
EXAMPLE: Si naturally occurs in three isotopes:
92.2% of 28Si (27.98 amu)4.7% of 29Si (28.98 amu)3.1% of 30Si (29.97 amu)
what is the atomic weight of Si?92.2100
4.7100
3.1100
0.922 x 27.98 amu
0.047 x 28.98 amu
0.031 x 29.97 amu
= 28.09 amu
addtogether
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2.5. The Periodic Table.
The Periodic Table is the most important tool that chemists use for organizing and remembering chemical facts. The chemical properties show a periodic pattern, hence ‘periodic table’. Mendeleev in 1869 came up with the arrangement we now essentially use, possibly from the arrangement of cards in ‘solitaire’.
Dmitri Ivanovich Mendeleev (1834-1907)
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Solitaire:
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Periodic Table
Elements are sorted according to increasing atomic number
C6
12.01
Atomic Number
Atomic weight
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Groups in the periodic table:
Group 1
Group 7Group 5
Group 3
Group 1B
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Groups:
The vertical columns in the periodic table are called GROUPS. Properties repeat down a group, such as Alkali metals (Group 1) are all soft silvery reactive metals that form basic hydroxides, while Noble gases (group 8) are all chemically unreactive gases, halogens (group 7) all exist as X2 molecules, and form salts where they are the anion X-.
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Group 1B, the coinage metals:
Copper penny American silver South African Eagle Gold Krugerrand
Cu Ag Au
They are all relatively inert, corrosion-resistant metals which have been used for minting coins. They are all excellent conductors of electricity. They all can form chlorides of thecomposition MCl (M = metal), such as CuCl, AgCl, and AuCl.
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Rows or Periods in the Periodic Table:(these run across the periodic table)
First period contains only H and He
third period
5th
period
NumbersDown sideIndicateperiod
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Periodic Table
Most elements are metals
Alkali Metals
Alkaline Earth Metals Noble Gases
Halogens
I AII A
III A IV A VA VI A VIIA
VIII A
Transition metals
HW: 37
Non-metals
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Molecules and Molecular Compounds
Some elements exist as diatomic molecules at room temp.
H2 N
2 O
2 F
2 Cl
2 Br
2 I
2
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Periodic table helps predict formulas of compounds:
e.g. fluorides of metallic elements:Group 1HF, LiF, NaF, KF, RbF, CsF
Group 2. BeF2, MgF2, CaF2, SrF2, BaF2
Group 3B. ScF3,YF3, LaF3, AcF3.
Group 3A. BF3,AlF3, GaF3, InF3
Group 2B. ZnF2, CdF2, HgF2
Metallic elements
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Periodic table helps predict formulas of compounds:
e.g. Hydrides (compounds with hydrogen) of
non-metallic elements: Group 7. HF, HCl, HBr, HI.Group 6. H2O,
H2S, H2Se, H2TeGroup 5. NH3, PH3, AsH3, SbH3
Group 4. CH4, SiH4, GeH4
inert
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Atoms of the first four elements in the periodic table:
Hydrogen Helium Lithium Beryllium
protons: 1 2 3 4Neutrons: 0 2 4 5Electrons: 1 2 3 4
1H 2He 3Li 4Be1 4 7 9
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Atoms and ions:
An atom can lose one or more electrons to become a positively charged cation, or gain electrons to become a negatively charged anion:
Ions are formed when a neutral atom
- gains electrons or- loses electrons
Li Li+
e-
Cation
BrBr
-Anion
e-
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This has an effect when we count electrons in elements and Ions
This has an effect when we count electrons in elements and Ions
number of electronsnumber of protons
(Z) number of neutrons
80Br-
40Ca2+
32S 16 32 – 16 = 1616
35 + 1 = 36
20 - 2 = 18
35
20
80 – 35 = 45
40 – 20 = 20
‘-’ charge results in gain of one electron
‘2+’ charge results in loss of two electrons
one extra compared to Ztwo less compared to Z
neutral atom, same as Z
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Molecules and Molecular Compounds
The periodic table helps predict how elements combine
Empirical Formulas Molecular FormulasOnly gives relative number of atoms in compound
Subscripts are smallestwhole-number ratios
Show actual number and types of atoms in a molecule
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Hydrogen peroxide
Empirical Formulas Molecular Formulas
H2O
2HO
ButaneC
4H
10C2H
5
Propane
C3H
8C
3H
8
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Empirical and molecular formulae of glucose
glucose
oxygen (red)
hydrogen (white)
carbon (cyan)
EmpiricalFormula: CH2O
MolecularFormula:
C6H12O6
(= empiricalformula x 6)
If you count theatoms in this modelof glucose you willsee 6 carbons, 12hydrogens, and 6oxygens
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Different representations of methane (CH4)
Perspectivedrawing
Space-filling model
Structural formula
Ball-and-Stick model
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Ions and Ionic Compounds
Ions are formed when a neutral atom - gains electrons or
- loses electrons
Li Li+
e-
Cation
BrBr
-Anion
e-
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Metals tend to form Cations
Which Elements form Anions, which Cations?
Nonmetals tend to form Anions
I A II A
III A IV A VA VI A VIIA
VIII A
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Cations and Anions can combine to form Ionic Compounds:
(Ionic Crystal – not a discrete molecule)
Sodium chloride
Sodium cation, Na+
Chloride anion, Cl-
Sodium chloride crystal
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Empirical Formulas for Ionic Compounds:
(A) determine charge on ions formed
(B) add ions so that compound is neutral overall
Na, O =>
2 Na+
2+ 2-
0
Na2O
Al, O =>
O2-Al3+
O2-
6+ 6-
0
Al2O
3
Na+ Al3+O2- O2-
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Mg2+MgCl
2
Al O3+ 2-
Al2O
3
O 2-
Ca2+
Ca2O
2
CaOsmallest possible ratio!
Cl-1
A simple approach: simply swap the charge onone ion to be the subscript on the other
charges
subscripts
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2.8 Naming Inorganic Compounds.
• There are now more than 19 million known chemical substances. We cannot use common names for all of them, e.g. ‘rock salt’ or ‘sugar’. We need a system of naming them that indicates unambiguously what they are.
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Organic and Inorganic substances:
• Organic substances are compounds of carbon and hydrogen (plus other elements such as N or S) associated with living things. Inorganic substances are substances not associated mainly with living things.
CH2CH2OH AlCl3
Ethanol aluminum chloride
Organic Inorganic
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Naming Ionic Compounds:
These consist of metal cations (e.g. Na+) combined with non-metal anions (e.g. Cl-).
1. Positive cations:
a) Cations formed from the metal are given the same name as the metal: e.g. Sodium, Aluminum, Zinc.
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b). If a metal can form cations of different charge, this is indicated with Roman numerals, e.g.
Fe2+ = iron (II) Fe3+ = iron(III)
Cu+ = copper(I) Cu2+ = copper(II).
An older method still widely used uses Latin names where the suffix –OUS indicates the lower charge, and –IC indicates the higher charge:
Fe(II) = ferrous Fe(III) = ferric
Cu(I) = cuprous Cu(II) = cupric
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2. Negative ions (Anions).
a) The names of monoatomic anions are formed by replacing the ending of the name of the element with ide.
H- = hydride ion Cl- = chloride O2- = oxide N3- = nitride S2- = sulfide
A few simple polyatomic anions also get the suffix ide:
OH- hydroxide CN- cyanide O22-
peroxide
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Oxo-anions (sulfate, sulfite, nitrate, nitrite)
b) Polyatomic anions containing oxygen end in ATE (more oxygens) or –ITE (fewer oxygens).
more oxygens
• NO3- nitrate SO4
2-
sulfate
• NO2- nitrite SO3
2- sulfite
fewer oxygens
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The oxo-anions of chlorine:
Prefixes are used when there are more than two different oxo-anions of the same element:
Cl- chloride ClO- hypochlorite fewer oxygens = ite ClO2
- chlorite ClO3
- chlorate more oxygens = ate ClO4
- perchlorate
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Carbonate and hydrogen carbonate:
• c) Anions derived from other anions by addition of hydrogen:
CO32- carbonate (+ H+) →
HCO3- hydrogencarbonate
O
CO--O
O
CO-HO
carbonate hydrogen carbonate
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3. Ionic compounds.
These are named with the cation name first, followed by the name of the anion:
NaCl = sodium chloride
Na2CO3 = sodium carbonate
Al(NO3)3 = aluminum nitrate
Cu(ClO4)3 = copper(II) perchlorate or
cupric perchlorate
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Names and Formulas of Binary Molecular Compounds.
Binary compounds are two-element compounds, e.g. carbon dioxide. Rules for naming these are as follows:
1. The name of the element further to the left in the periodic table is usually written first. Thus oxygen is always written last except when combined with fluorine. OF2 = oxygen difluoride, but Cl2O is dichlorine monoxide.
2. If elements are in same group, heavier element is written first, e.g. SO2 = sulfur dioxide.
3. the name of the second element is given an –IDE ending.
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Numerical prefixes:
4. Greek prefixes are used to indicate the number of atoms of each element in the formula:
SF6 = sulfur hexafluoride
NF3 = nitrogen trifluoride
N2O4 = dinitrogen tetroxide.
P4S10 = tetraphosphorus decasulfide.
(A table of Greek prefixes is given on next slide)
Note: here the ‘a’ in‘tetra’ is omitted
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Table of Greek prefixes: (bottom left p. 66) mono- = 1 di- = 2 tri- = 3 tetra- = 4 penta- = 5 hexa- = 6 hepta- = 7 octa- = 8 nona- = 9 deca- = 10 Note that if the name of the second element starts
with a vowel, e.g. oxide, then the -a is omitted for tetra-, penta-, and hepta-, so we have disulfur
tetroxide, phosphorus pentoxide, diiodine heptoxide.