Download Lecture 4. Structure of the Atom (Contd.)

Lecture 4. Chapter 2. Structure
of the Atom (Contd.)
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.
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 of
nucleus = 0.1 mm
.
Diameter
= 1 meter
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
Mass Number
The 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.
Nuclei of
Carbon atoms:
six
neutrons
(blue)
all have six protons (red)
carbon-12
carbon-13
carbon-14
eight
neutrons
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:
Atomic number:
14
6
C
‘carbon-14’
Usually, only the mass number is indicated,
and referred to as ‘carbon-14’.
Isotopes of Hydrogen (Z = 1)
one proton
in nucleus
Hydrogen
99.9844%
1
1
H
one proton,
one proton
one neutron two neutrons
Deuterium
Tritium
0.0156%
radioactive, unstable
2
1
H
3
1
H
The 12C atom:
nucleus –
contains 6
protons (Z =
6) and 6
neutrons
6 electrons
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 the
12C isotope is
exactly 12 amu.
Carbon-12
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 100
because ‘%’
= ‘per one hundred’
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?
92.2
100
4.7
100
3.1
100
0.922 x 27.98 amu
0.047 x 28.98 amu
0.031 x 29.97 amu
= 28.09 amu
add
together
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)
Solitaire:
Periodic Table
Elements are sorted according
to increasing atomic number
6
C
12.01
Atomic weight
Atomic Number
Groups in the periodic table:
Group 5
Group 1
Group 3
Group 1B
Group 7
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-.
Group 1B, the coinage metals:
Cu
Copper penny
Ag
Au
American silver
South African
Eagle
Gold Krugerrand
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 the
composition MCl (M = metal), such as CuCl, AgCl, and AuCl.
Numbers
Down side
Indicate
period
Rows or Periods in the Periodic Table:
(these run across the periodic table)
First period contains only H and He
third period
5th
period
HW: 37
Periodic Table
Most elements are metals
Alkaline Earth Metals
IA
VIII A
II A
Noble
Gases
III A IV A VA VI A VIIA
Transition metals
Nonmetals
Alkali Metals
Halogens
Molecules and Molecular Compounds
Some elements exist as diatomic molecules
at room temp.
H2
N2
O2
F2
Cl2
Br2
I2
Periodic table helps predict formulas of
compounds:
e.g. fluorides of metallic elements:
Group 1
HF, 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
Periodic table helps predict formulas of
compounds:
e.g. Hydrides (compounds with hydrogen) of
non-metallic elements:
Group 7.
Group 6. H2O,
H2S, H2Se, H2Te
Group 5. NH3,
PH3, AsH3, SbH3
Group 4. CH4,
SiH4, GeH4
HF, HCl, HBr, HI.
inert
Atoms of the first four elements in
the periodic table:
1
1H
Hydrogen
protons:
1
Neutrons: 0
Electrons: 1
4
2He
7
3Li
9
4Be
Helium
Lithium
Beryllium
2
2
2
3
4
3
4
5
4
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
e-
Li
Li
Cation
-
e-
Br
+
Br
Anion
This has an effect when we count
electrons in elements and Ions
neutral atom, same as Z
number of electrons
32S
80Br40Ca2+
one extra compared to Z
two less compared to Z
number of protons
(Z)
number of neutrons
16
32 – 16 = 16
35 + 1 = 36
35
80 – 35 = 45
20 - 2 = 18
20
40 – 20 = 20
16
‘-’ charge results in gain of one electron
‘2+’ charge results in loss of two electrons
Molecules and Molecular Compounds
The periodic table helps predict
how elements combine
Empirical Formulas
Only gives relative number
of atoms in compound
Subscripts are smallest
whole-number ratios
Molecular Formulas
Show actual number and
types of atoms in a molecule
Empirical Formulas
Molecular Formulas
HO
H2O2
Hydrogen peroxide
C2H5
C4H10
Butane
C3H8
C3H8
Propane
Empirical and molecular formulae of glucose
hydrogen
(white)
oxygen
(red)
CH2O
carbon
(cyan)
If you count the
atoms in this model
of glucose you will
see 6 carbons, 12
hydrogens, and 6
oxygens
Empirical
Formula:
Molecular
Formula:
C6H12O6
glucose
(= empirical
formula x 6)
Different representations of methane
(CH4)
Structural formula
Ball-and-Stick
model
Perspective
drawing
Space-filling model
Ions and Ionic Compounds
Ions are formed when a
neutral atom
- gains electrons or
- loses electrons
e-
Li
Li
Cation
-
e-
Br
+
Br
Anion
Which Elements form Anions, which Cations?
Metals tend to form Cations
Nonmetals tend to form Anions
VIII A
I A II A
III A IV A VA VI A VIIA
Cations and Anions can combine to
form Ionic Compounds:
Sodium
cation, Na+
Chloride
anion, Cl-
Sodium chloride
crystal
Sodium chloride
(Ionic Crystal – not a discrete molecule)
Empirical Formulas for Ionic Compounds:
(A) determine charge on ions formed
(B) add ions so that compound is neutral overall
Na, O => Na+
O2-
+
2 Na
2+
2-
0
Al, O =>
Al3+ O2Al3+ O2O26+
60
Na2O
Al2O3
A simple approach: simply swap the charge on
one ion to be the subscript on the other
charges
Al
3+
2-
Al2O3
O
subscripts
Mg
2+
2+
Ca
1
Cl
2-
O
-
MgCl2
Ca2O2
CaO
smallest possible
ratio!
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.
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
Ethanol
Organic
AlCl3
aluminum chloride
Inorganic
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.
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
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:
OHhydroxide CN- cyanide
O22- peroxide
Oxo-anions (sulfate, sulfite,
nitrate, nitrite)
b) Polyatomic anions containing
oxygen end in ATE (more oxygens) or –
ITE (fewer oxygens).
more oxygens
• NO3nitrate
SO42sulfate
• NO2nitrite
SO32sulfite
fewer oxygens
The oxo-anions of chlorine:
Prefixes are used when there are more than
two different oxo-anions of the same
element:
Clchloride
ClOhypochlorite
fewer oxygens = ite
ClO2chlorite
ClO3chlorate
more oxygens = ate
ClO4perchlorate
Carbonate and hydrogen
carbonate:
• c) Anions derived from other anions by
addition of hydrogen:
CO32- carbonate (+ H+) →
HCO3- hydrogencarbonate
O
-
O
C
carbonate
O
-
O HO
C
-
O
hydrogen carbonate
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
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.
Numerical prefixes:
4. Greek prefixes are used to indicate the
number of atoms of each element in the
formula:
SF6 = sulfur hexafluoride
Note: here the ‘a’ in
NF3 = nitrogen trifluoride ‘tetra’ is omitted
N2O4 = dinitrogen tetroxide.
P4S10 = tetraphosphorus decasulfide.
(A table of Greek prefixes is given on next slide)
Table of Greek prefixes: (bottom left p. 66)
mono=
1
di=
2
tritetrapentahexaheptaoctanonadeca-
=
=
=
=
=
=
=
=
3
4
5
6
7
8
9
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.