Download Adventures in Chemistry Julie T. Millard, Colby College

Learning Objectives:
a.
b.
c.
The model of atomic theory was 1st
proposed by John Dalton in 1803.
Atoms are composed of protons, neutrons,
and electrons.
All atoms of the same element contain the
same number of protons (and electrons) but
may vary in the number of neutrons
(isotopes).
d.
e.
f.
g.
Protons and neutrons are found inside the
tiny but dense nucleus, whereas electrons
are found in orbitals outside the nucleus.
The arrangement of electrons in the orbitals
is called the electronic configuration and
determines the chemistry of an atom.
The different types of atoms are called
elements, which are arranged
systematically in the periodic table.
Having eight valence electrons is
particularly desirable (“the octet rule”).
John Dalton (1766-1844)
The Atomic Theory
a.

Dalton’s Atomic Theory
1.
2.
3.
4.
All matter is composed of indivisible atoms.
All atoms of one element are identical to each
other but different than the atoms of other
elements.
Compounds are formed when atoms of different
elements combine in whole number ratios.
Atoms are rearranged during chemical reactions
but atoms cannot be created or destroyed.
Definitions
Law of conservation of matter states that matter is neither lost nor gained
during a chemical reaction.
Ex.
2H2 (g) + O2 (g)  2H2O(g)
2C(s) + O2 (g)  2CO(g)
Law of definite proportions states that in a compound, the constituent
elements are always present in a definite proportion by weight.
Ex.
Pure water, a compound, is always made up of 11.2% hydrogen and
88.8% oxygen by weight or table sugar always contains 42.1%
carbon, 6.5% hydrogen, and 51.4% oxygen by weight.
b.
Structure of the Atom
 Components
 Positive protons, negative electrons, and
neutral neutrons
 Atomic Number
 The number of protons in an atom, which
determines what element it is
 Mass Number
 Number of protons + the number of
neutrons
Definitions
Protons are positively charged subatomic particles found in the nucleus.
Electrons are negatively charged subatomic particles found in the space
around the nucleus.
Neutrons are electrically neutral subatomic particles found in the nucleus.
Nucleus is the small central core of the atom: contains the protons and
neutrons.
Separation of alpha, beta, and
gamma particles by applying
an electric field.
Ernest Rutherford Gold foil
experiment. A beam of
positively charged alpha
particles hits the gold foil.
Most particles passed
straight, some slightly
deflected and some deflected
back. The reason for
deflection were the positions
of the nucleus.
Ernest Rutherford (1871-1937)
Rutherford’s interpretation of the gold foil experiment done by
Geiger and Marsden.
Modern View of the Atom
For an atom, which always has no net
electrical charge, the number of negatively
charged electrons around the nucleus equals
the number of positively charged protons in
the nucleus.
Scientists have been able to
obtain computer-enhanced
images of the outer surface of
atoms using the scanning
tunneling microscope (STM)
and the atomic force
microscope (AFM).
c.
Structure of the Atom

Isotopes


Isotopes of the same element have the
same number of protons and electrons
but differ in the number of neutrons.
Atomic Mass

The atomic mass for each element on the
periodic table reflects the relative
abundance of each isotope in nature.
Definitions
Atomic number is the number of protons in the nuclei of the atoms of an element
Mass number or atomic mass is the number of neutrons plus number of
protons in the nucleus of an atom
Mass number = #’s of protons + #’s of neutrons
Ex. How many protons, neutrons, and electrons are in atom of gold (Au) with a
mass number of 197?
Protons = 79; Electrons = 79; Neutrons = 197 – 79 = 118;
Isotopes are atoms of the same element having different mass numbers.
Definitions cont
Atomic mass unit (amu) is the unit for relative atomic masses of the elements;
1 amu =1/12 the mass of carbon-12 isotope. 1 amu = 1.6605x10-24 grams
Atomic weight is the number that represents the average atomic mass of the
element’s isotopes weighted by percentage abundance.
Isotopes
1
1H
2
1H
3
1H
Learning Check
Write the nuclear symbols for atoms with the
following subatomic particles.
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
Solution
A. 8 p+, 8 n, 8 e-
16O
8
B. 17p+, 20 n, 17e-
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
Learning Check
1. Which of the following pairs are isotopes of the same
element?
2. In which of the following pairs do both atoms have
8 neutrons?
15X
A. 15X
8
B.
C.
12X
7
14X
6
6
15X
16X
7
8
Solution
B.
12X
6
14X
6
Both nuclear symbols represent isotopes of carbon
with six protons each, but one has 6 neutrons and the
other has 8.
C.
15X
7
16X
8
An atom of nitrogen (7) and an atom of oxygen (8)
each have 8 neutrons.
Isotopes of Magnesium
In naturally occurring magnesium,
there are three isotopes.
Isotopes of Mg
24Mg
12
25Mg
12
26Mg
12
Relative Masses of Atoms


Use atomic weights of the elements to
calculate molecular weights (MW) of
compounds
Relative masses provide a simple way of
comparing the masses of atoms. Ex. The
mass of neon atoms is 20.18 and the mass of
calcium atoms is 40.08.
The exact relationship between two masses
calculated
Ca atom mass = 40.08 = 1.986 Mass of Calcium
Ne atom mass
20.18
is 2x than Ne
He atom mass = 4.003 = 3.971 Mass of Helium
H atom mass
1.008
is 4x than
Hydrogen
Calculating the atomic weight of compounds

MW = CnHmOk
MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O)
H2O the MW is
MW = 2(at. Wt. H) + 1(at. Wt. O)
MW = 2(1.008 u) + 1(15.996 u)
MW = 18.012 u or 18.01 u for water
Learning Check
Use atomic weighs from the periodic table
inside the front cover of your book to determine
the molecular weight of urea, CH4N2O, the
compound by which much nitrogenous body
waste is excreted in the urine.
a. 58.02 u
b. 62.25 u
c. 60.06 u
Solution
The chemical formula for urea is CH4N2O,
MW = n(at. Wt. C) + m(at. Wt. H) + k(at. Wt. O)
MW = 1(12.01 u) + 4(1.008) + 2(14.01) + 1(16.00)
MW = 60.062 u or rounded off 60.06 u
d.
Models of the Atom

The Plum Pudding Model
 Electrons are embedded in a sphere of
positive charge.

The Nuclear Model
 All of the positive charge is in a tiny central
nucleus with electrons outside the nucleus.
 This model was developed by Rutherford
after his landmark experiments.
The Rutherford Experiment
Niels Bohr (1885-1962)
Models of the Atom (continued)
e.
Bohr Model or the Solar System Model





Niels Bohr in 1913 introduced his model of the
hydrogen atom.
Electrons circle the nucleus in orbits, which are also
called energy levels.
An electron can “jump” from a lower energy level to a
higher one upon absorbing energy, creating an
excited state.
The concept of energy levels accounts for the
emission of distinct wavelengths of electromagnetic
radiation during flame tests.
Bohr’s Orbit Model
Definitions
Quantum is the smallest increment of energy, for example, in an atom emitting or
absorbing radiation.
Ground state is the condition of an atom in which all electrons are in their
normal, lowest energy levels.
Excited state is an unstable, higher energy state of an atom.
A line spectrum for hydrogen
Neon (Ne)
Fig. 3-6a, p. 49
Neon, a partially evacuated
tube that contains neon gas
gives a reddish-orange glow
when high voltage is applied.
The line emission spectrum of neon is obtained when light from a neon source
passes through a prism.
Fig. 3-6b, p. 49
Potassium
burns with a violet
flame
Credit: Photo Researchers, Inc.
Lithium
burns with
a red flame
Electromagnetic Radiation
c = ln
c is the speed of light
l is wavelength
n is frequency
It is important to understand E (for energy), wavelength, and frequency
relationship:
As l decreases, n and E increases
As l increases, n and E decreases
Fig. 3-7, p. 50

Models of the Atom (continued)

The Orbital Model


Orbits are replaced with orbitals, volumes of space
where the electrons can be found.
The arrangement of electrons in the orbitals is the
electronic configuration of an atom, which
determines the chemistry of the atom.
Definitions
Electrons in the highest occupied energy level are the greatest stable distance
from the nucleus. These outermost electrons are known as valence
electrons.
Shell is a principal energy level defined by a given value of n, where n can be
1,2,3,4 etc… and is capable of holding 2n2 electrons.
An orbital is a region of three-dimensional space around an atom within
which there is a significant probability (usually shown as 90%) that a given
electron will be found.
Subshells have different energy levels (orbitals) within a given shell
Valence Electrons
The valence electrons
• determine the chemical properties of the elements.
• are the electrons in the highest energy level.
• are related to the group number of the element.
Example: Phosphorus has 5 valence electrons.
5 valence electrons
P Group 5A(15)
2, 8, 5
Groups and Valence Electrons
All the elements in a group have the same number of
valence electrons.
Example: Elements in group 2A(2) have two (2)
valence electrons.
Be
2, 2
Mg
2, 8, 2
Ca
2, 8, 8, 2
Sr
2, 8, 18, 8, 2
Periodic Table and Valence Electrons
Representative Elements Group Numbers
1
2
3
4
5
6
7
H
1
Li
2,1
Be
2,2
Li
Mg
2,8,1 2,8,2
Al
2,3
C
2,4
N
2,5
Ge
Si
2,8,3 2,8,4
P
2,8,5
O
2,6
F
2,7
S
Cl
2,8,6 2,8,7
8
He
2
Ne
2,8
Ar
2,8,8
Learning Check
State the number of valence electrons for each.
A. O
1) 4
2) 6
3) 8
B. Al
1) 13
2) 3
3) 1
2) 5
3) 7
C. Cl
1) 2
Solution
State the number of valence electrons for each.
A. O
2) 6
B. Al
2) 3
C. Cl
3) 7
Learning Check
State the number of valence electrons for each.
A. 2, 8, 5
B. 2, 8, 8, 2
C. 2, 7
Solution
State the number of valence electrons for each.
A. 2, 8, 5
5
B. 2, 8, 8, 2
2
C. 2, 7
7
Energy levels are spaced differently, like
ladder rungs
Credit: Foto-Search.com
Atomic energy
levels are like floors of a house
State transitions for
hydrogen
Table 3-2, p. 52
Atomic Orbitals.
Fig. 3-8, p. 54
Fig. 3-9, p. 55
The Orbital Model:
Electronic Configurations
Sample energy level diagram
Table 3-3, p. 55
Fig. 3-10a, p. 56
Fig. 3-10b, p. 56
f.
The Periodic Table
 Used to organize the elements by
recurring chemical properties.
 Elements in the same vertical column of
the periodic table have similar chemical
properties and are said to be in the same
group or family.
The Periodic Table
Dmitri Mendeleev (1834-1907)
Groups and Periods
On the periodic table,
• elements are arranged according to similar
properties.
• groups contain elements with similar properties in
vertical columns.
• periods are horizontal rows of elements.
Groups and Periods
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Group Numbers
Group Numbers
• use the letter A for the representative elements (1A to
8A) and the letter B for the transition elements.
• also use numbers 1-18 to number the columns from left
to right.
Names of Some Representative
Elements
Several groups of representative elements are known
by common names.
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Publishing as Benjamin Cummings
Alkali Metals
Group 1A(1), the alkali metals, includes lithium,
sodium, and potassium.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Halogens
Group 7A(17) the
halogens, includes
chlorine, bromine,
and iodine.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Learning Check
Identify the element described by the following.
A. Group 7A(17), Period 4
1) Br
2) Cl
3) Mn
B. Group 2A(2), Period 3
1) beryllium
2) boron
3) magnesium
C. Group 5A(15), Period 2
1) phosphorus 2) arsenic
3) nitrogen
Solution
A. Group 7A (17), Period 4
1) Br
B. Group 2A (2), Period 3
3) magnesium
C. Group 5A(15), Period 2
3) nitrogen
Metals, Nonmetals, and Metalloids
The heavy zigzag line
separates metals and
nonmetals.
• Metals are located to the
left.
• Nonmetals are located to
the right.
• Metalloids are located
along the heavy zigzag
line between the metals
and nonmetals.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Comparing a Metal, Metalloid, and
Nonmetal
Learning Check
Identify each of the following elements as
1) metal
2) nonmetal
3) metalloid
A. sodium
B. chlorine
C. silicon
D. iron
E. carbon
____
____
____
____
____
Solution
Identify each of the following elements as
1) metal
2) nonmetal
3) metalloid
A. sodium
B. chlorine
C. silicon
D. iron
E. carbon
1 metal
2 nonmetal
3 metalloid
1 metal
2 nonmetal
Learning Check
Match the elements to the description.
A. Metals in Group 4A(14)
1) Sn, Pb
2) C, Si
3) C, Si, Ge, Sn
B. Nonmetals in Group 5A(15)
1) As, Sb, Bi
2) N, P
3) N, P, As, Sb
C. Metalloids in Group 4A(14)
1) C, Si, Ge,
2) Si, Ge
3) Si, Ge, Sn, Pb
Solution
Match the elements to the description.
A. Metals in Group 4A (14)
1) Sn, Pb
B. Nonmetals in Group 5A(15)
2) N, P
C. Metalloids in Group 4A(14)
2) Si, Ge
Fig. 3-11, p. 58
Table 3-4, p. 59
Carbon
Gold
Sulfur
p. 60
The Octet Rule
g.



The noble gases of Group VIIIA do not
typically form compounds with other atoms.
Atoms with eight valence electrons are
particularly stable, an observation called the
octet rule.
Atoms form bonds with other atoms to
achieve a valence octet.
Electronic
Configuration of Noble Gases

Lewis Dot Structures


The number of valence electrons is equal to the
group number for most of the main group
elements.
In Lewis dot structures, the chemical symbol
represents the nucleus and the core electrons
and dots represent the valence electrons.
Writing Electron-Dot Symbols
Electron-dot symbols for
• groups 1A(1) to 4A(14) use single dots.
·
·
Na ·
· Mg ·
· Al ·
·C·
·
• groups 5A(15) to 7A(17) use pairs and single dots.
··
·P·
·
··
:O·
·
Groups and Electron-Dot Symbols
In a group, all the electron-dot symbols have the
same number of valence electrons (dots).
Example: Atoms of elements in Group 2A(2) each
have 2 valence electrons.
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·
Lewis Dot Structures
Learning Check

A.
X is the electron-dot symbol for
1) Na
2) K
3) Al

B.

X

1) B
is the electron-dot symbol of
2) N
3) P
Solution

A.
X is the electron-dot symbol for
1) Na
2) K

B.

X


is the electron-dot symbol of
2) N
3) P

Ionic Bonds


Ionic compounds result from the loss of
electrons by one atom (usually a metal) and the
gain of electrons by another atom (usually a
nonmetal).
Ionic bonds arise from the attraction between
particles with opposite charges (electrostatic
forces); e.g., Na+ Cl-.
Ionic Compounds
Atomic Size
Atomic size is described using the atomic radius; the
distance from the nucleus to the valence electrons.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Atomic Radius Within A Group
Atomic radius increases
going down each group of
representative elements.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Atomic Radius Across a Period
Going across a period left to right,
• an increase in number of protons increases attraction for
valence electrons.
• atomic radius decreases.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
Learning Check
Select the element in each pair with the larger atomic
radius.
A.
B.
C.
Li or K
K or Br
P or Cl
Solution
Select the element in each pair with the larger atomic
radius.
A. K
B. K
C. P
Key Words


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






Chemistry
Matter
Pure substance
Mixture
Element
Compound
Homogeneous mixture
Heterogeneous mixture
States of matter
Solid




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




Liquid
Gas
Physical changes
Chemical changes
Atom
Molecule
Periodic table
Periods
Groups or Families
Main group elements
Key Words (cont)











Transition elements
Metals
Nonmetals
Semimetals
Protons
Neutrons
Electrons
Atomic number
Mass number
Isotopes
Atomic Mass










Nucleus
Electromagnetic radiation
Wavelength
Energy level
Ground state
Excited state
Orbital
Electronic configuration
Valence electrons
Outer shell