Atoms, Elements and Molecules - 1
HNRS 227
Chapters 8 & 10: The Atom, Elements and Molecules
Outline
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
IX.
X.
Elements
Compounds and Molecules
Atoms
Particles
Atomic Structure
Electrons
Pauli Exclusion Principle and the Periodic Table
Valence Electrons
Valence Electrons and Bonding
a. Ionic Bond
b. Metallic Bond
c. Covalent Bond
Intermolecular Forces
a. Polarization and Hydrogen bonds
b. Van der Waal Forces
Take Home Messages
1. Atoms are the chemical building blocks of all matter
2. Structure of atoms (electrons, neutrons, protons and their
arrangement/distribution) determine the unique behavior/attributes of the
elements
3. Of the above (No. 2) the “place” and “pairing” of the electrons are the most
critical
4. Electrons reside in defined shells and orbits surrounding the nucleus of the atom
and the outermost shell/orbit, valence electrons, determine an atom’s chemical
reactivity
5. The utility and periodicity if the Periodic Table of Elements is a function of the
valence electrons in the outermost shell/orbit and the mass of the element
6. Matter consists of elements which combine to produce compounds (2 or more
elements) and/or molecules (1 element)
7. When atoms combine to produce molecules and compounds, expect the chemical
properties of the molecules/compounds to be far different than that of the
constituent atoms (principle of emergent properties)
8. Atoms bind together by re-arranging and sharing their electrons
a. Ionic bonds
b. Metallic bonds
c. Covalent bonds
d. Intermolecular forces (e.g., van der Waal forces, polarity)
9. Chemical reactions make and break bonds between elements
10. Weak chemical bonds play important roles in the chemistry of life
Atoms, Elements and Molecules - 2
I.
Elements
Substances that cannot be broken down by chemical reaction
Hierarchy is as follows:
Compound
Element
Atom
Subatomic Particles
92 naturally occurring elements in universe including gold, copper, helium,
nitrogen, sulfur, sodium, etc.
Additional 20 elements that are man made…ephemeral and exist only under
very unusual conditions (accelerators at national laboratories)
25 of 92 natural elements are essential for life and four of these comprise 96%
of all life forms…including you (carbon, oxygen, hydrogen and nitrogen)
Earth’s mass is predominantly only six elements (oxygen, sulfur, magnesium,
iron, aluminum and calcium) whereas the predominant elements in most
stars is hydrogen
Key: any element anywhere is the same in its chemical and physical
properties (stability in structure over time ands in space)
II.
Compounds and Molecules
See the above hierarchy
When elements combine, they do so in a very precise and specific manner
(predictable) that is repeated over and over again
e.g.,
Sodium (Na)
Metal
+
Chlorine (Cl)
gas
Produces sodium chloride (NaCl), which is an eatable solid
Discuss emergent properties and “creation” of new properties that cannot be
explained by simply the combination of the two elements
Note also that the atoms that comprise the compound are unchanged
III.
Atoms
Smallest unit of an element that still retains properties of the element
Period on a page has 106 atoms or 1,000,000 atoms
Period of sodium atoms.... Each atom would have the properties of sodium
that are identical to all the other atoms of sodium
Atoms, Elements and Molecules - 3
IV.
Particles
Atoms are composed of particles (subatomic particles)
100+ types of particles
Most stable are only 3
Neutrons
Protons
Electrons
Others include quarks, bosons, neutrinos, etc.
Charge
Mass
Neutron
neutral
1.7 x 10-24 g
1 Dalton
Proton
positive
1.7 x 10-24 g
1 Dalton
Electron
negative
5 x 10-28 g
160 lbs person weighs 68,000 g
V.
Atomic Structure
Atoms of the same element (e.g., helium or He) have the same number of
subatomic particles and by convention we abbreviate this information as
follows (use the Periodic Table):
2
Number of protons
He
Abbreviation for the element
4
Atomic mass (g/mole or Daltons)
Mass =
Neutrons
Protons
+
or/re-arranged
=
Mass -
Neutrons
Protons
Example: Sodium or Na
11
Na
23
Therefore, the mass is 23 Daltons (g/mole), number of protons is 11, number
of electrons is 11, and the number of neutrons is 12
If atom is neutral (not charged), number of protons always equals electrons
If atom is ionized (charged), number of protons does not equal electrons
(ions)
VI.
Electrons (energy barons)
Energy
=
ability to do work (earlier chapter)
Potential energy =
energy that is stored due to position or location in
Atoms, Elements and Molecules - 4
the form of gravitational energy or chemical energy
Analogy to water in a reservoir
Electrons are negatively charged particles that are constantly in motion within
an atom
Example of “capturing” an atom and its orbiting electron (smacking atom on
the board)
Hydrogen
+ (nucleus)
. – (Electron)
Note distance of electron from the proton
Now capture another hydrogen
+ (Nucleus)
. – (Electron)
Note:
distance is always the same between proton and electron but
location of electron differs (distance is synonymous with
energy)
Key:
Nucleus is stationary
Electron moves constantly
Distance of electron from nucleus is always the same due to the
orbit in which the electron sits (remember Bohr atom)
repeat the above to get a “picture” of the Bohr atom:
Orbit
Orbit is the “track” around the nucleus
Lithium as an example (go to Periodic Table)
3
Li
6
Atoms, Elements and Molecules - 5
Lithium: 2 orbits, 3 electrons
Keys:
1.
2.
3.
4.
Count the number of electrons (and compare with
abbreviation of element)
Electrons are negatively charged
Electrons are constantly moving
Electrons are in orbits around the nucleus
Another example (Sulfur)
16
S
32
5.
6.
7.
If electrons are moving, what does that tell you (energy?)?
The shell is determined by the distance from the nucleus
a. First shell has 1 orbit, is the lowest energy state, and
has a maximum of 2 electrons
b. Second shell has 4 orbits and a maximum of 8 electrons
(2 per orbit)
If electron posses energy, it can loose it or gain it
a. Loose energy….drops down to a lower shell
b. Gains energy…jumps up to a higher shell (excited)
(remember the Bohr atom)
How do you know this happens (i.e., electrons are moving between shells
and are a source of potential energy)?
Hot summer day, bright sun
Light energy absorbed by pigments on car top, electrons
jump to higher shell, and then drop back to lower
shell, giving off energy as heat
Banana, orange juice or bagel this AM?
Chlorophyll in a leaf absorbs energy and electron is excited
to a higher shell, electron is shuffled to a sugar or
carbohydrate molecule where it is stored until you
eat it…when you digest it, the energy is released in
Atoms, Elements and Molecules - 6
metabolism, which drives your body heat and runs
the machinery of your body
8.
9.
10.
11.
12.
13.
Can have 0, 1 or 2 electrons per orbit
No more than 2
1st Shell has only 1 orbit (maximum 2 electrons)
2nd Shell has 4 orbits and a maximum of 8 electrons
Electrons are distributed in shells and this distribution is
the key to understanding why elements behave
chemically the way they behave (Periodic Table)
The outer most electrons (outer most shell and orbit) are
called the valence electrons
Electrons obey the Pauli Exclusion Principle
No two electrons can occupy the same orbit
Counterclockwise and clockwise spin
Example:
H
He
Li
Na
VII.
1 electron
1st shell:
2nd shell:
1 electron
none
2 electrons
1st shell:
2nd shell:
2 electrons
none
3 electrons
1st shell:
2nd shell:
2 electrons
1 electron
11 electrons
1st shell:
2nd shell:
3rd shell:
2 electrons
8 electrons (4 orbits)
1 electron
Pauli Exclusion Principle (PEP) and the Periodic Table
Redefine PEP: no two electrons can occupy the same orbit at the same
time (clearly define shell versus orbit)
Relate to the structure of the atom
Energy levels of the electrons at defined distances from the nucleus (shells
and orbits)
Concept of most stable state is one in which the outer shell is filled to the
maximum with electrons
1st energy level or 1st Shell
1 orbit
Atoms, Elements and Molecules - 7
2 electrons
e.g., hydrogen (1 electron)
helium (2 electrons)
Therefore, most stable atoms is one with only 2 electrons and this
is why only 2 elements in the first row (Row 1; refer to
Periodic Table)
Explain clockwise/counterclockwise motion of electrons
2nd energy level or 2nd shell
4 orbits
2 electrons/orbit = 8 electrons total
spin/counterspin
Therefore, most stable atom is one with 10 electrons (2 + 8) (i.e.,
Neon with 10; refer to Periodic Table)
Row 2 has 8 elements
3rd energy level or 3rd shell
4 orbits
2 electrons/orbit = 8 electrons total
Therefore, the most stable atoms is one with 8 electrons in the 3rd
shell (i.e., Argon with 18)
Row 3 has 8 elements
4th energy level or shell
9 orbits or 18 electrons
Many more elements in the row as a function of the orbits being
full or half full
Most stable would be elements with all orbits filled
2 + 8 + 8 + 18 = 36 electrons (= krypton)
Periodic Table of Elements
Columns
Similar chemical properties
Function of the valence electrons
Rows
Number of elements/row is a function of the number of electrons
to fill the outer shell
Row 1
2
Row 2
8
Row 3
8
Row 4
18
Row 5
?
VIII.
Valence Electrons
Valence electrons: outermost electrons in shell (can be in multiple orbits)
Most stable number is one in which valence shell is filled
Most stable are the following
2 electrons
Helium
Atoms, Elements and Molecules - 8
10 electrons Neon
18 Electrons Argon
36 Electrons Krypton
All “inert” gases
Less stable number is anything other than a filled outer shell
IX.
Valence Electrons and Bonding
Prediction:
Interaction of 2 atoms’ electrons will tend to re-arrange to the lowest
energy state or situation where valence state is most stable
In so doing, electrons will be re-arranged into bonds
Three strategies for bonding:
Give away electrons
Accept electrons
Share electrons
A. Ionic Bonds: atoms give away electrons while other receive electrons
Example:
Lithium
Li gives up 1 electron in its interactions and in so doing now has 3
protons (+) and only 2 electrons (-) and so is negatively
charged
Referred to as an ion (explain as a charged atom)
Example:
Chlorine (17 electrons and note Periodic Table location)
One unpaired electron in valence shell…so that Cl tends to accept an
electron (sum charges as before and Cl is charged negative – ion)
Bonding via electrical attraction of charged ions Li+ and Cl-
Atoms, Elements and Molecules - 9
B. Metallic Bond
C. Covalent Bond
X.
Intermolecular Forces
a. Polarization and Hydrogen bonds
b. Van der Waal Forces