On to Chemistry
• Having established the existence of
molecules and atoms, we now take several
chapters to explore their properties.
– What varieties are there?
– What are the natures of the pure elements?
Why?
– How do atoms interact and combine together
when they form molecules?
PS 110A Hatch Ch. 17 -3
PS 110A Hatch Ch. 17 -4
Chemical properties can vary widely even
when other physical properties are similar
H
He
Chemical properties can vary widely even
when other physical properties are similar
Chlorine
Nitrogen
PS 110A Hatch Ch. 17 -5
John Dalton proposed the
atomic theory (1803)
Matter is composed of small indivisible
particles called atoms
An element contains only a single kind of
atom. Atoms of a given element are
identical in every respect, including
mass and chemical behavior.
Atoms of different elements have different
mass and chemical reactivity
Chemical compounds are composed of two
or more atoms that are joined together in
fixed ratios
Chemical reactions correspond to the
rearrangement of atoms to form a
different compound.PS 110A Hatch Ch. 17 -7
PS 110A Hatch Ch. 17 -6
Chemical Elements
Boyle (1661): an element is any substance that
cannot be separated into different components
by any known (chemical) methods
modern definition: an element is a group of atoms
all having the same # of protons in nucleus
metals: copper(Cu), tin(Sn), aluminum(Al), nickel(Ni)
non-metals: hydrogen(H), helium(He), carbon(C),
nitrogen(N), oxygen(O), sulfur(S)
PS 110A Hatch Ch. 17 -8
Designating a Specific Atom
Mass number
Atomic number
Al
27
13
3+
Ionic state
Early work led to the law of
constant composition:
Compounds contain a fixed definite proportion of the
elements.
Chemical symbol
Atomic number = number of protons.
Mass number = number of protons and neutrons
Ionic state = total charge of atom = the number of missing
(or added) electrons.
atomic weight (atomic mass): the relative mass of naturally
occurring atoms of an element compared to carbon-12 atoms
which have arbitrarily been given a mass of twelve (often called
atomic mass units)
PS 110A Hatch Ch. 17 -9
Example: Water
By mass: 2 amu of hydrogen for 16 amu of oxygen
An oxygen is 16 times more massive than a single
hydrogen atom, so we have two hydrogen atoms per
oxygen
PS 110A Hatch Ch. 17 -10
Arrangement in the
periodic table
Mendeleev
According to atomic number, not
atomic weight (27Co, 28Ni) (transparency)
Similar chemical
properties in same
column
Dimitri Ivanovich Mendeleev ordered the elements by atomic
weight (mass number) and established that there were reoccurring patterns in the ways that elements combined with
other elements. But the first attempt had a few problems.
Order the elements by atomic number and the periodicity is
PS 110A Hatch Ch. 17 -11
perfect.
The wave model and the
periodic chart
To understand the main features of the periodic
table we must use the wave model. It explains
Atomic diameters
Periodicities
The octet rule for covalent bonding
Molecular shapes
PS 110A Hatch Ch. 17 -13
Properties change
across same row
PS 110A Hatch Ch. 17 -12
Why the periodicity?
☺
The chemical properties of the elements are
governed by their electrons.
☺
The number of electrons in a neutral atom is given by the
atomic number.
☺
The chemical properties are governed mostly by the nature of
the valence (outermost) electrons; i.e. the valence electrons’
energy shells and subshells (arising from the wave model of the
atom).
☺
Elements in a family have same number and orbital type of
valence electrons & same number of unfilled states in their
outermost shell
☺
Moving across a particular row corresponds to gradually filling
up a particular shell.
PS 110A Hatch Ch. 17 -14
Expected Filling order of States
Filling sequence
(Periodic Table transparency)
- lowest energy orbitals fill first
4S
4P
3S
3P
2S
2P
4F
4D
3D
E
1S
Notice what happens to the 1s shell!
PS 110A Hatch Ch. 17 -15
PS 110A Hatch Ch. 17 -16
Filling order of Electron States
Filling Orbitals
(Periodic Table transparency)
• At the third shell elements start filling
higher shell before finishing lower shells
– see K and Ca, added 4s before the 3d orbitals
– then from Sc to Zn the 3d orbitals are filled
• The s and p orbitals strongly determine
chemical behavior, s on the left of table and
p on the right.
transparency of filled levels
•Orbitals fill the lowest energy
first.
However, this does not always
mean that the lowest shell is
completely filled before electrons
begin filling orbitals of the next
shell.
•valence electrons: the electrons
in the outermost occupied shell
(the outside shell is often not
completely filled).
PS 110A Hatch Ch. 17 -17
PS 110A Hatch Ch. 17 -18
Energy Well
Orbitals of Valence Electrons
P2: Which element is it? What orbital type is its valence
electron(s)?
etc
3
Filled shell 2
s
f
d
p
2
14
10
6
PS 110A Hatch Ch. 17 -19
p
s
p
s
Energy
1
Shells
Filled shell
s
Orbital sets
PS 110A Hatch Ch. 17 -20
The elements group together in families
of similar chemical properties.
The elements group together in families
of similar chemical properties.
At the left of the periodic table are the alkalai metals:
Lithium Li, Sodium Na, Potassium K, etc.
They have a single S valence electron which is easily
lost. All react energetically with water.
At the right of the table are the noble gasses:
Helium He, Neon Ne, Argon Ar, etc.
Have filled shells. These do not easily combine
with other elements.
Na
Rb
K
Cs
Video: alkalai
metals
PS 110A Hatch Ch. 17 -21
The elements group together in families
of similar chemical properties.
The second column from the right is the
halogens: Flourine F, Chlorine Cl, Bromine
Br, etc. Need only one electron to fill shell.
Highly reactive. All form salt compounds
PS 110A Hatch Ch. 17 -23
smaller
PS 110A Hatch Ch. 17 -22
The “A” Columns
•These columns represent filling the S and P
shells.
•These two outer valence shells are the
primary determiner of chemical behavior.
•As we move from Sc to Zn the 3d orbitals
are filled. The “B” columns of the table.
PS 110A Hatch Ch. 17 -24
Sizes
(radius in pm)
Periodic
Trends:
P3: Which family
smaller
Video: noble gasses
appears as the
peaks in the
atomic size plot?
Transparency
PS 110A Hatch Ch. 17 -25
Atomic Diameters
Atomic diameters increase suddenly when the new
electron is in the next shell.
As you move along the row, the outer electrons of
elements have their electrons in the same shell;
they fill different orbitals, but the diameters of
orbitals in the same shell are comparable.
However, more protons pulling on more electrons
tends to squeeze the electron probability clouds
in closer to the nucleus, decreasing the diameter.
PS 110A Hatch Ch. 17 -26
Ionization energy: energy required to completely
remove an outermost electron from an atom
Ionization Energy
freedom
etc
3
2
1
Shells
d
p
s
p
s
elements with ionization energies less than 8
electron volts are metals (electrons not
tightly bound)
shiny, malleable, solid, good conductors,
chemically active
Ionization
Energy
elements with ionization energies greater
than 10 electron volts are nonmetals
s
Orbital sets
missing at least one of the properties of metals
PS 110A Hatch Ch. 17 -27
PS 110A Hatch Ch. 17 -28
Ionization Energies
Ion. Energy
Ion. Energy
Note: in same family
Ionization Energies:
General trends
P4: Element 118 will be discovered. How does its ionization energy
compare with Radium (Ra)?
~5 ev
PS 110A Hatch Ch. 17 -29
Familiarity
PS 110A Hatch Ch. 17 -30
Regularities
Alkali Metals (column IA)
-single s electron, easily lost
-very reactive
-ionization energy low
PS 110A Hatch Ch. 17 -31
P5: Why does the ionization energy of the valence
electron in the metals decrease as you go down the column?
(see filling sequence transparency)
PS 110A Hatch Ch. 17 -32
Regularities
Regularities
Halogens (VIIA)
Inert gases (VIIIA)
-filled shells
-difficult to extract electrons
-not reactive
-high ionization energies
-single electron to fill shell
-very reactive
-grab an electron to fill the shell
-give off energy when they grab
electron
PS 110A Hatch Ch. 17 -33
Group Characteristics
Metals on left
Non-metals on right
Noble gases
valence electrons same
for all members of family
Video: fluorine reactions
PS 110A Hatch Ch. 17 -35
As the elements react:
• produce molecules and “compounds”
• formula of the compound shows how
many atoms of each element are in
the compound
(or how many dozen, or how many
moles)
PS 110A Hatch Ch. 17 -37
PS 110A Hatch Ch. 17 -34
Mole
like the word “dozen”
12
a little larger number
mole
6 × 1023
“Avogadro’s Number”
Number of molecules or atoms or donuts
Why such an unusual number?
Just the right number of 12C atoms to
weigh 12 g
PS 110A Hatch Ch. 17 -36
Triumphs of the Periodic Table
Predicted the existence of then undiscovered
elements and what their chemical properties
would be.
Gave simplicity and order to chemistry.
Firmly established the existence of atoms.
Verified the wave model of the atom.
PS 110A Hatch Ch. 17 -38
P6: Which of the following elements do you expect
to be chemically most similar to Fluorine
a)
b)
c)
d)
Oxygen
Nitrogen
Helium
Bromine
Neon
Sodium
Oxygen
Nitrogen
PS 110A Hatch Ch. 17 -39
Chemical Elements
~ 92 natural elements today
mass number
ionization number
the number of nucleons
in the atom
tells the number of excess
or deficient electrons
(charge on atom)
2+
4
He
2
atomic number
the number of protons
in the atom
P7: Which of the following do you expect
to have the largest ionization energy?
alpha particle
PS 110A Hatch Ch. 17 -41
PS 110A Hatch Ch. 17 -40