The Periodic Table
1.
2.
The elements in columns have very similar chemical properties.
The first column, the alkali metals are extremely reactive in air and are seldom found
uncombined.
3.
Elements in the second column, the alkaline earths are considerably less reactive.
4.
Elements in the first 12 columns are considered metals.
5.
The non-metals include all the elements included in the columns listed above the
diagonal
defined by the elements of B, Si, As, Te and At.
6.
The last column contains the inert gases. While most elements in the periodic
table are
found combined with each other, the inert gases are found uncombined
and monatomic.
7.
The number at the top of each square is called the atomic number. This number
defines
the number of protons in the nucleus of each atom. The nucleus, which
consists of
protons positive charged particles) and neutrons (neutral particles) make
up better than 99% of the mass of an atom. The sum of the number of protons and
neutrons is called the atomic weight. This number is found at the bottom of each square
in the periodic table. Atoms that have the same number of protons but different number
of neutrons are called isotopes. Isotopes have the same chemistry but different masses.
The atomic weights listed in the periodic table are weighted averages of all the isotopes
of each element. In a neutral atom, the number of electrons that surround the nucleus
equals the atomic
number. The volume the electrons occupy make up more than
99+% of the volume of an
atom.
8.
The chemistry of the elements in the periodic table is governed by the number of
electrons that surround the nucleus. Much of the chemistry of the elements can be
understood in terms of their desire to mimic the electronic environment found in
the inert
gases.
He
2 electrons
Ne
2) 8 electrons
Ar
2) 8) 8 electrons
I. Types of bonding found within molecules (molecule: two or more atoms bound
together to form a single entity)
1. metallic bonding: typically found in various metals; usually characterized by good
conductivity; bonding viewed as metal ions immersed in a sea of
electrons.
ex: metals such as sodium, iron, magnesium
2. ionic bonding:
typically found in salts; usually characterized by good conductivity
when
dissolved in water, bonding viewed as transfer of
electrons from a
metal to a non-metal to form a
cation (+) and anion (-).
ex: table salt: NaCl; NaF
3. covalent bonding: typically found in compounds formed from non-metals; non-conducting,
organic molecules.
ex: sugar, cinnamaldehyde (oil of cinnamon); eugenol
II. Examples of ionic and covalent bonding
1. Ionic bonding that results from the transfer of one or more electron(s)
.
Na
+
..
:Cl :
.
Na+
.. + : Cl :
..
Examples of compounds with ionic bonding
KCl
CaCl2
NaI
2. Covalent bonding that results from sharing of two or more electrons (note that
a sharing of two electrons is denoted by a line _ , sharing of four electrons by two
lines (=)).
Case A. each partner contributes equally
H
.
+
..
:Cl :
.
..
H __Cl
:
..
Note: two elements may contribute equally but the electrons may not be shared equally.
Examples of covalently bound molecules where each partner contributes equally
H2
NH3
C2H4
H2O
C2H6
C2H2
H2S
O2
C3H8
CH4
N2
C3H6
Case B. Unequal contribution of electrons
..
S:
..
- +
.. .. ..
_
:O
.. S = O
.
+ 2 :O:
.
Case C. A combination of both ionic and covalent bonding
..
:O :
.
2 Na
.
+ 2 :O:
.
+
.
. C.
.
2 Na+
-2
..
O
=C
..
:O
.. :
Rule 1. In order to understand bonding, remember hydrogen wants a total of 2
electrons in the electron hotel and most of the remaining elements of interest to us
want a total of eight.
See if you can draw structures for each of the following compounds that satisfy this rule.
CO2
C3H6
NaHCO3
C2H7N
C2H6O
C2H6S
Rule 2. The geometry of groups about a central atom (a group is either a pair of
electrons not involved in bonding or another atom; this other atom may be a single
atom or have other groups attached to it)
Central atoms with two groups attached:
Central atom with three groups attached:
Central atom with four groups attached:
Predict the geometry of the following molecules:
BCl3
NH3
CO2
CH4
Exercises
1.
Try to make models for each of the following molecules. First draw a structure
that satisfies Rules 1 and 2. Then make the model. More than one correct answer is
possible for some compounds.
CH4O
C2H6
H2S
C3H8
C2H6O
CH5N
Summary of Rule 2.
i. Central atoms containing two groups (groups as defined in Rule 1) are linear;
ii. central atoms with three groups are planar and triangular (the type of triangle
depends on the nature of the groups;
iii. central atoms with four groups attached are tetrahedral.