Atoms, Molecules, and
Ions
The Atomic Theory
The Greek Philosopher Democritus expressed the belief that all
matter consists of tiny, indivisible particles. He named these
particles atomos, which means indivisible. In 1808, John Dalton
formulated a definition of the atom and thus Dalton’s Atomic theory
says this:
1)-Elements are made up of extremely small particles called
atoms. All atoms of a given element are identical having the same
mass, size, and chemical properties; however, the atoms of
differing elements are different.
2)-Compounds are composed of atoms of more than one
element and in any compound the ratio of the number of atoms of
any two elements present is an integer or simple fraction.
3)-A chemical reaction involves the separation, combination, or
rearrangement of atoms, however atoms are not created or
destroyed in any chemical reaction.
The Structure of the Atom
According to Dalton’s atomic theory, an atom is the basic unit
of an element that can enter into a chemical reaction. Dalton
believed that the atom was extremely small and indivisible,
however it was later found that the atom actually posses internal
structure and is made up of even smaller particles called
subatomic particles, mainly electrons protons and neutrons.
Radiation is the study of the emission and transmission of energy
through space in the form of waves. The cathode ray tube was used
to study this phenomenon. It is a glass tube from which most of the
air has been evacuated. Two metal plates are connected to a high
voltage source and the negatively charged plate called the cathode
emits an invisible ray. This cathode ray is drawn to the positively
charged plate called the anode, where it passes through a hole where
it strikes a special surface and produces a bright light. When a
magnet and two electrically charged plates are added to the outside
of the cathode ray tube, the direction of the ray can be controlled.
According to electromagnetic theory, a moving charged body
behaves like a magnet and can interact with electric and magnetic
fields. So the ray, as a moving charged body, is affected by the
electric filed of the two oppositely charged plates and the magnetic
filed of the magnet. The cathode ray is attracted to the positive plate
and repelled by the negative plate, so it must consist of negatively
charged particles called electrons.
Cathode Ray Tube
Becquerel, a French physicist discovered that when he
exposed thickly wrapped photographic plates to a certain
uranium compound, it caused them to darken. In fact he
had found that the radioactivity, or spontaneous emission
of particles or radiation cause the plated to darken. There
are three types of rays that are produced by the decay of
radioactive substances as uranium. Alpha rays are
positively charges particles called alpha() particles. Beta
rays are electrons called beta() particles. Gamma rays
are high-energy rays otherwise known as gamma() rays.
In the 1900s Thomson proposed that an atom is essentially
a charged positive sphere with electrons embedded in it. In
1910, Rutherford carried out experiments in which he
targeted alpha particles at thin foils of gold as seen in Figure
2. He noticed that most of the particles penetrated the foil
with no or small deflection. However, he also noticed that
occasionally an alpha particle bounced back. Based on
these findings, Rutherford developed a new model of the
atom in which most of the atom is empty space. The atom’s
positive charges, called protons are concentrated in the
nucleus, the dense central core of the atom. The charge of
the proton is equal to the charge of the electron except it is
positive. The mass of the proton is 1.67262*10-24 g., 1840
times bigger than the mass of the electron. Also, while
typical atomic radius is 1 pm, the atomic nucleus is only
5*10-3 pm.
Rutherford Experiment
Atomic Number, Mass Number, and Isotopes
Atoms can be identified by the number of protons and
neutrons they contain. The atomic number, Z, is the number
of protons in the nucleus of each atom of an element. Atoms
with no charge have the same number of electrons as protons.
An atom with an atomic number of seven has seven protons
as well as seven electrons and is identified as the element
nitrogen.
The mass number, A, is the total number of neutrons and
protons present in the nucleus of an atom of an element. The
mass number is equal to the number of protons plus the
number of neutrons.
Atoms of an element often have isotopes-atoms
that have the same atomic number but different
mass numbers. The mass number, A, of an
element, X, is written in the upper left hand corner
of the element as such: AX. The atomic number of
an element, Z, is written in the lower left hand
corner of the element, X, as such: ZX. Since the
chemical properties of an element are determined
mostly by the protons and electrons in its atoms,
isotopes of the same element have similar
chemistries.
The Periodic Table
The periodic table is a chart in which elements having
similar chemical and physical properties are grouped together.
It was made from the need to organize large amounts of
information about the structure and properties of elemental
substances. Thus, elements are grouped according to similar
physical and chemical behavior. Figure 3 shows the periodic
table of the elements. The elements are arranged by atomic
number in horizontal rows called periods and in vertical columns
known as groups or families, according to their similarities in
chemical properties.
The elements are divided into metals, nonmetals, and
metalloids. Metals are good conductors of heat and
electricity. Nonmetals are poor conductors of heat and
electricity. Metalloids have properties that are intermediate
between those of metals and nonmetals. The elements
change from metallic to nonmetallic as you go across the
table left to right. Elements can be referred to collectively by
their group number such as 2A or special names that the
groups have been give. Group 1A elements are called alkali
metals, group 2A elements are called alkaline earth metals,
group 7A elements are known as halogens, and group 8A
elements are called noble gasses. We can make easy use of
the periodic table to predict chemical behavior between
elements.
Periodic Table
Molecules and Ions
A molecule is an aggregate of at least two atoms in a
definite arrangement held together by chemical bonds. The
molecule may contain atoms of the same element or atoms of
different elements. Molecules are not always compounds,
which are made up of two or more elements. Molecules that
contain more than two atoms are called polyatomic molecules.
Ammonia, NH3 is a polyatomic molecule as well as is ozone, O3.
An ion is an atom or a group of atoms hat has a net
positive or negative charge. The number of protons in the
nucleus of an atom always remains the same during
ordinary chemical reactions, however electrons may be
gained or lost by certain elements. A cation is an ion with a
positive charge resulting from the loss of electrons.
Sodium, Na, can lose an electron to become sodium
cation, Na+. An anion is an ion with a negative charge
resulting from the gain of electrons. Chlorine, Cl, can gain
an electron to become a chlorine anion, Cl-. Together
sodium and chlorine can combine to form sodium chloride,
table salt. This type of reaction is called an ionic
compound because it is formed from two ions. Monatomic
ions are ions that form from only one element such as Na+.
Polyatomic ions are ions that form from more than one
element such as OH-.
Chemical Formulas
Chemists make use of chemical formulas to express
the composition of molecules ad ionic compounds in terms of
chemical symbols. A molecular formula shows the exact
number of atoms of each element in the smallest unit of a
substance. So H2 is the molecular formula for hydrogen and
H2O is the molecular formula for water. An allotrope is one of
two or more distinct forms of an element. O2 and O3 are
allotropes of oxygen since they are two different forms of the
same element, oxygen.
Since molecules are too small for us to observe directly, we
can use molecular models to observe them physically. In balland-stick models the atoms are represented by plastic or wooden
balls with holes in them. Sticks or springs are used to represent
chemical bonds. The angles of these bonds approximate the
bond angles in actual molecules. Except for the hydrogen atom,
the balls are the same size and each atom is represented with a
different color. In space-filling models, atoms are represented by
truncated balls held together by snap fasteners. The bonds are
not visible. Figure 4 shows a ball-and-stick model of a molecule
on top and a space-filling model of the same molecule on the
bottom. The structural formula shows how atoms are bonded to
one another in a molecule. In a water molecule, two hydrogen
atoms are bonded to oxygen so its structural formula would be
written as H-O-H. The lines connecting the two atomic symbols
represent chemical bonds.
The empirical formula tells which elements are present and
the simplest whole number ration of their atoms. The
molecular formula for hydrogen peroxide is H2O2. The ratio of
H to O is 2:2 or in simplest whole number terms 1:1. Thus,
the empirical formula for H2O2 is HO. The empirical formula
for N2H4 is NH2. Empirical formulas are the simplest chemical
formulas while molecular formulas are the true formulas of
molecules. Determining a compounds empirical formula is the
first step in determining its molecular formula through
experimentation. For water, H2O, its ratio of elements cannot
be reduced further, so it is already an empirical formula as
well as well as a molecular formula.
Naming Compounds
Except for the ammonium ion, most cations are derived from
metal ions. Metal cations are named simply by naming the
metal and adding the word ion after it. So sodium in its ionic
state would be named sodium ion. Aluminum in its ionic state
would be named aluminum ion. For ionic compounds formed
from just two elements, these are referred to as binary
compounds. In binary compounds the first element named is
the metal cation, followed by the nonmetallic anion. The anion
is named by taking the first part of the element name and
adding “ide” to the end. So chlorine would be called chloride
and bromine would be called bromide. So the binary
compound NaCl would be called sodium chloride. Certain
anions such as hydroxide, OH- and cyanide, CN- also use the
“ide” ending. So the compound LiOH is called lithium
hydroxide. This compound consists of three elements and so
is called a ternary compound.
Molecular compounds are usually made up of nonmetallic
elements. Many molecular compounds are binary and naming
these compounds is similar to naming binary compounds. The
first element is named and then the second element is named by
adding “ide” to the end of the name. So HCl would be named
hydrogen chloride and HBr would be named hydrogen bromide.
When one pair of elements forms more than one compound,
Greek prefixes are used to denote the number of each element
present. The Greek prefixes can be seen below:
Prefix
MonoDiTriTetraPentaHexaHeptaOctaNonaDeca-
Meaning
1
2
3
4
5
6
7
8
9
10
An acid is a substance that yields hydrogen ions, H+, when
dissolved in water. Acids contain hydrogen atoms and an
anionic group. If the anionic group ends in “ide”, then acids are
formed with a “hydro” prefix and an “ic” ending. HCl is called
hydrogen chloride in the gaseous or liquid state. However,
when HCl is dissolved in water, H+ ions are released and the
molecule becomes hydrochloric acid.
A base is a substance that yields hydroxide ions, OH-, when
dissolved in water. NaOH, KOH, and Ba(OH)2 are all bases.
Ammonia, NH3 does not even contain an OH however, when
it dissolves in water, it reacts with water to yield among other
products OH- ions. So it is indeed a base.
Hydrates are compounds that have a specific number of water
molecules attached to them. Copper(II) sulfate has five water
molecules attached to it, so the systematic name for the
compound is copper(II) sulfate pentahydrate. Its formula is
CuSO45H2O. The water molecules can be evaporated by
heating and the resulting substance would be called
anhydrous copper(II) sulfate. Its formula is CuSO4.
BaCl22H2O is another hydrate named barium chloride
dihydrate. One more hydrate is LiClH2O named lithium
chloride monohydrate. The amount of water molecules
present is named using the Greek prefixes seen in Table 1.