ATOMS, MOLECULES, AND IONS

Ron Angelo R. Gatinga

Special Science Teacher I
ATOMIC THEORY OF MATTER
•The theory that
atoms are the
fundamental building
blocks of matter
reemerged in the
early nineteenth
century, championed
by John Dalton.
LAW OF CONSERVATIONS OF MASS

• The total mass of substances present at the end of a chemical process is

the same as the mass of substances present before the process took place.
• This law was one of the laws on which Dalton’s atomic theory was
based.
LAW OF MULTIPLE PROPORTIONS
• If two elements, A and B, form more than one compound,
the masses of B that combine with a given mass of A are in
the ratio of small whole numbers.
• Dalton predicted this law and observed it while
developing his atomic theory.
• When two or more compounds exist from the same
elements, they can not have the same relative number of
atoms.
DISCOVERY OF SUBATOMIC PARTICLES
•In Dalton’s view, the atom was the smallest
particle possible. Many discoveries led to the
fact that the atom itself was made up of smaller
particles.
•Electrons and cathode rays
•Radioactivity
•Nucleus, protons, and neutrons
THE ELECTRON (CATHODE RAYS)
• Streams of negatively
charged particles were
found to emanate from
cathode tubes, causing
fluorescence.
• J. J. Thomson is credited
with their discovery
(1897).
• Thomson measured the
charge/mass ratio of the
electron to be 1.76 x 108
coulombs/gram (C/g).
MILLIKAN OIL-DROP EXPERIMENT

• Once the charge/mass ratio of the electron was known, determination of

either the charge or the mass of an electron would yield the other.
• Robert Millikan determined the charge on the electron in 1909.
THE MODEL OF THE ATOM, 1900
•The prevailing theory was
that of the “plum pudding”
model, put forward by
Thomson.
•It featured a positive sphere
of matter with negative
electrons embedded in it.
RADIOACTIVITY
• Radioactivity is the
spontaneous emission of high-
energy radiation by an atom.
• It was first observed by Henri
Becquerel.
• Marie and Pierre Curie also
studied it.
• Its discovery showed that the
atom had more subatomic
particles and energy associated
with it.
RADIOACTIVITY

• Three types of radiation were discovered by Ernest Rutherford:

• α particles (positively charged)
• β particles (negatively charged, like electrons)
• γ rays (uncharged)
THE NUCLEAR MODEL OF THE ATOM
• Ernest Rutherford
shot α particles at a
thin sheet of gold
foil and observed
the pattern of
scatter of the
particles.
• Since some particles
were deflected at
large angles,
Thomson’s model
could not be
correct.
THE NUCLEAR MODEL OF THE ATOM
• Rutherford postulated a very
small, dense nucleus with the
electrons around the outside of
the atom.
• Most of the volume is empty
space.
• Atoms are very small; 1 – 5 Å or
100 – 500 pm.
• Other subatomic particles
(protons and neutrons) were
discovered.
SUBATOMIC PARTICLES

• Protons (+1) and electrons (–1) have a charge; neutrons are neutral.
• Protons and neutrons have essentially the same mass (relative mass 1). The
mass of an electron is so small we ignore it (relative mass 0).
• Protons and neutrons are found in the nucleus; electrons travel around the
nucleus.
ATOMIC MASS AND ATOMIC WEIGHT
MEASUREMENT
• Atoms have extremely small masses.
• The heaviest known atoms have a mass of
approximately 4 × 10–22 g.
• A mass scale on the atomic level is used,
where an atomic mass unit (amu) is the
base unit.
• 1 amu = 1.66054 × 10–24 g
• Atomic and molecular weight can be
measured with great accuracy using a mass
spectrometer.
• Masses of atoms are compared to the
carbon atom with 6 protons and 6 neutrons
(C-12).
ATOMIC NUMBER, ATOMIC MASS, AND
ISOTOPES
• The atoms of each element have a characteristic number of protons. The number of protons in an
atom of any particular element is called that element’s atomic number.
• Because an atom has no net electrical charge, the number of electrons it contains must equal
the number of protons.
• The atomic number is indicated by the subscript; the superscript, called, the mass number is the
number of protons and neutrons
ATOMIC NUMBER, ATOMIC MASS, AND
ISOTOPES
• Isotopes have the same atomic
number, but different mass numbers.
• Isotopes are atoms of the same
element with the same number of
protons, but different numbers of
neutrons.
• Isotopes of the same element have the
same chemical properties, although
there will be very small differences in
their physical properties.
• Most elements have more than one
isotope. Some isotopes are radioactive.
ATOMIC WEIGHT CALCULATIONS
•Atomic weight is the natural mixture of isotopes
found in nature
•To calculate the atomic weight of an element is to
use the following equation:
Atomic Weight = Σ[(isotope mass) x (fractional
isotope abundance)]
EXAMPLE
Three isotopes of silicon occur in
28
nature: Si (92.23%), atomic mass
29
27.97693 u; Si (4.68%), atomic mass
30
28.97649 u; and Si (3.09%), atomic
mass 29.97377 u. Calculate the atomic
weight of silicon.
THE PERIODIC TABLE
• As the list of elements
began to grow
throughout history,
certain elements were
noticed to have similar
traits as other elements.
• For example, Na and K
had similar properties
while He and Ne also had
similar properties to one
another.
• These similarities
became known as
periodic trends or
families
METALS, NONMETALS, AND METALLOIDS
MOLECULES AND
MOLECULAR COMPOUNDS
• A molecule is an aggregate of two or more
atoms in a definite arrangement held
together by chemical forces.
• A diatomic molecule contains only two
atoms: H2, N2, O2, Br2, HCl, CO
• A polyatomic molecule contains more than
two atoms: O3, H2O, NH3, CH4
• Molecules that contain more than one type
of atom are called molecular compounds
MOLECULAR FORMULA AND EMPIRICAL
FORMULA
• A molecular formula
shows the exact
number of atoms of
each element in the
smallest unit of a
substance.
• An empirical formula
shows the simplest
whole-number ratio of
the atoms in a
substance.
PICTURING MOLECULES
• Structural formulas shows which atoms are attached to which
• Atoms can be represented by their symbols and bonds by lines
• This type of modeling does not show the actual geometry of the molecule

• The ball-and stick model has the advantage of showing the actual
geometry of the atoms in relation to one another.
• The space filling model shows the relative sizes of atoms and their
geometries.
IONS
• An ion is an atom, or group
of atoms, that has a net
positive or negative charge.
• Positive Ion – ion with a
positive charge. If a neutral
atom loses one or more
electrons it becomes a
positive ion. It is called a
cation.
• Negative Ion – ion with a
negative charge. If a neutral
atom gains one or more
electrons it becomes a
negative ion. It is called an
anion.
TYPES OF ION

•A monatomic ion contains only

+ - 2+ 2- 3+
one atom: Na , Cl , Ca , O , Al ,
N 3-

•A polyatomic ion contains more

- -
than one atom: OH , CN , NH4 ,+
NO3 -
IONIC COMPOUNDS
• Ionic compounds are a compounds made up of anions and cations
• Ionic compounds are generally metals and nonmetals.
CRISS-CROSS METHOD
• Since there are not discrete ionic “molecules” we are only
able to write empirical formulas for these compounds
• The criss-cross method take the charges of each ion and
puts it down at the subscript of the opposite ion
NAMING INORGANIC COMPOUNDS
• Chemical Nomenclature comes from the Latin word nomen (name) and
calare (to call)
• The rules of chemical nomenclature are based on the division of substances
into categories
• Organic compounds contain carbon and hydrogen often with oxygen, nitrogen,
and other elements
• All other compounds are inorganic compounds
CATIONS
a) Cations formed from metal atoms have the same name as the metal:

b) If a metal can form cations with different charges, the positive charge is indicated by a Roman
numeral in parentheses following the name of the metal:

An older method still widely used for distinguishing between differently charged ions of a metal
uses the endings -ous and -ic added to the root of the element’s Latin name:
CATIONS
c) Cations formed from molecules
composed of nonmetal atoms have
names that end in -ium:
ANIONS
a) The names of monatomic anions are formed by replacing the ending of the name of the
element with –ide

A few polyatomic anions also have names ending in -ide:

b) Polyatomic anions containing oxygen have names ending in either -ate or -ite and are called
oxyanions. The -ate is used for the most common or representative oxyanion of an element,
and -ite is used for an oxyanion that has the same charge but one O atom fewer:
ANIONS
Prefixes are used when the series of oxyanions of an element extends to four members, as with the
halogens. The prefix per- indicates one more O atom than the oxyanion ending in -ate; hypo-
indicates one O atom fewer than the oxyanion ending in -ite
ANIONS
c) Anions derived by adding H+ to an
oxyanion are named by adding as a
prefix the word hydrogen or
dihydrogen, as appropriate:

Notice that each H+ added reduces the

negative charge of the parent anion by
one. An older method for naming some
of these ions uses the prefix bi-. Thus,
the HCO3- ion is commonly called the
bicarbonate ion, and HSO4- is
sometimes called the bisulfate ion.
NAMING IONIC COMPOUNDS
Names of ionic compounds consist of the cation name followed by the anion name:

In the chemical formulas for aluminum nitrate and copper(II) perchlorate, parentheses followed by
the appropriate subscript are used because the compounds contain two or more polyatomic ions.
YOUR TURN
Name these ionic Provide the formula for
compounds: these ionic compounds:
(a) K2SO4 (a) sodium hydride

(b)Ba(OH)2 (b) lead (II) cyanide

(c) FeCl3 (c) potassium permanganate

(d) ferric chlorate
(d)NH4ClO4
(e) aluminum phosphate
(e)Al2(Cr2O7)3
NAMES AND FORMULAS OF ACIDS
a) Acids containing anions whose names end in -ide are named by changing the -ide ending to -ic,
adding the prefix hydro- to this anion name, and then following with the word acid:

b) Acids containing anions whose names end in -ate or -ite are named by changing -ate to -ic and
-ite to -ous and then adding the word acid. Prefixes in the anion name are retained in the name
of the acid:
YOUR TURN
Name these acids: Provide the formula for
(a) HBr these acids:
(b)HCN (a) acetic acid

(c) HNO3 (b)carbonic acid

(d)H2SO3 (c) hydroiodic acid

(d) nitrous acid
BINARY MOLECULAR COMPOUNDS
The procedures used for naming binary (two-element) molecular compounds are similar to
those used for naming ionic compounds:
1. The name of the element farther to the left in the periodic table (closest to the metals)
is usually written first. An exception occurs when the compound contains oxygen and
chlorine, bromine, or iodine (any halogen except fluorine), in which case oxygen is
written last.
2. If both elements are in the same group, the one closer to the bottom of the table is
named first.
3. The name of the second element is given an -ide ending.
4. Greek prefixes indicate the number of atoms of each element. (Exception: The prefix
mono- is never used with the first element.) When the prefix ends in a or o and the
name of the second element begins with a vowel, the a or o of the prefix is often
dropped.
YOUR TURN
Name these binary Provide the formula for
molecular compounds: these binary molecular
compounds:
(a) PCl5
(a) sulfur hexaflouride
(b) BF3
(b)nitrogen monoxide
(c) Cl2O3 (c) xenon tetraiodide
(d)SO2 (d) tetraarsenic decoxide