Download Atomic Structures Part

STRUCTURE OF AN ATOM
BACKGROUND KNOWLEDGE
Atomic - Molecular
Matter
Theory of
The Atomic - Molecular Theory of Matter states
that all matter is composed of small, fast
moving particles called atoms. These atoms
can join together to form molecules.
This theory is really thousands of individual
theories that provide evidence for the whole
theory.
Matter
• Since the atom is too small to be
seen even with the most powerful
microscopes, scientists rely upon
models to help us to understand the
atom.
• Believe it or not this
is a microscope.
Even with the world’s
best microscopes we
cannot clearly see
the structure or
behavior of the atom.
Where did it all begin?
The word “atom” comes
from the Greek word
“atomos” which means
indivisible.
The idea that all matter
is made up of atoms
was first proposed by
the Greek philosopher
Democritus in the 5th
century B.C.
History of the Atom
• The concept of atoms as
proposed by Democritus
remained relatively unchanged
for over 2,000 years. In the late
18th century new discoveries
were made that led to a better
understanding of atoms and
Niels Bohr is one of many
chemistry. Many scientists since
scientists that have given us a
that time have contributed new
better understanding of Atoms.
evidence for the Atomic Molecular Theory.
WHAT IS AN ATOM?
o The smallest unit of
an element.
o Consists of a central
nucleus surrounded
by one or more
electrons.
WHAT IS THE NUCLEUS?
o The central part of
an atom.
o Composed of
protons and
neutrons.
o Contains most of an
atom's mass.
WHAT IS A PROTON?
o Positively charged
particle.
o Found within an
atomic nucleus.
WHAT IS A NEUTRON?
o Uncharged particle.
o Found within an
atomic nucleus.
WHAT IS AN ELECTRON?
o Negatively charged
particle.
o Located in shells
that surround an
atom's nucleus.
Atomic Structure
Nucleus - the central portion of the atom.
Contains the protons and neutrons.
Electron Cloud - area around the nucleus
where electrons are found. Electrons are
arranged within the electron cloud in energy
levels (Energy levels are sometimes called
shells or orbits).
Subatomic Particles
Proton - positive charged particle found
in the nucleus. Mass = 1 amu. (a.m.u Atomic Mass Unit)
Neutron - particle with no
charge.
Found in the nucleus.
Mass = 1 amu.
Electron - negative charged particle
found within the electron cloud . Mass
= 1/1836 amu.
Why are all Atoms are
Electrically Neutral?
• Normally in an atom the
number of electrons within
the electron cloud is equal
to the number of protons in
the nucleus. The positive
and negative charges
cancel each other out.
Therefore, the atom is said
to be electrically neutral.
• If an atom gains or
loses electrons the
atom is no longer
neutral . This can
happen if the atom
absorbs or releases
energy The atom is
then called an ION.
IMPORTANT
In all ATOMS the number of positively
charged protons is always equal to
the number of negatively charged
electrons.
Particle
Mass (amu)
Charge
Electron (e-)
0.00054858
-1
Proton (p,p+)
1.0073
+1
Neutron(n,n0)
1.0087
0
The Discovery of Electrons
• Late 1800’s & early 1900’s
Cathode ray tube experiments showed that
very small negatively charged particles are
emitted by the cathode material.

1897 – J. J. Thomson
Modified the cathode ray tube and measured the charge to
mass ratio of these particles. He called them electrons.
(Nobel prize in physics, 1906)
16
The Discovery of Electrons


1909 – Robert A. Millikan
Determined the charge and the mass of the electron from
the oil drop experiment.
(The second American to win
Nobel prize in
physics in 1923)
1910 – Ernest Rutherford
Gave the first basically correct picture of the atom’s
structure.
(Nobel prize in chemistry in 1908)
17
The Discovery of Neutrons

1932 – James Chadwick
recognized existence of massive neutral particles which
he called neutrons
(Nobel prize in physics in 1935)
 The atomic mass of an element is mainly determined
by the total number of protons and neutrons in the
nucleus
 The atomic number of an element is determined by
the total number of protons in the nucleus
18
The Discovery of Protons

1913 – H.G.J. Moseley
Realized that the atomic number
element:
defines the
 Each element differs from the preceding element by
having one more positive charge in its nucleus

Along with a number of observations made by
Rutherford and some other physicists, this led to the
discovery of the proton
 The elements differ from each other by the number
of protons in the nucleus
19
Rutherford’s Atom
 The atom is mostly empty space
 It contains a very small, dense center
called the nucleus
 Nearly all of the atom’s mass is in the
nucleus
 The nuclear diameter is 1/10,000 to
1/100,000 times less than atom’s radius
20
CLASS ASSIGNMENT
• Draw a table for the comparison of
protons, neutrons and electrons.
MASS NUMBER AND ATOMIC
NUMBER
REPRESENTATION
EXAMPLE
HOW TO CALCULATE
Neutrons
EXAMPLE
Atomic Number
The atomic number
• is specific for each element.
• is the same for all atoms of an element.
• is equal to the number of protons in an atom.
• appears above the symbol of an element.
Atomic Number
Symbol
11
Na
Atomic Number and Protons
Examples of atomic number and number of protons:
• Hydrogen has atomic number 1; every H atom has
one proton.
• Carbon has atomic number 6; every C atom has six
protons.
• Copper has atomic number 29; every Cu atom has
29 protons.
• Gold has atomic number 79; every Au atom has 79
protons.
Electrons in An Atom
An atom
• of an element is electrically neutral; the net charge of
an atom is zero.
• has an equal number of protons and electrons.
number of protons = number of electrons
Aluminum has 13 protons and 13 electrons. The net
(overall) charge is zero.
13 protons (13+) + 13 electrons (13 -) = 0
Mass Number
The mass number
• represents the number of particles in the nucleus.
• is equal to the number of protons + the number of
neutrons.
Study Tip: Protons and
Neutrons
Number of protons = Atomic number
Number of protons + neutrons = Mass
number
Number of neutrons = mass number – atomic
number
(protons)
CLASS ASSIGNMENT
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
2) 35
3) 65
B.
How many neutrons are in the zinc
atom?
1) 30
2) 35
3) 65
C. What is the mass number of a zinc atom
that has
37 neutrons?
1) 37
2) 65
3) 67
Solution
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
(atomic number 30)
B. How many neutrons are in the zinc
atom?
2) 35
(65 – 30 = 35)
C. What is the mass number of a zinc atom that
has 37 neutrons?
3) 67
(30 + 37 = 67)
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
Solution
An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34 (14 + 20 = 34)
PERIODIC TABLE
What are columns of elements called?
Columns of elements
1
2
groups
3
4
5
6
7
0
Rows of elements
What are rows of elements called?
1
2
3
4
5
6
7
•
periods
Groups and periods
• How many electrons fit in each shell
around an atom?
• The maximum number of electrons that
can occupy a specific energy level can be found
using the following formula:
•
Electron Capacity = 2n2
• Energy Level
Capacity
•
1
•
2
•
3
•
4
•
5
•
6
Shell Letter
K
L
M
N
O
P
Electron
2
8
18
32
50
72
• Electrons revolve around the nucleus
in different energy levels or shells
and each shell is associated with
definite energy. The energy of the K shell
is the least while those of L, M, N and O
shells increases progressively. We also
know that any system that has least
energy is the most stable.1st energy level
is K shell2nd energy level is L
shell3rd energy level is M shell4th energy
level is N shell and so on
Rules
• Maximum number of electrons that can be
accommodated in a shell is given by 2n2 where
n = shell number
• For 1st energy level, n = 1Maximum number of
electrons in 1st energy level = 2n22 x (1) 2 = 2
• For 2nd energy level n = 2Maximum number of
electrons in the 2nd energy level = 2n22 x 22 = 2 x 4 =
8
• For 3rd energy level n = 3Maximum number of
electrons in the 3rd energy level = 2n2= 2x(3) 2= 2 x 9
= 18
• For 4th energy level n = 4Maximum number of
electrons in the 4th energy level = 2n2= 2x(4) 2= 2x16
= 32
Octet Rule
• The outermost shell of an atom cannot
accommodate more than 8 electrons, even
if it has a capacity to accommodate more
electrons. This is a very important rule and
is also called the Octet rule. The presence
of 8 electrons in the outermost shell
makes the atom very stable.
Electronic Configurations of Some
Important Elements
Geometric Representation of
Atomic Structure
Example 1 : Magnesium
Atomic number:12
Mass Number:24
• Steps:
• The first 2 electrons will go to the 1st shell
= K Shell (2n2)
• The next shell L takes a maximum of 8
electrons (2n2)
• In this way 2 + 8 = 10 electrons have been
accommodated. The next 2 electrons go to
the M Shell.
Class Assignment
• Draw the geometric representation of
first eighteen elements.
Check the structures