Download Atomic structure

History of the Atom
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small
particles called atoms. All atoms of a given
element are identical. The atoms of one element
are different from the atoms of all other elements.
2. Compounds are composed of atoms of more
than one element. The relative number of atoms
of each element in a given compound is always
the same.
3. Chemical reactions only involve the rearrangement
of atoms. Atoms are not created or destroyed in
chemical reactions.
2.1
16 X
+
8Y
8 X2Y
2.1
HISTORY OF THE ATOM
1904
Thompson found that atoms could sometimes eject a far
smaller negative particle which he called an Electron.
Thompson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere surrounded
by a soup of positive charge to balance the electron's charge
like plums surrounded by pudding.
PLUM PUDDING
MODEL
1.In 1897 JJ.Thomson devised an experiment with cathode rays that resulted in the discovery of
the electron.
2.Up to this time it was thought that the hydrogen atom was the smallest particle in existence.
Thomson demonstrated that electrons comprising cathode rays were nearly 2,000 times smaller in
mas than the lightest known particle the hydrogen ion.
3.When a high voltage is placed across a pair of plates, they become charged relative to each
other. The positively charged plate is the anode and the negatively charged plate the cathode.
4.Electrons pass from the surface of the cathode and accelerate toward the oppositely charged
anode. The anode absorb many electrons, but is the anode has slits some electrons will pass
through.
5.The electron travel into an evacuated tube, where they move in a straight line until striking a
fluorescent screen. this screen is coated with a chemical that glows when electrons strike it.
J.J. Thomson, measured mass/charge of e(1906 Nobel Prize in Physics)
2.2
HISTORY OF THE ATOM
1910
Ernest Rutherford
Fired alpha particle at a piece of gold foil which was only
a few atoms thick.
they found that although most of them passed through.
About 1 in 10,000 hitRutherford’s new evidence allowed
him to propose a more detailed model with a central
nucleus.
He suggested that the positive charge was all in a central
nucleus. With this holding the electrons in place by
electrical attraction
(1908 Nobel Prize in Chemistry)
 particle velocity ~ 1.4 x 107 m/s
(~5% speed of light)
1. atoms positive charge is concentrated in the nucleus
2. proton (p) has opposite (+) charge of electron
3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)
2.2
HELIUM ATOM
Shell
proton
+
-
N
N
+
electron
What do these particles consist of?
-
neutron
ATOMIC STRUCTURE
Particle
Charge
Mass
proton
+ charge
1
neutron
No charge
1
electron
- charge
nil
Atoms have no overall electrical charge so,
an atom must have as many electrons as there are protons
in its nucleus.
The atomic number of an element also equals the number of
electrons
in a neutral atom of that element.
Atomic Number (Z): is the number of protons
in the nucleus of the atom.
Z=#p
The number of protons (atomic number) determine
the identity of an element.
The atomic number of an element also equals
the number of electrons
in a neutral atom of that element.
Ex: Sodium
• What is the atomic number of Sodium?
• How many protons does sodium have?
• How many electrons does sodium have?
Mass Number (A):
• The sum of the protons and neutrons in the
nucleus.
A=#p + #n
ATOMIC STRUCTURE
He
2
4
Atomic number
the number of protons in an atom
Atomic mass
the number of protons and
neutrons in an atom
number of electrons = number of protons
14
Atomic Mass Unit
is a unit used to compare the masses of atoms
and has the symbol u or amu.
1 AMU is approximately equal to the mass of a single
proton or neutron.
Isotopes
of an element have
different mass numbers
because
they have
different numbers of neutrons,
but they have
the same atomic number.
Atomic number (Z) = number of protons in nucleus
Mass number (A) = number of protons + number of neutrons
= atomic number (Z) + number of neutrons
Isotopes are atoms of the same element (X) with different
numbers of neutrons in the nucleus
Mass Number
A
ZX
Atomic Number
1
1H
235
92
2
1H
U
Element Symbol
(D)
238
92
3
1H
(T)
U
2.3
Do You Understand Isotopes?
How many protons, neutrons, and electrons are in 146
C?
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are in 116
C?
6 protons, 5 (11 - 6) neutrons, 6 electrons
2.3