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Grade 9
Atoms
 What is an atom?
It is the smallest particle of an element having the same properties of that element.
 Development of the atomic model:
Matter is made up of tiny “indivisible” particles which he called atoms.
1- Democritus
(2400 years ago or 400B.C.)
Atomos = indivisible (in Greek)
2- Dalton
Proposed the Atomic theory.
(1803)
a. All elements are made up of atoms.
b.
Atoms of each element are exactly alike.
c. Atoms of different elements are different.
d. Atoms of two or more elements can combine to form compounds.
Dalton’s model (atom is a hard sphere)
3- J.J Thomson
(1904)
a. Atoms are not simple solid spheres; they are made up of smaller particles.
b. Discovered the electron (negative charge particles and knew its charge
and mass).
c. Suggested the “Plum-pudding model”.
Plum-pudding model:
Atoms are made up of negative charged particles (electrons) embedded in a positively charged jelly-like
fluid sphere like raisins in a cake.
Thomson’s model:
d. Number of negative charges = positive charges of the atom making the
atom neutral (the net charge of the atom = 0).
4- Rutherford: did the following experiment to test Thomson’s model:
(1911)
Rutherford experiment:
4- Rutherford
suggested
a. Protons found in nucleus (size of nucleus is small since very few rays bounce
(1911)
back).
b. Electrons surround the nucleus.
c. Mostly, the atom is empty-spaced.
d. Net charge of the nucleus is +Ze (Z = number of protons).
Rutherford’s model:
5- Bohr
Planetary model
(1913)
Electrons move around the nucleus in orbits (energy level) just like planets
move around the sun.
Bohr’s model:
6- Modern atomic model
(1926)
Electrons are found in the electronic cloud: space around the
nucleus where electrons are most probably to be found.
Fundamental particles of the atom
Atom
Nucleus
Electronic cloud
(Located in the center (very small space)
Protons (+ve)
(Located in the space around the nucleus (very large space)
neutrons (null)
electrons (-ve)
Protons, neutrons and electrons are called the subatomic particles of an atom.
Particle
Symbol
Mass in kg
Atomic mass:
Charge in
Relative
mass in amu
coulomb
charge
Location
Proton
Electron
Neutron
* Coulombs is a unit of electricity.
* amu is atomic mass unit.
*1u → 1.67 x 10-27Kg
Remark:

mass of the atom is located in its nucleus (mass of electron is negligible).

In a neutral atom: Number of protons = number of electrons
 Net charge of atom = 0.
Number
 The atomic number:
Atomic number (Z): it is the number of protons inside the nucleus of an atom. Atomic number is
represented by the capital letter Z.
Z= atomic number= number of protons.
Z is unitless.
Z is the identity of the element.
Since the atom is neutral, number of protons = number of electrons= Z.
Remark:
Nucleic charge is due to the charge of the protons since the charge of neutrons is zero.
q=ze
q:charge of nucleus.
Z:atomic number
e: charge of 1proton (=+1 or 1.6 x 10-19C)
☺ Memorize page 25 & 171 in book.
 The mass number:
Mass number (A): it is the number of protons and neutrons inside the nucleus of an atom. It is represented
by the capital letter A.
A is unitless .
A = Z + N = (number of nucleons)
N: (number of neutrons) ≠ from n0 symbol of neutron.
A is the number of nucleons.
Nucleons: particles inside the nucleus.
Nucleons
Protons
Atomic mass= nuclear mass (mass of e- is negligible).
= mass of nucleons.
= mass number A in u.
Atomic mass=A(u).
Neutrons
 Atomic symbol, (Representation of an atom):
We can represent an atom by writing its nuclide (nuclear symbol)
A
Z
A
Z
X
.
X : A= mass number / z= atomic number/ X is the symbol of the element.
Example:
23
11
Na p=11, n=23-11=12
35
17
Cl
p=17, n= 35-17=18
Another notation is X – A.
Example:
C-12, O-16.
☺ Application:
I.
23
11
Na
Determine the nb of protons, electrons, and neutrons in the above element.
II.
19
9
F
1. Determine the charge of the nucleus; then deduce it in coulombs.
2. Find the charge of the atom.
3. Determine the mass of nucleus; deduce it in Kg.
4. Calculate the atomic mass.
III. Complete the table.:
P+
e-
A
Z
Nuclear
Atomic charge
Mass of nucleus in u
Atomic mass in u
charge
39
19
K
31
15
P
1. Find the mass of 1 phosphorus atom in Kg.
2. Given that an atom has an electric charge of -2.24 x 10-18C, find the name of that atom.
IV. The nucleus of an atom has a charge of + 20.8 x 10-19C, and its atomic mass is 45.09 x 10-27Kg.
1. Determine its atomic number.
2. Determine its mass number.
3. Find the nb of protons, electrons, neutrons.
4. Identify this element.
V. The nucleus of an atom has a charge of + 3.2 x 10-18C.
1. Determine the mass number, knowing that it contains the same nb of protons and neutrons. Identify
this atom and write its nuclide.
2. Indicate the charge of its electronic cloud.
 Exercise p 28, nb I, II & p. 29, nb V.
Extra problems sheet