3. Matter is made of atoms (cannot be further divided), the word atom
comes from the Greek word “atomos” which means indivisible.
Democritus
4. • Atoms were tiny particles that could not be divided
• John Dalton proposed that an atom is a sphere of
matter that is the same throughout and can’t be seen
by human eyes
• Marble Model
5. • Came up with the idea of the electron negatively charged particle
• Thomson discovered that all atoms contain electrons, which are tiny,
negatively charged particles. Thomson proposed that an atom is a
sphere of positive charge. The electrons are mixed uniformly in the
sphere.
• Cookie Model
6. Ernest Rutherford
• He hypothesized that almost all the mass and all the
positive charge of an atom is concentrated in an extremely
tiny nucleus at the center of the atom.
7. • Thought electrons traveled in fixed paths around the
nucleus called energy levels.
12. •They are the negatively charged particles orbiting the
nucleus.
•It is much smaller than the protons and the neutrons .
13. What is the net charge of
the atom ?
• The atom is formed of positively charged proton in the
nucleus and negatively charged electrons in the energy
level ,but they are equal to each other in number ,making
the net charge of the atom “neutral”.
No of positive protons = No of negative electrons
14. • They are charged atoms
• Number of positive protons = Number of negative electrons.
Positive
ion
Negative
ion
15. If the atom lost an electron ,therefore the number of positive protons
are more than the negative electrons making the atom positively
charged
Positive Ion
16. If the atom gained an electron ,therefore the number of negative
electrons
are more than the positive protons making the atom negatively
charged
Negative Ion
17. • Atomic number is the number of Protons in an atom
• All atoms of a particular element have the SAME number of protons (All
Carbon atoms have 6 protons)
• In a neutral atom,
The atomic number =The number of electrons
18. • Also called atomic weight or Mass Number
• Sum weight of protons and neutrons in an atom
21. Calculating Neutrons:
• Mass number = protons + neutrons
so…..
• Neutrons = mass number – protons
What is the number of neutrons of the following atom ?
A
21
45
22. • In 1869 Dimitri Mendeleev was working on a way to
classify elements.
•At the time, more than 60 elements had been
discovered.
•And he arranged the elements according to their
atomic mass
Developing a Periodic Table
23. 100 table The periodic table is
a chart of the
elements arranged
according to their
atomic number in
rows and columns
according to their
physical and chemical
properties.
27. •The periodic table is a chart of the elements
arranged into rows and columns according to
their physical and chemical properties.
•It can be used to determine the relationships
among the elements.
What is the periodic table?
28. Groups and Periods
- Columns in the periodic table represents “groups”
- There are 18 groups in the periodic table.
- Elements of the same the same group have the same
chemical properties as the all have the same number of
electron/s in the outer most energy levels.
29. - Rows in the periodic table represent “periods”.
- There are 7 periods in the periodic table .
- Elements of the same period have the same number
of energy levels
31. Periodic Table
A great deal of information about an element can be gathered
from its position on the period table.
Understanding the organization and plan of the periodic table will
help you obtain basic information about each of the 118 known
elements.
32. Mass Number
Atomic Mass (Mass Number) is the number
…………….
__________
Atom p+ n0 e- Mass #
Oxygen - 8
- 33 42
- 31 15
___ ___ ______
______ ___ ___ ___
______ ___ ______
33. Recall - How are P, N, e- related?
• atomic number = # protons
• # protons = …………….
• # protons + # neutrons = atomic mass
# neutrons = …………….
…………….
34. • How many protons, neutrons and electrons are found in
an atom of Cs?
• _____________
• …………….
• …………….
• Therefore, # Neutrons = …………….
_____________
_____________
55
Cs
133
Example 1
36. Example 2
If an element :Magnesium
a) number of protons
b) number of neutrons
c) number of electrons
d) complete symbol
=_____________
= _____________
= _____________
_____________
43. Compounds-Are substances made of atoms of two or more elements
that are chemically combined.
-The chemical formula of a compound shows the elements
that are in it
-Example:
SiO₂ (sand): It is formed of 1 Silicon atom
2 oxygen atoms
44. -Compounds don’t have the same properties of the
elements forming them
Example:
The ordinary table salt is a compound called NaCl
It is formed of :
1 atom of Sodium and 1 atom of chlorine
Where “Sodium” is a metal
45. • And “Chlorine” is a gas
But when they combine together they give a product of
different properties which is the table salt
