atomic theory

1. Atomic Theory

2. Greek Philosophers
•Aristotle
4Elements of matter
-Earth
-Fire
-Air
-Water

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.

8. Atomic Structure
• Arrangement and number of sub-atomic particles
•Protons
•Neutrons
•Electrons

9. •Center or core of an atom
•Contains 99.9% of the weight of an atom
•Contains protons and neutrons

10. •Positively Charged Sub-atomic particles
•Found in the nucleus
•All protons are a like

11. •Neutrally charged (no charge) sub-atomic particle
•Found in nucleus

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

19. Calculate the mass number of each of
the following atoms

20. O
8
16
Atomic
number
Mass
number
Symbol
OxygenName
Atomic number= Number of protons= Number of electrons
Mass number= Number of protons + Number of neutrons

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. • Atoms of the same element with different numbers of neutrons.