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1. ATOMS
Presentation by
Ruby group

4. Atomic Structure
 Atoms are composed of 2 regions:
 Nucleus: the center of the atom that contains the mass of the
atom
 Electron cloud: region that surrounds the nucleus that contains
most of the space in the atom
Nucleus
Electron
Cloud

5. Three subatomic particles make up every atom:
Subatomic
Particle
Charge Location Charge
(coulombs)
mass
Proton Positive (+) Nucleus or
“Core”
+1.6 x 10-19 1.67 x 10-24
Neutron No Charge (0) Nucleus or
“Core”
0 1.67 x 10-24
Electron Negative (-) Electron Cloud -1.6 x 10-19 9.1 x 10-28

6. Notice the smaller
particles that make
up this neutron
after you take a
closer look.
+
Notice the smaller
particles that make up
this proton after you
take a closer look.

8. Democritus (460 - 370 BC)

10. 4. Atoms cannot be created, divided into smaller
particles, or destroyed. The law of conservation of mass
**In a chemical reaction, atoms of different elements are separated,
joined or rearranged. They are never changed into the atoms of
another element.
5. Atoms combine in definite whole number ratios to
make compounds (you can’t have a ½ of a Carbon bonding
with Oxygen; it’s a whole atom or no atom)
Dalton’s Atomic Theory (1803)

11. Atomic Theory
TimelineScientist Information Model
John
Dalton
All matter is made of
atoms. Atoms are too
small to see, indivisible
and indestructible. All
atoms of a given element
are identical. And are not
further divisible.

12. John Dalton (1766-1844)
• Dalton is the “Father of Atomic
Theory”
• John Dalton was first modern
scientist to give the concept of
atom
• Dalton’s ideas were so brilliant
that they have remained
essentially intact up to the
present time and has only
been slightly corrected.

13. Atomic Theory
TimelineScientist Information Model
J.J
Thompson
Discovered the negative
electron, and predicted
that there also must be a
positive particle to hold
the electrons in place.

14. JJ Thomson (1856-1940)
• Used cathode rays to
prove that Dalton’s Solid-
ball model could be broken
into smaller particles
•Thomson is credited with
discovering electrons

15. Thomson’s Plum Pudding Model
•Thomson’s Plum Pudding model is a + charge
sphere that has (- )charged electrons scattered
inside, like “raisins” in “plum pudding”.
•Overall, the atom is neutral atom because the
atom had the same number of positive and
negative charges.
•From Thomson’s experiments, scientists concluded that
atoms were not just neutral spheres, but somehow were
composed of electrically charged particles.

16. Atomic Theory
TimelineScientist Information Model
Ernest
Rutherford
Discovered the nucleus of an
and named the positive particles
in the nucleus “protons”.
Concluded that electrons are
scattered in empty space around
the nucleus.

17. Ernest Rutherford (1871-1937)
Learned physics in J.J.
Thomson’ lab.
Noticed that ‘alpha’
particles were
sometime deflected
by something in the
air.
Gold-foil experiment

18. Rutherford’s alpha scattering experiment.
source of
alpha particles
thin gold foil
photographic film
beam of a particles
most pass
straight
through
1 in 20 000
deflected

19. Rutherford’s explanation.
atoms of gold
in gold foil
Path of an a particle
nucleus of
gold atom
All these a particles pass
straight through the empty
space that makes up most of
the atom.
This a particle
passes close
enough to
nucleus to be
repelled
As a particles are positively charged,
the nucleus must contain positively
charged protons.

20. atoms of gold
in gold foil
nucleus of
gold atom
As only 1 in every 20 000 a
particles are repelled the
nucleus must be very small.
Size of the nucleus.

21. Rutherford’s model of the atom.
Tiny nucleus
containing the
positively charged
protons.
Rest of atom is mostly empty
space. The negatively charged
electrons are spinning round
the nucleus.

22. Review of the Rutherford's experiment
What were Rutherford’s conclusions from the Gold Foil Experiment?
 Atom has a positively charged nucleus
 electrons are outside,
 atoms are mostly empty
 Nucleus contains most of the mass.

23. Rutherford’s Planetary Model
• To explain his
observations, Rutherford
developed a new model
called atomic planetary
model
•The electrons orbit the
nucleus like the planets
revolve around the sun.

24. Atomic Theory Timeline
Scientist Information Model
James
Chadwick
Discovered that neutrons were also
located in the nucleus of an atoms
and that they contain no charge.
Neutron
s

25. Atomic Theory
TimelineScientist Information Model
Neils
Bohr
Concluded that electrons
are located in planet-like
orbits around the nucleus
certain energy levels.

26. Neils Bohr (1885-1962)
Worked in Rutherford’s lab
Wondered why – electrons are not
attracted to the + nucleus and
cluster around it
Disproved Rutherford’s Planetary
Model
Experimented with light and its
interaction with matter to develop
a new model.

27. Bohr’s Energy Level Model
Energy Level Model: Electrons are arranged in
circles around the nucleus. Each circle has a
different energy.
•Electrons are in constant motion, traveling
around the circle at the speed of light.
•Electrons can “jump” from one circle to the next
•But they can’t go to the nucleus they traveling
too fast to be fully attracted.

28. Nucleus
1st shell
2nd shell
3rd shell

29. How to calculate the number of
electrons in each shell?
There is a formula to calculate it, the formula is 2n^2.
For first shell the number of electron will be= 2 x 1^2 = 2 electrons

30. ELECTRONS IN ORBIT ABOUT
THE NUCLEUS

31. Atomic Structure

32. Bohr’s Energy Level Model
 He proposed the following:
4. The electrons must absorb or emit a fixed amount of energy
to travel between these energy levels. He also purposed the idea
of shells around.
3. The electrons orbit the nucleus at fixed energy levels.
2. Electrons can only be at certain distances from the nucleus.
1. Protons and neutrons are in the nucleus

33. Scientist Information Model
(Many
Scientists!)
The Modern
Atomic Theory
Electrons do not orbit the
nucleus in neat planet-like
orbits but move at high
speeds in an electron
around the nucleus.
Atomic theory timeline

34. Atomic TheoryChanges over time…

35. Symbols of the element of an atom
First idea of symbols was given by Dalton. He used some symbols as given
below.

36. Modern symbols
Symbols used by Dalton were very complicated to be used in a chemical
equations. Therefore modern symbols are written from their starting first, first
and second or by first and third letter
For example
(i) hydrogen, H
(ii) aluminium, Al and not AL
(iii) cobalt, Co and not CO.

37. What is atomic mass?
Atomic mass is the mass of an single atom. It is dependent on
mass of protons and neutrons of the atom but not on electrons
because it’s mass is negligible
Atomic mass

38. What is atomic number?
Atomic number is equal to number of protons in the atom of
element. The elements are arranged according to the atomic
number in the periodic table.
Atomic number

40. Step 1
Use your periodic table to find
an element

41. Step 2
 To find The number of Protons (+ charge)
 The atomic number = number of protons
 Therefore, Kr has 36 protons

42. Step 3
 To find The number of Electrons (- charge)
 Remember atoms have no overall charge.
 Atoms must have an equal number of protons and
electrons
 Number of protons= number of electrons
 Therefore, Kr has 36 electrons

43. Step 4
To find The number of Neutrons (no charge)
Check the mass of the element
 Neutrons = atomic Mass of the element – number of Protons
Neutrons = 84 – 36
Neutrons = 48
Therefore number of neutrons are 48

44. conclusion

47. Gravitational Force
 The force of attraction of
objects due to their masses
 The amount of gravity
between objects depends on
their masses and the distance
between them

48. Electromagnetic Force
 The force that results from
the repulsion of like charges
and the attraction of opposites
 The force that holds the
electrons around the nucleus
-
+
+
+
--
Notice how the particles
with the same charge move
apart and the particles with
different charges move
together.

49. Strong Force
 The force that
holds the atomic
nucleus together
 The force that
counteracts the
electromagnetic
force
+
+
+
+
Notice how the electromagnetic
force causes the protons to repel
each other but, the strong force
holds them together.

50. -
n
Weak Force
 This force plays a key role in the
possible change of sub-atomic
particles.
 For example, a neutron can change into
a proton(+) and an electron(-)
 The force responsible for radioactive
decay.
 Radioactive decay  process in which
the nucleus of a radioactive (unstable)
atom releases nuclear radiation.
+
Notice how the original
particle changes to something
new.