1. STRUCTURE OF AN ATOM
BACKGROUND KNOWLEDGE

2. Atomic - Molecular Theory of Matter
The Atomic - Molecular Theory of Matter states that all matter is composed of small, fast moving particles called atoms. These atoms can join together to form molecules. This theory is really thousands of individual theories that provide evidence for the whole theory.

3. Matter
Since the atom is too small to be seen even with the most powerful microscopes, scientists rely upon models to help us to understand the atom.

4. Believe it or not this is a microscope
Believe it or not this is a microscope. Even with the world’s best microscopes we cannot clearly see the structure or behavior of the atom.

5. Where did it all begin?
The word “atom” comes from the Greek word “atomos” which means indivisible.
The idea that all matter is made up of atoms was first proposed by the Greek philosopher Democritus in the 5th century B.C.

6. History of the Atom
The concept of atoms as proposed by Democritus remained relatively unchanged for over 2,000 years. In the late 18th century new discoveries were made that led to a better understanding of atoms and chemistry. Many scientists since that time have contributed new evidence for the Atomic - Molecular Theory.
Niels Bohr is one of many scientists that have given us a better understanding of Atoms.

7. WHAT IS AN ATOM? The smallest unit of an element.
Consists of a central nucleus surrounded by one or more electrons.

8. WHAT IS THE NUCLEUS? The central part of an atom.
Composed of protons and neutrons.
Contains most of an atom's mass.

9. WHAT IS A PROTON? Positively charged particle.
Found within an atomic nucleus.

10. WHAT IS A NEUTRON?
Uncharged particle.
Found within an atomic nucleus.

11. WHAT IS AN ELECTRON? Negatively charged particle.
Located in shells that surround an atom's nucleus.

12. Atomic Structure
Nucleus - the central portion of the atom. Contains the protons and neutrons.
Electron Cloud - area around the nucleus where electrons are found. Electrons are arranged within the electron cloud in energy levels (Energy levels are sometimes called shells or orbits).

13. Subatomic Particles
Proton - positive charged particle found in the nucleus. Mass = 1 amu. (a.m.u - Atomic Mass Unit)
Neutron - particle with no charge. Found in the nucleus Mass = 1 amu.
Electron - negative charged particle found within the electron cloud . Mass = 1/1836 amu.

14. Why are all Atoms are Electrically Neutral?
Normally in an atom the number of electrons within the electron cloud is equal to the number of protons in the nucleus. The positive and negative charges cancel each other out. Therefore, the atom is said to be electrically neutral.
If an atom gains or loses electrons the atom is no longer neutral . This can happen if the atom absorbs or releases energy The atom is then called an ION.
IMPORTANT
In all ATOMS the number of positively charged protons is always equal to the number of negatively charged electrons.

16. The Discovery of Electrons
Late 1800’s & early 1900’s
Cathode ray tube experiments showed that very small negatively charged particles are emitted by the cathode material.
1897 – J. J. Thomson
Modified the cathode ray tube and measured the charge to mass ratio of these particles. He called them electrons.
(Nobel prize in physics, 1906)

17. The Discovery of Electrons
1909 – Robert A. Millikan
Determined the charge and the mass of the electron from the oil drop experiment.
(The second American to win Nobel prize in physics in 1923)
1910 – Ernest Rutherford
Gave the first basically correct picture of the atom’s structure.
(Nobel prize in chemistry in 1908)

18. The Discovery of Neutrons
1932 – James Chadwick
recognized existence of massive neutral particles which he called neutrons
(Nobel prize in physics in 1935)
The atomic mass of an element is mainly determined by the total number of protons and neutrons in the nucleus
The atomic number of an element is determined by the total number of protons in the nucleus

19. The Discovery of Protons
1913 – H.G.J. Moseley
Realized that the atomic number defines the element:
Each element differs from the preceding element by having one more positive charge in its nucleus
Along with a number of observations made by Rutherford and some other physicists, this led to the discovery of the proton
The elements differ from each other by the number of protons in the nucleus

20. Rutherford’s Atom The atom is mostly empty space
It contains a very small, dense center called the nucleus
Nearly all of the atom’s mass is in the nucleus
The nuclear diameter is 1/10,000 to 1/100,000 times less than atom’s radius

21. CLASS ASSIGNMENT
Draw a table for the comparison of protons, neutrons and electrons.

23. MASS NUMBER AND ATOMIC NUMBER

24. REPRESENTATION

25. EXAMPLE

26. HOW TO CALCULATE
Neutrons

27. EXAMPLE

28. 11 Na Atomic Number Atomic Number Symbol The atomic number
is specific for each element.
is the same for all atoms of an element.
is equal to the number of protons in an atom.
appears above the symbol of an element. 11 Na
Atomic Number
Symbol

29. Atomic Number and Protons
Examples of atomic number and number of protons:
Hydrogen has atomic number 1; every H atom has one proton.
Carbon has atomic number 6; every C atom has six protons.
Copper has atomic number 29; every Cu atom has 29 protons.
Gold has atomic number 79; every Au atom has 79 protons.

30. Electrons in An Atom An atom
of an element is electrically neutral; the net charge of an atom is zero.
has an equal number of protons and electrons.
number of protons = number of electrons
Aluminum has 13 protons and 13 electrons. The net
(overall) charge is zero.
13 protons (13+) electrons (13 -) = 0

31. Mass Number The mass number
represents the number of particles in the nucleus.
is equal to the number of protons + the number of neutrons.

32. Study Tip: Protons and Neutrons
Number of protons = Atomic number
Number of protons + neutrons = Mass number
Number of neutrons = mass number – atomic number (protons)

33. CLASS ASSIGNMENT An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) ) ) 65
B. How many neutrons are in the zinc atom?
C. What is the mass number of a zinc atom that has
37 neutrons?
1) ) ) 67

34. Solution An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30 (atomic number 30)
B. How many neutrons are in the zinc atom?
2) 35 (65 – 30 = 35)
C. What is the mass number of a zinc atom that has 37 neutrons?
3) 67 ( = 67)

35. An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) ) ) 34
B. Its mass number is

36. Solution An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34 ( = 34)

38. PERIODIC TABLE What are columns of elements called? Columns of elements1 2
groups 3 4 5 6 7

39. Rows of elements What are rows of elements called? periods1 2 3 4 5 6 7

40. Groups and periods

41. How many electrons fit in each shell around an atom?
The maximum number of electrons that
can occupy a specific energy level can be found using the following formula:
Electron Capacity = 2n2
Energy Level Shell Letter Electron Capacity K L M N O P

42. Electrons revolve around the nucleus
in different energy levels or shells
and each shell is associated with
definite energy. The energy of the K shell is the least while those of L, M, N and O shells increases progressively. We also know that any system that has least energy is the most stable.1st energy level is K shell2nd energy level is L shell3rd energy level is M shell4th energy level is N shell and so on

44. Rules
Maximum number of electrons that can be accommodated in a shell is given by 2n2 where
n = shell number
For 1st energy level, n = 1Maximum number of electrons in 1st energy level = 2n22 x (1) 2 = 2
For 2nd energy level n = 2Maximum number of electrons in the 2nd energy level = 2n22 x 22 = 2 x 4 = 8
For 3rd energy level n = 3Maximum number of electrons in the 3rd energy level = 2n2= 2x(3) 2= 2 x 9 = 18
For 4th energy level n = 4Maximum number of electrons in the 4th energy level = 2n2= 2x(4) 2= 2x16 = 32

45. Octet Rule
The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the Octet rule. The presence of 8 electrons in the outermost shell makes the atom very stable.

46. Electronic Configurations of Some Important Elements

47. Geometric Representation of Atomic Structure Example 1 : Magnesium Atomic number:12 Mass Number:24

48. Steps:
The first 2 electrons will go to the 1st shell = K Shell (2n2)
The next shell L takes a maximum of 8 electrons (2n2)
In this way = 10 electrons have been accommodated. The next 2 electrons go to the M Shell.

49. Class Assignment
Draw the geometric representation of first eighteen elements.

50. Check the structures