1. Chapter 4
Atomic Structure

2. Early Models of the Atom
Democritus’s Atomic Philosophy
Greek philosopher (460 B.C.-370 B.C.)
1st to suggest the existence of atoms
Said that atoms were indivisible and indestructible

3. Early Models of the Atom
Dalton’s Atomic Theory
English chemist and school teacher ( ).
1. All elements are composed of indivisible ‘solid sphere’ atoms.
All atoms of a given element are identical.
Atoms are different elements differ in their masses.
Different atoms combine in simple whole number ratios to form compounds.

4. Defining the Atom All elements are composed of atoms
All atoms of the same element are identical
Atoms of different elements combine to form compounds
Atoms of one element can never be changed into another element

5. Structure of the Nuclear Atom
Atoms are divisible
Three kinds of subatomic particles are:
Electrons
Protons
Neutrons

6. Electrons Orbit the nucleus of the atom Have a negative (-1) charge
Mass ~0 atomic mass unit (amu)
Shorthand symbol (e-)
J.J. Thomson discovered the
electron using the cathode-ray
tube

7. A cathode ray produced in a discharge tube travels from left to right
A cathode ray produced in a discharge tube travels from left to right. The ray itself is invisible, but the fluorescence of a zinc sulfide coating on the glass causes it to appear green
CATHODE RAY

8. When the polarity of the magnet is reversed, the ray bends in the opposite direction
CATHODE RAY

9. The cathode ray is bent downwar d when the south pole of the bar magnet is brought toward it

10. JJ Thomson’s Atom
The plum pudding model

11. 2. Protons Charge of +1 (positive) Mass of 1 amu (atomic mass unit)
Located inside the nucleus
Shorthand symbol (p+)

12. 3. Neutrons Consists of charge of 0 (neutral) Mass of 1 amu
Located inside the nucleus
Shorthand symbol (n0)

13. Discovering the Atomic Nucleus
When the particles were discovered, scientists wondered how these particles were put together.
Ernest Rutherford made it possible to understand how the subatomic particles were assembled

14. Figure: 02-10

15. Figure: 02-11

16. Figure: 02-12

17. Rutherford’s Gold-Foil Experiment
Proposed that the atom is made up of empty space
The mass and positive charge of the atom is located in the nucleus (a dense region)

18. The Nuclear Atom Nucleus Contains positive electrical charge
Makes up the most of the mass of the atom
There are two subatomic particles: protons and neutrons
The electrons are distributed around the nucleus and occupy almost all the volume of the atom

19. Figure: 02-08

20. Figure: 02-04

21. Atomic Number
Elements are different because they contain different number of protons
Atomic number is the number of protons in the nucleus of an atom of that element
Atomic Number = protons or electrons
Is usually written as a subscript
Ex: 12Mg or Mg12

22. What element has 11 protons?
How many protons does potassium have?
Sodium 19

23. Complete the Table Element Atomic Number Protons Electrons K 19 5 S 16V 23

24. Mass Number Mass Number = Protons + Neutrons
Mass number is also known as atomic mass
The number of neutrons in an atom is the difference between the mass number and the atomic number
# of neutrons = mass number – atomic number
Usually located at the top of the element symbol
Ex: 12C C12 C-12

25. Sample Problems
How many protons, electrons, and neutrons are in each atom?
Beryllium (Be) -
Neon (Ne) -
Sodium(Na) -
4, 4, 5
10, 10, 10
11, 11, 12

26. How many neutrons are in each atom?
47Ag108
82Pb207
16 – 8 = 8
108 – 47 = 61
207 – 82 = 125

27. Express the composition of each atom in shorthand form.
Nitrogen -14
Sodium - 23
Phosphorus -31
p+ = 7, n0 = 7, e- = 7
p+ = 11, n0 = 12, e- = 11
p+ = 15 , n0 = 16, e- = 15

28. Atomic #
Mass #
# of proton
# of Neutron
# of e-
Symbol 9 10 14 15 47 22 55 25

29. Isotopes

30. Sample Exercise
Determine the number of protons, neutrons and electrons in the following:
1H1
1H2
1H3
p+ = 1, n0 = 0, e- = 1
p+ = 1, n0 = 1, e- = 1
p+ = 1, n0 = 2, e- = 1

31. Natural Abundance of Stable Isotopes of Some Elements
Name
Symbol
Natural Percent Abundance
Mass (amu)
Average Atomic Mass
Nitrogen
7N14
7N15
99.63
0.37
14.003
15.000
14.007
Chlorine
17Cl35
17Cl37
75.77
24.23
34.969
36.966
35.453
Helium
2He3
2He4
0.0001
3.0160
4.0026
Sulfur
16S32
16S33
16S34
16S36
95.002
0.76
4.22
0.014
31.972
32.971
33.967
35.967
32.06

32. Uncovering Atomic Mass
Atomic mass # is an average of atom’s naturally occurring isotopes
Copper has 2 isotopes Cu -63 and Cu Which isotope is most abundant?
There are 3 isotopes of Silicon with mass numbers of 28, 29, and 30. Which is more abundant? 63
Si -28

33. Practice Exercise
Boron has two isotopes: boron – 10 and boron – 11. Which is more abundant, given that the atomic mass of boron is 10.81?
Answer: B-11

34. Calculating Atomic Mass
Multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

35. Sample Exercise
An element consists of two isotopes. Isotope A has an abundance of 75%, and its mass is a.m.u. Isotope B has an abundance of 25% and its mass is a.m.u. What is the atomic mass of the element?
Ans: 14.25

36. Sample Exercise
Boron in nature has two isotopes. Boron -10 has a mass of a.m.u. and abundance is 19.9%. B – 11 has a mass of a.m.u., and abundance is 80.1%. Calculate the atomic mass of boron.
Ans: a.m.u

37. Sample Exercise
There are four isotopes of Lead: Pb – 204, Pb – 206, Pb – 207, Pb – Their weights are 1.37%, 26.26%, 20.82%, and 51.55% respectively. Using this data calculate the atomic mass of lead.

38. Calculate the atomic mass of bromine
Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of amu (50.69%) and amu (49.31%).

39. Ions An atom with an electrical charge Occurs 2 ways:
Oxidation – loss of an e-
results in a (+) charge
Reduction – gain of an e-
results in a (-) charge
Mnemonic: ‘LEO says GER’
Loss of electrons = oxidation
Gain of electrons = reduction

40. Examples of Ions Lost 1e- H+ Cl- As3- Gained 1e- Mg2+ Gained 3e-
Oxidation State –
Number found on the periodic table
Shows the number of e- gained or lost
Many elements have more than one number
Ions & Subatomic Particles
Lost 1e-
Gained 1e-
Gained 3e-
Lost 2e-

41. Review and Assessment
Would you expect two electrons to attract or repel each other?
What charge does the atom become when it loses/gains an electron?
What were the results of Rutherford’s experiment?
What is the charge – positive or negative – of the nucleus of every atom?

42. Review and Assessment
Which subatomic particles are located in the nucleus?
Name two ways that isotopes of an element can differ?
What is the atomic mass of an element?
How are the elements arranged in the modern periodic table?