1. The Structure of the Atom

2. Objectives
Compare and contrast the atomic models of Democritus and Dalton
Define an atom.
Distinguish between the sub atomic particles in terms of relative charge and mass.
Describe the structure of the nuclear atom, including the locations of the subatomic particles.

3. Objectives
Explain the role of atomic number in determining the identity of an atom.
Define an isotope and explain why atomic masses are not whole numbers.
Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.

4. Dalton’s Atomic Theory
Each element consists of individual particles called atoms
Atoms can neither be created nor destroyed
All atoms of a given element are identical
Atoms combined chemically in definite whole-number ratios to form compounds
Atoms of different elements have different masses

5. History
The electron was discovered in 1897 by Thomson. He imagined the atom as a “raisin pudding” with electrons stuck in a cake of positive charge
In 1912, Rutherford discovered the nucleus (proton). He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun

6. History
Rutherford’s gold foil experiment was used to show that the nucleus was small, dense, and positively charged
Thomson used the cathode ray to discover electrons
The neutron was then discovered in 1932 by Chadwick.

7. Subatomic Particles and the Nuclear Atom

8. Development of Atomic Structure
• About 1900 Sir J. J. Thomson and Earnest Rutherford established the modern model of the atom.
• The atom is composed of three subatomic particles.
1. Electrically positive (+1) protons.
2. Electrically neutral (0) neutrons.
3. Electrically negative (-1) electrons.
• Atoms have a net charge of zero.
• The number of protons equals the number of electrons.

9. Parts of the atom

10. Parts of an Atom
Each element in the Periodic Table has a different number of protons in its nucleus
Change the number of protons  change elements
The element also has the same number of electrons
Change the number of electrons  ionize the element
Some elements also have neutrons
They are in the nuclei of atoms p e n

11. 1H The Hydrogen Atom One electron orbiting a nucleus
1 proton = Z = atomic number
0 neutrons = N
Total mass = A = Z+N =1
Singly ionized Hydrogen is missing one electron = 1H+
Add a neutron and you have Deuterium = 2H = D p e 1H

12. 4He The Helium Atom Two electrons orbiting a nucleus
2 protons = Z = atomic number
2 neutrons = N
Total mass = A = Z+N =4
Singly ionized Helium is missing one electron = 4He+ e p n n p e
4He

13. 3H (Tritium) 3He Isotopes and Elements
If Helium loses one of its protons (and one of its electrons), it becomes a different element p n n
3H (Tritium) p n e
If Helium loses one of its neutrons, it becomes an isotope
3He

14. Why Learn about Atomic Structure?
Knowing the structure of atoms tells us about their
chemical properties
light-emitting properties
light-absorbing properties
From this information we can learn about galaxies, stars, planets, asteroids, based on the light they emit or reflect.
An example of absorption spectra
from many different types of stars.

15. How Atoms Differ
Section 4.3

16. Atomic Structure
An atom is composed of a dense core called a nucleus and surrounding this nucleus one or more negatively charged electrons.
The nucleus is composed of positively charged protons and neutral neutrons.

17. Atomic Structure
An atom is mostly empty space because the electron moves around the nucleus at such a great distance.
A chemical element is determined based on how many protons the nucleus contains (Hydrogen has 1, Carbon has 6, Oxygen has 8 protons).

18. An atom has the same number of electrons as protons, an atom has no net charge.
If an atom loses or gains one or more electrons it has become ionized.
With less electrons, the atom has a positive charge (Cation), more electrons it has a negative charge (Anion).
Ions

19. Parts of the atom Particle Charge Mass (g) Mass (amu) Proton +1
1.672 x 10-24 1
Neutron
1.675 x 10-24
Electron -1
9.110x10-28
1/1839

20. Atomic Number & Atomic Mass
• The number of protons in the nucleus of an atom is called the atomic number (Z).

21. Atomic Number & Atomic Mass
Mass Number
• The atomic mass of an atom is estimated from the total number of protons and neutrons in the nucleus.
• The sum of the number of protons and neutrons in an atom is called the atomic mass number (A).
Element Symbol
Mass Number
Atomic Number

22. Isotopes
Isotope - Atoms with the same atomic number but different mass numbers.
• Isotopes have the same number of protons but differ in number of neutrons.

23. Isotopes
Isotope Abundance
• Naturally occurring sample of most elements will contain a mixture of isotopes.
• Percent abundance is used to quantify the amount of each isotope in an elemental sample.
Example: Oxygen
Isotope
% Abundance
# in 100,000
16O
99.759
99,759
17O
0.037 37
18O
0.204
204

24. Atomic Mass Atomic mass unit is a relative unit
1/12 the mass of a 12C atom
1 atomic mass unit (amu) = 1.66 X kg

25. Atomic Mass and Isotopes
The atomic mass which appears on the periodic table is the weighted average of the masses of all the isotopes.
weighted fractional isotopic
average = isotopic X mass
abundance S

26. Atomic Mass and Isotopes
Boron has two isotopes
10B abundance 19.9% mass u
11B abundance 80.1% mass u
Atomic mass = (0.199 x 10.01) + (0.801 x 11.01)
Atomic mass = u

27. Atomic Weight (Mass) – The average mass of a representative sample of atoms.
Example 1: Calculate the atomic weight of oxygen given the following data.
Isotope
% Abundance
Atomic Weight
16O
99.762
17O
0.038
18O
0.200

28. Atomic Weight (Mass) – The average mass of a representative sample of atoms.
Example 2: Calculate the atomic weight of chlorine given the following data.
Isotope
% Abundance
Atomic Weight
35Cl
75.77
36Cl
0.0
37Cl
24.23

29. Atomic number and mass number
+/-q Z X
X=symbol
Z=atomic number (# of protons)
A=mass number (# of protons + # neutrons)
q= charge :+=electrons<protons,
:-=electrons>protons

30. Atomic Structure
Atomic number (Z) is the number of protons in the nucleus of an atom. It is also known as the proton number.
No. of protons always equals the no. of electrons in any neutral atom of an element.
Mass number (A) is the sum of the number of protons and the number of neutrons in the nucleus of an atom.

31. Atomic Structure - Example
So how can you work out the number of neutrons in an atom?
Example
No. of neutrons = Mass number – atomic number
No. of neutron = Mass No. – Atomic No.
= 23 – 11
= 12

32. Atomic Mass from Isotopic Composition

33. Atomic Mass from Isotopic Composition
20.00 (0.9092) +
21.00 (0.0026)
22.00 (0.0882)
20.18 amu
A.M. Ne = 20.18g/mol

34. X Symbols Mass number Atomic number
Contain the symbol of the element, the mass number and the atomic number
Mass
number X
Atomic
number

35. F Symbols 19 9 Find the number of protons number of neutrons
number of electrons
Atomic number
Mass Number 19 F 9

36. Br Symbols 80 35 Find the number of protons number of neutrons
number of electrons
Atomic number
Mass Number 80 Br 35

37. Symbols
if an element has an atomic number of 34 and a mass number of 78 what is the
number of protons
number of neutrons
number of electrons
Complete symbol

38. Symbols if an element has 91 protons and 140 neutrons what is the
Atomic number
Mass number
number of electrons
Complete symbol

39. Symbols if an element has 78 electrons and 117 neutrons what is the
Atomic number
Mass number
number of protons
Complete symbol