1. Unit 2
Atomic Structure

2. Atomic Theory

3. Greek Model Democritus Greek philosopher Idea of ‘democracy’
“To understand the very large, we must understand the very small.”
Democritus
Greek philosopher
Idea of ‘democracy’
Idea of ‘atomos’
Atomos = ‘indivisible’
‘Atom’ is derived
No experiments to support idea, his ideas were forgotten for thousands of years.
Atomists; they argued for a completely materialistic universe consisting of atoms moving in a void.
Since mere fragments of the ideas of Leucippus are known, his pupil, Democritus of Abdera (c B.C.)
is considered the elaborator of this concept.
Aaron J. Ihde The Development of Modern Chemistry, Dover Publishing, 1984 pg 6
It should also be noted that the Romans were not a scientific people and made almost no scientific contributions of their own.
“To understand the very large, we must understand the very small.”
-Democritus
The world
Reality to Democritus consists of the atoms and the void. Atoms are indivisible, indestructible, eternal, and are in constant motion. However, they are not all the same as they differ in shape, arrangement and position. As the atoms move they come into contact with other atoms and form bodies. A thing comes into being when the atoms that make it up are appropriately associated and passes away when these parts disperse.
This leaves no room for the intelligent direction of things, either by human or divine intelligence, as all that exists are atoms and the void. Democritus stated, "Nothing occurs at random, but everything occurs for a reason and by necessity."
The soul
Although intelligence is not allowed to explain the organization of the world, according to Democritus, he does give place for the existence of a soul, which he contends is composed of exceedingly fine and spherical atoms. He holds that, "spherical atoms move because it is their nature never to be still, and that as they move they draw the whole body along with them, and set it in motion." In this way, he viewed soul-atoms as being similar to fire-atoms: small, spherical, capable of penetrating solid bodies and good examples of spontaneous motion.
Democritus’s model of atom
No protons, electrons, or neutrons
Solid and INDESTRUCTABLE

4. Alchemy After that, chemistry was ruled by alchemy.
They believed that that could take any cheap metals and turn them into gold.
Alchemists were almost like magicians.
Elixirs: physical immortality
Philosopher’s stone: change lead to gold

5. Alchemy
Alchemical symbols for substances…
. . .
. . .
GOLD
SILVER
COPPER
IRON
SAND
transmutation: changing one substance into another D
In ordinary chemistry, we cannot transmute elements.

6. Contributions of alchemists: Information about elements
- the elements mercury, sulfur, and antimony were discovered
- properties of some elements
Develop lab apparatus / procedures / experimental
techniques
- alchemists learned how to prepare acids.
- developed several alloys
- new glassware

7. The Atomic Theory of Matter
In 1803, Dalton proposed that elements consist of individual particles called atoms.
His atomic theory of matter contains four hypotheses:
1. All matter is composed of tiny indivisible particles called atoms.
2. All atoms of an element are identical in mass and chemical properties.
3. A chemical compound is a substance that always contains the same atoms in the same ratio.
4. In chemical reactions, atoms from one or more
compounds or elements rearrange in to form one
or more new compounds. Atoms themselves do
not change of identity in chemical reactions.
Some parts of his theory were later proven wrong.

8. Modern atomic theory Subatomic particles Electrons:
Negatively charged subatomic particles
Discovered by J.J. Thomson in 1897
Discovered by observing deflection of cathode rays.
Occupy most of the volume of an atom
YouTube - Thomson

9. Electron Microscopy
The electron microscope is a type of microscope that uses a beam of electrons to create an image of the specimen. It is capable of much higher magnifications and has a greater resolving power than a light microscope, allowing it to see much smaller objects in finer detail.
Typical light microscope magnifies up to 1000 x and an electron microscope magnifies over 100,000 x
SEM - Image Gallery

10. Protons : positively charge particles
Neutrons: no charge particles, mass similar to protons.
The atomic nucleus:
Protons and neutrons are located in the center of the atom
Through the gold-foil experiment, Rutherford determined:
The atom is mostly empty space
Positive charge and most of atom’s mass is concentrated in a small region, called the nucleus (composed of protons and neutrons)

11. Distinguishing Among Atoms
Elements are different because they contain different number of protons.
Atomic number: indicates the number of protons in the nucleus of an element.
Since atoms are electronically neutral:
# protons= # electrons
Atomic number:
Carbon has 6 protons
Carbon has 6 electrons 6 C

12. Learning Check: complete the following table
Element
Symbol
Atomic number
# protons
# electrons 1.
Tin 2. 16 3. 81 4. 76 5. Gd

13. Au Mass number gold-197 Mass number 197 79
Most of the mass of an atom is concentrated in its nucleus and depends on the number of protons and neutrons.
Mass number: total number of protons and neutrons in an atom.
Mass number= protons + neutrons
# neutrons= mass number – protons
Representing atoms:
Mass number
Mass number Au
197 79
gold-197
Atomic number

14. Learning check: determine the number of protons, neutrons and electrons for the following atoms.
Carbon-12
Fluorine-19
Beryllium-9

15. Isotopes
Isotopes are atoms of the same element that have the same number of protons, but different number of neutrons.
Isotopes of an element have the same atomic number but different number of neutrons, thus have different mass numbers.
Hydrogen has 3 isotopes:
Hydrogen-1 or simply hydrogen
Hydrogen-2 or deuterium
Hydrogen-3 or tritium

16. Classwork: isotope notation handout

17. Atomic Mass Mass of proton or neutron: 1.67x10-24g
Mass of electron : x10-28g
These values are impractical to work with, so scientists compare relative masses of atoms using a reference isotope : carbon-12
An atomic mass unit (amu) is defined as 1/12 the mass of carbon-12

18. Atomic mass (continued)
The atomic mass of an element is not a whole number because the isotopes of an element and its natural abundance is taken in consideration.
The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.

19. Atomic mass (continued)
Ex. 1 The atomic mass of copper is amu. Which of copper’s two isotopes is more abundant: copper-63 or copper-65?
Since is closer to 63 than 65, the most abundant isotope is copper-63.

20. Atomic mass (continued)
Ex. 3 There are 3 isotopes of silicon; they have mass numbers of 28, 29, and 30. The atomic mass of silicon is amu. Comment on the relative abundance of these 3 isotopes.
Silicon-28 must be the most abundant.

21. Atomic mass (continued)
Ex. 2 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%)
1. Divide the percentages by 100: and
atomic mass= (0.5069x78.92) + (0.4931x80.92) =
= amu

22. Atomic mass (continued)
Classwork p103 # 12,13; p 105 #33-34