1. Atoms, Molecules, and Ions
Unit 2
Atoms, Molecules, and Ions

2. The Atomic Theory of Matter
Democritus is generally credited with putting forth the first ideas about atomic structure
Stated that all of matter consisted of indivisible particles called “atoms”

3. Dalton’s Atomic Theory
Dalton proposed the first serious theories on the structure of matter
Most of his postulates still hold true today and are known as Dalton’s atomic theory

4. Dalton’s Atomic Theory
Matter is composed of small indivisible particles called atoms. The atom is the smallest unit of an element that has all the properties of that element.
An element is composed entirely of one type of atom. The chemical properties of all atoms of any element are the same.
A compound contains atoms of two or more different elements. The relative number of atoms of each element in a particular compound is always the same.
Atoms do not change their identities in chemical reactions. Chemical reactions rearrange only how atoms are joined together.
Essentially a restatement of the Law of Conservation of Mass

5. Chemical Laws Associated with Dalton’s Atomic Theory
Law of Constant Proportions
The relative numbers and kinds of atoms remain constant for a given compound
Ex: Water (H2O) always has two hydrogens for every one oxygen
Law of Multiple Proportions
If two elements are capable of combining to form more than one type of product, they do so in a ratio of small whole numbers
i.e. There is H2O and H2O2, but no H2O1.5

6. Atomic Composition and Structure
Dalton also said that the atom was indestructable, which is untrue
Actually made up of subatomic particles
Protons
Neutrons
Electrons
The electron was the first subatomic particle to be discovered as a result of experiments performed with cathode ray tubes

7. Discovery of the Electron
J.J. Thomson is credited with discovery of the electron through his work with cathode ray tubes
Referred to as “cathode rays” because they originated from the positively charged cathode
Quantified a value of 1.76 x 108 C of charge per gram of electrons

8. Millikan’s Oil Drop Experiment
Determined that each electron possessed
-1.60 x C of charge.

9. Mass of an Electron
With Thomson’s mass to charge ratio and Millikan’s charge per electron, the mass of a single electron could now be calculated:

10. Plum Pudding Model
Positively charged blob with individual electrons randomly distributed throughout

11. Rutherford’s Gold Foil Experiment
Proved the existence of a positively charged nucleus
According to Thomson’s model, a scattering angle of 1 was expected; however scattering at much larger angles was also observed
“It was almost as if you fired a fifteen inch shell at a piece of tissue paper and it bounced back and hit you.”

12. Experimental Design

13. Explanation of Scattering Data
Extremely dense, positively charged center of mass at the core of each atom
Later dubbed the nucleus

14. The Neutron Summary of subatomic particles:
Discovered by James Chadwick in 1932
Held together with protons by the strong nuclear force inside the nucleus of atoms
Summary of subatomic particles:

15. Describing Atoms and Ions

16. Isotopes
Isotopes differ from other atoms only by the number of neutrons contained in the nucleus
For example:
Symbol
# of Protons
# of Electrons
# of Neutrons
24Mg 12
25Mg 13
26Mg 14

17. Example
Write the symbol that describes each of the following isotopes:
An atom that contains 7 protons and 8 neutrons
An atom that contains 31 protons and 39 neutrons
An atom that contains 14 protons and 14 neutrons

18. Ions
Formation of ions involves the gain or loss of electrons from outside the nucleus
Never involves the gain or loss of protons
A cation is created from the loss of an electron (positively charged)
An anion is created from the gain of an electron (negatively charged)

19. Symbols of Ions Give the chemical symbol, including mass number,
for each of the following
ions:
The ion with 22 protons, 26 neutrons, and 19 electrons
The ion of sulfur that has 16 neutrons and 18 electrons
How many protons, neutrons, and electrons does the 79Se2- ion possess?
48Ti3+
32S2-
34 protons, 45 neutrons, 36 electrons

20. Atomic Masses
Atoms have extremely small masses (largest atom has m = 4 x g)
Atomic Mass Unit (amu) was implemented to avoid expression of such cumbersome numbers
Defined as 1/12 the mass of a 12C atom
Particle
Charge
Mass(amu)
Proton
Positive (+1)
1.0073
Neutron
Neutral
1.0087
Electron
Negative(-1)
5.486 x 10-4

21. Average Atomic Mass
Because most elements exist in nature as a mixture of isotopes, we must express a large sample of these elements in the form of an average atomic mass (sometimes referred to as atomic weight
Schematic of a mass spectrometer
which is designed to measure
atomic mass distributions

22. Example
Data obtained with a mass spectrometer show that, in a sample of an element, 60.11% of the atoms have masses of u, whereas the remaining 39.89% of the atoms have masses of u. Calculate the atomic mass of this element and give its name and symbol. u
Gallium (Ga)