1. Unit 2: History and Structure
Atoms
Unit 2: History and Structure

2. Atomic Structure

3. Subatomic particles Particle Charge Location Mass (amu) Electron
Negative
Outside the nucleus/electron cloud
1/1840 atomic mass unit (negligible)
Proton
Positive
Nucleus
1 amu
Neutron
Neutral

4. Atomic Number the number of protons in an atom
This is unique to each element
Found at the top of each element cell

5. Mass Number proton + neutrons This is NEVER a rounded number
Cannot split neutrons or protons
Is not on the period table
In isotope notation it is the top number (A)

6. Isotope Same element with DIFFERENT numbers of neutrons
Neutrons = mass number – atomic number
N= 1-1
N=0
N= 2 -1
N=1
N= 3 -1
N=2

7. Average atomic Mass
The mass of an atom considers the abundance of the all naturally occurring isotopes
Weighted average
How to solve
Make a table of values that include
Isotope
Mass
Percent
Covert the percent to decimals (divide by 100)
Multiple by the mass of the isotope
Add the answers together

8. Sample Problem (0.692) (62.93)amu = 43.55 (0.308) (64.93)amu = 19.998
The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for a mass of 62.93amu and 30.8% for a mass of 64.93amu. Calculate the average atomic mass of copper.
Isotope
Percent
Mass
Cu - 63
69.2 %
0.692
62.93 amu
Cu – 65
30.8 %
0.308
64.93 amu
(0.692) (62.93)amu = 43.55
(0.308) (64.93)amu =
63.55 amu

9. ions Same elements with different number of electrons
Cation: positive ion
lower number of electrons than protons
Anion: negative ion
higher number of electrons than protons

10. Nuclear Decay Isotopes can have an unstable nucleus
Stable nuclei have a 1:1 ratio of protons and neutrons
Nucleus will emit different particles until it becomes stable
stability can take many transmutations
Nuclear reactions
During nuclear decay, the identity of the element can change

11. Particles

12. transmutations −1 0 β 92 238 U 92 235 U 23993Np  23994Pu + _________
14) 42He + _________  Pu n n
15) 10n __________  U U U

13. Half Life
The amount of time it takes for half the nuclear material within a substance to transmute/ decay
Calculated using a time and mass chart
Always start with time zero

14. Half Life example
A rock that originally had a mass of 1.00 gram of uranium-238 now has only grams. How old is the rock if the half-life of uranium-238 is 4.5 billions of years.
Time
Mass (g)
1.00 grams
4.5 billion years
0.50 grams
Half life 2

15. The Ancients Aristotle Democritus Matter is continuous
All matter was composed of tiny particles
Atomos
“particles “ were thought to be indivisible
Aristotle
Matter is continuous
Everything in the universe is indefinitely indivisible

16. Dalton Had 5 principles to his Atomic theory
All matter is composed of particles called atoms
Atoms of a given elements have the same properties, atoms of different elements have different properties
Atom cannot be subdivided, created, or destroyed
Atoms of different elements combined in small whole numbers
Chemical reactions are when atoms are combined, separated, and rearranged.

17. Disproving Dalton Not all of Dalton’s principles were correct
Atoms can be subdivide in to subatomic particles
Neutron
Proton
Electron
Elements can combine with themselves to form diatomic molecules

18. J.J. Thompson
The discovery of the first subatomic particle took place in the late 1800’s.
A power source was attached to two metal ends of an evacuated glass tube, called a cathode ray tube.
A beam of “light” appears between the two electrodes called a cathode ray.

19. Electric Current
cathode ray
cathode
anode
Cathode Ray

20. Observations and Conclusions
Scientists determined the ray was negatively charged.
Electron negatively charged
Must be a positive particle
Cathode rays were deflected by a magnetic field
The rays were deflected away from a negatively charged object

21. Negative particles embedded in a sphere of positive plasma-like matter.
THINK…
Chocolate Chip Cookie Mg + -

22. Ernest Rutherford Gold Foil Experiment
directed a narrow beam of alpha particles at a very thin sheet of gold foil.
Alpha particles (a) are He atoms that have been stripped of their electrons

24. Observation Alpha particles Went straight through
Deflected significantly at an angle
Bounced back

25. Conclusions Scientists determined
Atom is made up of mostly empty space
There is a positive particle (like repels like)
Proton positively charged
There is a dense mass in the center of an atom
Nucleus
Must be another particle to account for the mass

26. Chadwick
In 1932, the English physicist James Chadwick discovered yet another subatomic particle.
the neutron is electrically neutral
It’s mass is nearly equal to the proton
Therefore the subatomic particles are the electron, proton, and neutron

27. Bohr Why don’t the electrons crash into the nucleus?
Energy levels –Planetary model
Electrons orbit on specific energy levels
Electrons can absorb/release a specific amount of energy, quanta, to move up or down an energy level
Produces a emission spectra

28. Emission Spectra
An electron absorbs a photon of energy and jumps to a higher energy level.
Becomes excited
moves to an excited state
An electron releases a photon in the form of wavelength
De-excites back down to it normal energy state
Ground state
The wavelength is seen as color

29. Forces
Strong: works over small distances, so can bring about interactions between particles colliding at high kinetic energy
Weak: involved in certain decays and interactions, and its involvement is signaled by interactions over seconds or longer
Electromagnetic: acts between all charged particles and is signaled by photon emissions or absorption. It may be involved in the internal re-arrangement of electron within an atom.