1. 2
Atomic Structure

2. Subatomic Particles?
If you cut a piece of graphite from the tip of a pencil into smaller and smaller pieces, how far could you go?
You would eventually end up with atoms (translates to “indivisible” in greek) of pure carbon.
You can not divide a carbon atom into smaller pieces and still have carbon

3. Atomic Structure
We have established that matter is comprised of atoms. But what are atoms made of?
There are three types of sub-atomic particles that make up the atom are known as:
electrons
protons
neutrons

4. Setting up the Cathode Ray Exp.
The electron was the first subatomic particle discovered.
In the late 1800’s, J.J Thomson sought to understand the strange “cathode ray” phenomena, which involved the observation of a “strange, flowing energy” through gases.
He developed the cathode ray experiment, which was comprised of:
Glass tube from which most of the air was removed
Two metallic plates, an anode and a cathode, connected to a high voltage power supply.
The cathode is connected to the negative terminal of the power source. The anode has a small hole drilled through its center.

5. Observation of Cathode Rays
When the connections were made, these mysterious cathode rays flow from the cathode to the anode, and some of these rays escape through the hole in the anode.
The rays are invisible, so a phosphorescent screen lines the back of the tube, which exhibits a glowing spot when struck by the beam.

6. What Are Cathode Rays?
Thompson soon realized that the cathode rays could be deflected by electric and magnetic fields. The image below shows a cathode ray beam being deflected upwards toward a positive pole.
He also found that the mass of the cathode remained virtually unchanged. What does this mean???
The beam is not energy, but rather, charged, nearly massless particles, and the particles are negative!

7. Plum-Pudding Model
Following the discovery of the electron, it became obvious that positive charges, called protons, must also exist since matter is electrically neutral
However, scientist had no idea how these particles were arranged in the atom. The first proposed model was the “plum pudding model”, which described electrons as being spread out in a proton “sea”

8. Determining the Nuclear Model
Following the discovery of radioactivity, the “gold foil experiment” was designed to test the PP model.
A thin sheet of gold foil was placed in a phosphorescent ring. A radioactive emitter of positive particles was placed in front.
If the PP model was correct, then the positive α-particles would pass through the foil unimpeded. But…
The particles were actually deflected!! How?

9. Expected Results
Expected results from “plum-pudding” model.

10. Rutherford’s Experiment
The experiment not only disproved the PP model, but also suggested that a very dense, very positive “core” exists at the center of the atom, in which all positive charges are found.
This came to be known as the nucleus.

11. Neutrons
Rutherford’s model was incomplete. For example, a hydrogen atom has one proton and one electron, but is only ¼th the mass of a helium atom which has two electrons and two protons.
If all of the mass of an atom comes from its sub-atomic particles, how do we explain the unaccounted for mass?
The answer is neutrons, particles that are equal in mass to protons, but with no electrical charge. While scientists knew that neutrons had to exist, they were not officially discovered until 1932.

12. The Nuclear Atom
Positively charged center of an atom, containing nearly all of the atom’s mass
About 1/10,000 the size of the atom

13. About the Nucleus Atomic Mass Units (amu)
Unit used to express the relative masses of atoms and subatomic particles
Equal to 1/12 of a carbon atom

14. C Elemental Symbols 6 Atomic #
Carbon
Atomic #
The number of protons in an atom is called the atomic number. An element is defined by its atomic number. (ex. only carbon has 6 protons)
For a given element, the number of protons DOES NOT CHANGE
In a neutral atom, the number of protons is equal to the number of electrons.

15. C Elemental Symbols 6 Mass #
Carbon
Mass #
The mass number of an element is the sum of its protons and neutrons.
The mass #’s listed on the periodic table are averages, in units of amu
These averages are used because numerous variations of elements called isotopes exist in nature.

16. Isotopes
Isotopes are variations of the same element having different numbers of neutrons.
Isotope symbols are shown below for the two isotopes of nitrogen with their % abundances in nature. The 14N and 15N isotopes have 7 and 8 neutrons, respectively.
Atomic Mass = total number of “nucleons” (protons, neutrons) in the nucleus X A Z
Atomic Number (Z) = the number of protons
𝟕 𝟏𝟒 𝑵 (99.636%)
𝟕 𝟏𝟓 𝑵 (0.346%)

17. Group Work
Complete the missing information in the table. 23 ?

18. Transitional Page
Avg. atomic mass is obtained using the % abundance and the isotope mass.
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑡𝑜𝑚𝑖𝑐 𝑚𝑎𝑠𝑠= 𝑖𝑠𝑜𝑡𝑜𝑝𝑒 𝑚𝑎𝑠𝑠 𝑥 (% 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒)

19. Group Work
Using the given abundances and isotope masses, calculate the average atomic mass of C. Does it match the reported value?
Boron has two isotopes, 10B and 11B. Using the given isotope masses, determine the % abundances of each isotope. Hint: total abundance must equal 100%
ISOTOPE
% A
Mass (amu)
𝟔 𝟏𝟐 𝑪
98.93 12
𝟔 𝟏𝟑 𝑪
1.07
𝟔 𝟏𝟒 𝑪 ~0
ISOTOPE
% A
Mass (amu)
𝟓 𝟏𝟎 𝑩
10.013
𝟓 𝟏𝟏 𝑩
11.009

20. Proton-Neutron Ratio
The nuclei of most naturally occurring isotopes are very stable, despite the massive repulsive forces that exist between the protons in the nucleus.
A strong force of attraction between neutrons and protons known as the nuclear force counteracts this repulsion.
As the number of protons increases, more neutrons are required to stabilize the atom. Stable nuclei up to atomic number 20 have equal numbers of protons and neutrons.
For nuclei with atomic number above 20, the number of neutrons exceeds the protons to create a stable nucleus.

21. Proton-Neutron Ratio
Radioactive isotopes are unstable (high in energy). This instability is attributed to a neutron/proton ratio that is either too high or too low.
To become stable, they spontaneously release particles or radiation to lower their energy.
This release of energy is called radioactive decay.

22. Radioactivity
The three most common types of radioactive decay are alpha, beta, and gamma
Property α β γ
Reason for process
Too many protons, too few neutrons (n/p ratio too low)
Too few protons, too many neutrons (n/p ratio too high)
Too much energy in nucleus
Charge 2+ 1-
Mass
6.64 x g
9.11 x g
Emitted Radiation Type
2 protons and 2 neutrons ( 2 4 𝐻𝑒 )
High energy electron.
Pure energy (Radiation)
Penetrating Power
Low. Stopped by paper. Blocked by skin.
Moderate. Stopped by aluminum foil. (10α)
High. Can penetrate several inches of lead. (10000α)

23. Radioactive Decay
For example, the 𝑈 isotope undergoes alpha decay to increase its n/p ratio:
𝑈→ 𝑇ℎ+ 2 4 𝐻𝑒
238−92 𝑛 92 𝑝 =1.58 𝑛/𝑝
234−90 𝑛 90 𝑝 =1.60 𝑛/𝑝

24. Radioactive Decay
The Thorium-234 isotope undergoes beta decay which lowers the n/p ratio:
In beta decay, a neutron is converted to a proton and an electron. This causes the proton count to increase:
𝑇ℎ→ 𝑃𝑎+ −1 0 𝑒
234−90 𝑛 90 𝑝 =1.60 𝑛/𝑝
234−91 𝑛 91 𝑝 =1.57 𝑛/𝑝
0 1 𝑛→ 1 1 𝑝+ −1 0 𝑒

25. Ions
Thus far, we’ve learned that each element has an exact number of protons.
For example, Hydrogen has only one proton. If you force a second proton onto the atom, you no longer have hydrogen… you now have Helium.
We have also learned that atoms can have variable numbers of neutrons (isotopes).
Next, we will discuss ions.

26. Ions
Ions are electrically charged atoms, resulting from the gain or loss of electrons.
Positively charged ions are called cations. You form cations when electrons are lost
Negatively charged ions are called anions. You form anions when electrons are gained

27. Ion Nomenclature
A cation is named by adding the word “ion” to the end of the element name
Anions are named by adding the suffix –ide to the end of an element
𝑳𝒊 +
Lithium ion
Sodium ion
Magnesium ion
Aluminum ion
𝑪𝒍 −
Chloride
Sulfide
Oxide
Phosphide
𝑵𝒂 +
𝑺 𝟐−
𝑴𝒈 𝟐+
𝑶 𝟐−
𝑨𝒍 𝟑+
𝑷 𝟑−

28. Group Work Fill in the missing information below ISOTOPE P N E 16 32 𝑆
16 32 𝑆 2- ?? 13 14 10
?? ?? 𝑃𝑡 4+ 95