Introduction to Atomic Structure and Radioactive Isotopes
Quantum Mechanical Model of the Atom
Electrons are organized into groups around the nucleus known as shells.
The shell closest to the nucleus is designated as shell 1.
Outer shells are larger, accommodate more electrons, and have higher energy levels.
Electron Configuration
The maximum number of electrons that can occupy an energy level can be calculated using the formula:
2n^2 where n represents the energy level.
Energy Levels and Maximum Electron Capacity
Energy Level (n)
Energy Level (n)
Maximum Number of Electrons (2n²)
1
2
2
8
3
18
4
32
Example Question on Electron Configuration
Question: How many electrons are in the 3rd energy level (n=3) of sulfur?
a. 4 b. 6 c. 8 d. 16
Valence Electrons
Valence Electrons Defined: Valence electrons are the electrons located in the outermost occupied energy level (valence shell) of an atom.
Valence electrons are crucial for predicting element properties.
Valence Electron Trends:
Group 1A = 1 valence electron
Group 3A = 3 valence electrons
Group 8A = 8 valence electrons
Electron Dot Structures
Valence electrons are represented visually by dots in electron dot structures.
Representative Elements in the Periodic Table
Groups of Representative Elements:
Group 1A: Metals and some Nonmetals
Group 2A: Metals
Group 3A: Metals, Semimetals, and Nonmetals
Group 4A: Varied types of elements
Group 5A, 6A, 7A, 8A: Increase in nonmetals towards Group 8A
Example of Key Elements:
Group
Element
Symbol
Atomic Weight
1A
Hydrogen
H
1.00794
2A
Beryllium
Be
9.012182
3A
Aluminum
Al
26.98154
4A
Silicon
Si
28.0855
5A
Phosphorus
P
30.97376
6A
Sulfur
S
32.065
7A
Chlorine
Cl
35.453
8A
Neon
Ne
18.99840
Example Question on Valence Electrons
Question: Arsenic (As) has ___ total # of electrons and ___ # of valence electrons?
a. 33, 3 b. 33, 5 c. 41, 3 d. 41, 5
Radioactive Isotopes
Definition: Radioactive isotopes are atoms that are unstable and emit nuclear radiation.
A nuclear change involves a transformation in the atomic nuclei, typically creating a new type of atom.
More than 300 naturally occurring isotopes have been identified.
Nuclear Radiation
Definition: Nuclear radiation refers to the particles and energy released during a nuclear change.
Atoms emitting nuclear radiation are called radioisotopes.
Four Common Forms of Nuclear Radiation:
Alpha Particle (α):
Makeup: 2 protons + 2 neutrons
Charge: 2+
Velocity: 5-10% of the speed of light
Penetrating Ability: Low
Beta Particle (β):
Makeup: Electron
Charge: 1-
Velocity: Up to 90% of the speed of light
Penetrating Ability: Moderate
Positron (β+):
Makeup: Positively charged electron
Charge: 1+
Velocity: Up to 90% of the speed of light
Penetrating Ability: Moderate
Gamma Particle (γ):
Makeup: Electromagnetic radiation
Charge: 0
Velocity: Light speed
Penetrating Ability: High
Light speed: 3 imes 10^8 ext{ m/s} (meters/second) = 186,000 mps (miles per second).
Alpha Particles
An alpha particle is identical to the nucleus of a helium-4 atom, consisting of two protons and two neutrons, with a charge of 2+.
Due to its velocity, it possesses high energy but relatively low penetrating ability.
Alpha Particle Emission
When a radioactive isotope emits an alpha particle, the resulting product has two fewer protons and neutrons.
Balanced Nuclear Reaction: The total mass numbers and total charges must remain constant on both sides of the reaction. Example:
\text{ }^{230}{90}\text{Th} \rightarrow \text{ }^{226}{88}\text{Ra} + \text{ }^{4}_{2}\alpha
Beta Particles
A beta particle is defined as an electron ejected from the nucleus of a radioisotope.
It travels at approximately 90% of the speed of light and has identical charge and mass as electrons.
Beta Particle Emission
When a radioactive isotope emits a beta particle, the resulting product has one more proton and one less neutron. Example:
\text{ }^{12}{5}\text{B} \rightarrow \text{ }^{12}{6}\text{C} + \text{ }^{0}_{-1}β
Positrons
A positron has the same mass as a beta particle but carries a charge of 1+.
Positrons are emitted at 90% of the speed of light and are utilized in medical imaging procedures like Positron Emission Tomography (PET).
Positron Emission
When a radioactive isotope emits a positron, the final product has one less proton and one more neutron. Example:
\text{ }^{18}{9}\text{F} \rightarrow \text{ }^{18}{8}\text{O} + \text{ }^{0}_{1}β+
Gamma Rays
A gamma ray is characterized as a high-energy form of electromagnetic radiation.
Gamma radiation often accompanies the release of alpha or beta radiation when the nucleus stabilizes. Example:
\text{ }^{131}{53}\text{I} \rightarrow \text{ }^{131}{54}\text{Xe} + \text{ }^{0}{-1}β + \text{ }^{0}{0}γ