CHEMISTRY: Chapter 2 - Atoms, Molecules, and Ions
Chapter 2: Atoms, Molecules, and Ions
Lecture #14: Problem-Solving
- Subatomic Particles in the Nucleus: The subatomic particles contained within the nucleus of an atom are protons and neutrons.
- Isotope Symbol Notation:
- The symbol for an isotope is represented as _{Z}^{A}\text{X}, where:
- A is the Mass Number (total number of protons and neutrons).
- Z is the Atomic Number (number of protons).
- \text{X} is the chemical symbol of the element.
- Example: The isotope having atomic number Z=88 and mass number A=226 has the symbol _{88}^{226}\text{Ra}.
- Calculating Average Atomic Mass:
- The average atomic mass of an element is calculated by taking the weighted average of the masses of its naturally occurring isotopes, considering their respective percent abundances.
- Formula: Average Atomic Mass = \sum \left( \text{Isotope Mass} \times \text{Fractional Abundance} \right)
- Example Problem: An element has two naturally occurring isotopes with the following masses and percent abundances:
- Isotope 1: 41.02 \text{ amu}, 72.00\% abundance
- Isotope 2: 42.98 \text{ amu}, 28.00\% abundance
- Solution:
\left( 41.02 \text{ amu} \times 0.7200 \right) + \left( 42.98 \text{ amu} \times 0.2800 \right)
= 29.5344 + 12.0344
= 41.5688 \approx 41.57 \text{ amu}
Lecture #15: Learning Objectives & Atomic Structure Details
Atomic Numbers, Mass & Electron Counts
- Atomic Number (Z):
- Defines the number of protons in an atom.
- For a neutral atom, the number of protons is equal to the number of electrons.
- Example (Zirconium, Zr): Zirconium has an atomic number of 40.
- It contains 40 protons.
- A neutral Zr atom contains 40 electrons.
- Mass Number (A):
- Represents the total number of protons and neutrons in an atom's nucleus.
- Calculating Neutrons: The number of neutrons can be determined by subtracting the atomic number from the mass number: Number of neutrons = \text{Mass Number} - \text{Atomic Number}.
- Example (Zr atom): To find the number of neutrons in a Zr atom, its specific mass number (for a given isotope) must be provided. If we consider ^{92}\text{Zr}, the number of neutrons would be 92 - 40 = 52.
- Atomic Mass (Average Atomic Mass):
- The atomic mass listed on the periodic table is the average atomic mass, reflecting the weighted average of all naturally occurring isotopes of an element.
- Example (Zirconium Isotopes): Zirconium (Zr) has multiple isotopes, each contributing to its average atomic mass:
- ^{90}\text{Zr} (51.50\% abundance)
- ^{91}\text{Zr} (11.20\% abundance)
- ^{92}\text{Zr} (17.20\% abundance)
- ^{94}\text{Zr} (17.30\% abundance)
- ^{96}\text{Zr} (2.80\% abundance)
- Using their mass numbers as an approximation:
\left( 90 \times 0.5150 \right) + \left( 91 \times 0.1120 \right) + \left( 92 \times 0.1720 \right) + \left( 94 \times 0.1730 \right) + \left( 96 \times 0.0280 \right)
= 46.35 + 10.20 + 15.82 + 16.26 + 2.69 = 91.32 - Note: For more precise calculations, the accurate atomic masses of each isotope, rather than just their integer mass numbers, are necessary.
Electron Arrangement
- Electrons are structured into distinct layers, also known as energy shells, which surround the atomic nucleus.
The Periodic Table
- Organization: The periodic table organizes elements primarily based on their increasing atomic number.
- Groups (Vertical Columns):
- There are 18 groups in total.
- Elements within the same group often share similar chemical properties because they typically have the same number of valence electrons.
- Valence Electrons: These are the electrons located in the outermost electron shell of an atom, critical for chemical bonding.
- Group Classifications:
- Main-group elements (or Representative elements): These include Groups 1\text{A} - 8\text{A} (also known as 1, 2, \text{ and } 13-18).
- Transition elements (or Transition metals): These span Groups 1\text{B} - 8\text{B} (also known as 3-12).
- Periods (Horizontal Rows):
- There are 7 periods in the periodic table.
- Elements belonging to the same period have the same number of occupied electron shells.
- **Classification of Elements (Metals, Nonmetals, Semimetals):
