Chemistry: Atomic Mass and Subatomic Particles

Key Topics Discussed in Exam Preparation

Atomic Mass Unit (AMU)

• Definition: The atomic mass unit is a standard unit of mass that quantifies mass on an atomic or molecular scale.
• Importance of AMU: Each element has its atomic mass indicated on the periodic table, akin to how a person's weight is stated on a driver's license.
• Conversion from Grams to AMU:
• Be prepared to perform conversions between grams and AMU for calculations on the exam, specifically focusing on unit cancellation for accurate measurements.
• Example: The atomic mass of a proton is approximately 1.673 \times 10^{-24} grams, which can be converted to AMU using the conversion 1 \text{ AMU} = 1.6606 \times 10^{-24} grams.

Subatomic Particles

• Protons: Found in the nucleus with a positive charge. The mass of 1 proton is about 1.007 \text{ AMU}.
• Neutrons: Discovered by James Chadwick in 1932, also found in the nucleus and with a mass almost equivalent to protons but with no charge. Mass also about 1.008 \text{ AMU} for a neutron.
• Electrons: Negatively charged particles found in orbital clouds outside the nucleus, with negligible mass (around 9.11 \times 10^{-28} grams). Their mass is about \frac{1}{1840} the mass of a proton, making them practically massless for atomic mass calculations.

Example Problem:

• If an atom has a mass number of 35 and contains 18 neutrons, determine the element:
• Use formula: Mass number (A) = Neutrons (N) + Protons (Z)
• Calculation: 35 = 18 + Z
  ightarrow Z = 17, which corresponds to the element Chlorine (Cl) on the periodic table.

Electron Energy Levels

• Principle: Electrons occupy energy levels related to their distance from the nucleus which can be thought of like residential levels in a building.
• Energy levels are denoted by integers (n=1, 2, 3…) and increase with distance from the nucleus.
• Each energy level can hold a specific number of electrons:
• n=1: 2 electrons
• n=2: 8 electrons
• n=3: 18 electrons
• n=4: 32 electrons

Periodic Trends

• Group number indicates the number of valence electrons for representative elements (Group 1A and 2A).
• Period number corresponds to the number of occupied energy levels in an atom.
• Valence electrons affect the chemical reactivity of elements; more valence electrons typically grant greater reactivity potential.

Bohr Atomic Model

• Describes the arrangement of electrons in concentric energy levels around the nucleus.
• Core electrons are closer to the nucleus while valence electrons are in the outermost energy levels, influencing the atom's reactivity.
• Example: Helium is unique as it has only two electrons in the outermost shell, which achieves stability and contributes to its inert nature.

Study Recommendations:

• Familiarize yourself with conversions between grams and AMU, the characteristics of subatomic particles, and how to calculate atomic structure details like protons, neutrons, and electrons.
• Review periodic trends carefully, especially the relevance of group and period numbers in determining electron configurations.
• Practice conceptualizing the Bohr model for various atoms and how this relates to their chemical properties.

Questions to Consider:

• What is the significance of mastering unit conversions for this exam?
• How do the properties of subatomic particles define the behavior of elements?
• Why are valence electrons crucial in understanding chemical reactivity?