CHE101 Chapter 2

Definition of an Atom: The smallest amount of any element that retains its properties. Breaking an atom leads to subatomic particles, which no longer have the characteristics of that element.
- Subatomic Particles: Protons, neutrons, and electrons.

Elements Count: As of the current date, 118 elements on the periodic table.
Periodic Table Structure:
- Tiles typically contain:
- Top Left Corner: Atomic Number (Z) - whole number representing the number of protons in the element.
- Center: Chemical Symbol (one or two letters); first letter capitalized, second letter lowercase (e.g., C for Carbon, Co for Cobalt).
- Bottom Number: Atomic Mass (not discussed here in detail).

Periods: Horizontal rows (indicated by numbers at the bottom of the periodic table) where elements are organized by increasing atomic number.
Groups: Vertical columns that contain elements with similar chemical properties (e.g., Group 17: Halogens - Fluorine, Chlorine, etc.).
Periodic Trends: As you move left to right across a period, elements display trends in properties such as atomic size, ionization energy, etc. This phenomenon is known as periodicity.

Metal and Nonmetals: The periodic table is divided by a bold black line.
- Left of the Line: Metals (e.g., malleable, conductive);
- Right of the Line: Nonmetals (e.g., insulating).
Metalloids: Elements located along the line that exhibit properties of metals and nonmetals.
Noble Gases: Group 18, characterized as nonreactive gases (e.g., Helium, Neon).
Specific Groups:
- Group 1: Alkali Metals
- Group 2: Alkaline Earth Metals
- Group 1B: Coinage Metals (e.g., Copper, Silver, Gold)
- Group 17: Halogens

Atomic Model: Visualized as a sphere with a dense nucleus at the center containing protons (positive charge) and neutrons (neutral charge).
- Nucleus: Contains nearly all the mass of the atom.
- Electrons: Orbit the nucleus and carry a negative charge; mass is negligible compared to protons and neutrons.

Protons: Positively charged, mass of approximately 1 AMU.
Neutrons: Neutral charge, mass of approximately 1 AMU.
Electrons: Negligible mass, negative charge (-1); on the atomic scale, their mass is not pivotal.

Nuclear Symbol: Depicted with an elemental symbol and numbers describing protons (Z) and mass number (A).
- Mass Number (A): Total number of protons and neutrons (A = Z + # of Neutrons).
- Example of Nuclear Symbol: For nitrogen, \mathbf{\text{N}^14_7} indicates that nitrogen has 7 protons and 7 neutrons (14 total).

Finding Neutrons: Given mass number (A) and atomic number (Z):
- Number of Neutrons = A - Z

Definition: Isotopes are atoms of the same element with different numbers of neutrons, leading to different mass numbers.
- Example: Carbon-12 (6 protons, 6 neutrons), Carbon-14 (6 protons, 8 neutrons).
Natural Abundance: Mixture of different isotopes found in nature can be used e.g., in carbon dating, where Carbon-14 decays to Carbon-12.

Definition: The AMU is defined based on the mass of Carbon-12, with 1 AMU being approximately equal to the mass of one proton or neutron. For more complex elements, the atomic mass is often equal to the weighted average of the isotopes, also measured in AMUs.

Calculate average atomic weight as a weighted average of isotopes based on natural abundance:
(A{avg} = (mass{isotope 1} imes abundance{1}) + (mass{isotope 2} imes abundance_{2}))
Practice Question: Given isotopes of Boron with specific AMUs and natural abundance, calculate weighted atomic weight using natural percentage values.

Role of Electrons in Chemistry: Most chemical reactions involve manipulation of the electrons surrounding an atom, which are easier to manipulate than protons or neutrons due to their negligible mass.
- Common examples: Rusting of iron, burning of fuels, processes in biological systems.

Reiterate the importance of recognizing the periodic trends, atomic number, mass number, isotopes, and their real-world applications (e.g., carbon dating).
Remind students to review material for quizzes emphasizing calculations and understanding of atomic structure.