Atomic Structure and Subatomic Particles

Atom History and Structure Overview

• Chapter Focus: This short chapter covers the periodic table and the atomic model, expected to be completed within the week.
• Mandatory Pre-requisite: Students must watch the Chapter 2 video by the end of the day. This video details:
  • The history of the atom.
  • Key experiments from the early $20$th century that elucidated the atom's structure.
  • Students are responsible for understanding this content.
• Current Lecture Scope: Provides an initial breakdown of the periodic table, discussing how its shape reveals information, and offers a summary of the atomic model derived from the Chapter 2 video.
• Model Accuracy: The atomic model discussed serves as a good approximation of current understanding. It will be further refined in Chapter $8$ to align with a more modern view of the atom.

Definition and Components of an Atom

• Atom Definition: An atom is defined as a single particle of an element, representing the smallest stable subdivision of any elemental substance.
• Subatomic Particles: Discoveries in the $20$th century revealed that atoms are composed of three fundamental subatomic particles. These particles are not stable individually but combine to form stable atoms of various elements.
• Key Subatomic Particles and Their Properties:
  • Proton ($ext{p}^{+}$):
    • Charge: Positively charged ($+ ext{charge}$).
    • Location: Found within the nucleus.
    • Mass: Contributes significantly to the atom's mass.
  • Neutron ($ext{n}^{0}$):
    • Charge: Neutral (no charge).
    • Location: Found within the nucleus.
    • Mass: Contributes significantly to the atom's mass.
  • Electron ($ext{e}^{-}$):
    • Charge: Negatively charged ($- ext{charge}$).
    • Location: Orbits the nucleus, existing in the space surrounding it (not within the nucleus).
    • Mass: Possesses very little mass, less than $0.1\%$ of a proton's mass. For practical purposes, its mass is considered negligible compared to protons and neutrons.
• Core Takeaway: Protons are positive, electrons are negative, and protons and neutrons account for almost all of the atom's mass.

Atomic Structure and Scale

• The Nucleus:
  • A dense, central region within the atom containing protons and neutrons.
  • Elemental Identity: The number of protons directly defines the element (e.g., $7$ protons define Nitrogen). This number is unique to each element.
  • Mass Concentration: Nearly all of the atom's mass is concentrated in this tiny nucleus, as protons and neutrons are the primary mass-contributing particles.
• Electron Behavior (Chapter 8 Preview):
  • While often depicted as orbiting the nucleus like planets, this model is an simplification.
  • In reality, the exact location of electrons is unknown; they have a probability of existing in the space around the nucleus.
• Atomic Size and Scale (Conceptual Understanding):
  • Atom Diameter: Approximately $1$ to $5$ angstroms ($10^{-10} ext{ meters}$) across.
  • Scale Comparisons:
    • Human hair: $10^{-4} ext{ meters}$
    • Red blood cell: $5 imes 10^{-6} ext{ meters}$
    • DNA: $10^{-9} ext{ meters}$
  • Nucleus Diameter: Roughly $4$ orders of magnitude smaller than the atom itself.
  • Density of the Nucleus: Due to its small size and concentrated mass, the nucleus is incredibly dense.
  • Analogy (Atom as a Football Stadium): If an atom were the size of a football stadium, its nucleus would be equivalent to a marble placed at the center of that stadium. This highlights that most of an atom is empty space.
  • Density Analogy (Baseball): If a baseball were as dense as an atomic nucleus, it would weigh approximately $23,000,000,000$ tons.
  • Note: Specific numerical values relating to scale are provided for conceptual benefit and are not required for memorization.

Nuclide Symbol Notation for Subatomic Particles

• Purpose: To clearly communicate the composition of an atom in terms of its subatomic particles.
• Nuclide Symbol Format: ${}_{Z}^{A} ext{X}$
  • X: The elemental symbol (e.g., $ext{N}$ for Nitrogen, $ext{Cl}$ for Chlorine, $ext{Ag}$ for Silver).
  • A (Mass Number, Top Number):
    • Represents the total number of mass-carrying subatomic particles in the nucleus, rounded to a whole number.
    • $A = ext{number of protons} + ext{number of neutrons}$.
    • Electrons are excluded due to their negligible mass.
  • Z (Atomic Number, Bottom Number):
    • Defines the element (e.g., $7$ is always Nitrogen).
    • Corresponds to the number of protons.
    • $Z = ext{number of protons}$.
    • Typically found in the upper left corner of an elemental symbol on the periodic table.
• Calculating Subatomic Particles for Neutral Elements:
  • Number of Protons: Directly equal to the atomic number $(Z)$. This dictates the element's identity.
  • Number of Electrons: For a neutral atom (no net charge), the number of electrons must equal the number of protons to balance the positive nuclear charge with an equal negative charge.
  • Number of Neutrons: Calculated by subtracting the atomic number from the mass number: $ext{Number of Neutrons} = A - Z$.
• Charge Carriers Reminder:
  • Protons carry a positive charge.
  • Neutrons are neutral (no charge).
  • Electrons carry a negative charge.

Chemistry vs. Nuclear Chemistry

• Chemistry (as studied in this course):
  • Primarily involves the manipulation and interaction of electrons.
  • Examples include common processes like wood burning, which is a rearrangement of electrons.
• Nuclear Chemistry:
  • Involves the manipulation and reactions within the nucleus of an atom.
  • Examples include atomic bonds (in the context of nuclear forces), nuclear power, and nuclear weapons.
  • This is a specialized branch of chemistry that will be briefly covered in $102$ but is not the main focus of this course.

Practice: Identifying Subatomic Particles from Nuclide Symbols

• Example 1: ${}_{17}^{35} ext{Cl}$ (Chlorine)
  • Protons: $17$ (from atomic number $Z=17$)
  • Electrons: $17$ (for a neutral atom)
  • Neutrons: $35 - 17 = 18$
• Example 2: ${}_{47}^{108} ext{Ag}$ (Silver)
  • Protons: $47$ (from atomic number $Z=47$)
  • Electrons: $47$ (for a neutral atom)
  • Neutrons: $108 - 47 = 61$
• Example 3: ${}_{19}^{39} ext{K}$ (Potassium)
  • Protons: $19$ (from atomic number $Z=19$)
  • Electrons: $19$ (for a neutral atom)
  • Neutrons: $39 - 19 = 20$
• Example 4: ${}_{14}^{28} ext{Si}$ (Silicon)
  • Protons: $14$ (from atomic number $Z=14$)
  • Electrons: $14$ (for a neutral atom)
  • Neutrons: $28 - 14 = 14$
• Summary: The atomic number ($Z$) directly gives the number of protons and electrons (for neutral atoms). The number of neutrons is found by subtracting the atomic number ($Z$) from the mass number ($A$).