Atoms and Subatomic Particles (Notes)

Electric Charge and Neutral Atoms

  • Atoms contain three main sub-atomic particles: protons (positive), neutrons (neutral), and electrons (negative).
  • The nucleus of an atom contains protons and neutrons; electrons move around the nucleus.
  • The overall charge of an atom depends on the balance of charges from protons and electrons.
  • For a neutral atom, the number of protons equals the number of electrons, so the total charge is zero.
  • Neutrons contribute no electrical charge.
  • Useful relation for charge: Q = (+e) imes Np + (-e) imes Ne = eigl(Np - Neigr).
    • If the atom is neutral, Np = Ne and Q = 0.
  • Ions (charged atoms) occur when Np eq Ne, giving a net charge.

Sub-atomic Particles and Isotopes

  • Proton: positively charged particle located in the nucleus.
  • Neutron: neutral particle located in the nucleus.
  • Electron: negatively charged particle that moves around the nucleus.
  • In a neutral atom, the numbers of protons and electrons are equal; neutrons can vary independently, creating isotopes.
  • Isotopes: atoms with the same number of protons (same element) but different numbers of neutrons.
    • Examples from Table 8.3:
    • Carbon: \text{Protons} = 6, \text{Electrons} = 6, \text{Neutrons} = 6, 7, or 8.
    • Nitrogen: \text{Protons} = 7, \text{Electrons} = 7, \text{Neutrons} = 7 \text{ or } 8.
    • Uranium: \text{Protons} = 92, \text{Electrons} = 92, \text{Neutrons} = 143 \text{ or } 146.
  • Number of protons defines the element (atomic number, Z); different Z values distinguish elements.

Mass of an Atom

  • The atom’s mass is mainly due to its nucleus (protons and neutrons).
  • Electrons have mass, but it is very small compared with protons and neutrons; thus, electron mass is often considered negligible in the total atomic mass.
  • Approximate relation: m{\text{atom}} \approx mp\,Np + mn\,Nn, where mp \approx mn and me \ll mp, mn.

Structure of the Atom

  • Three sub-atomic particles: proton, neutron, electron.
  • Nucleus: contains protons and neutrons; positively charged due to protons.
  • Electrons: move around the nucleus; around the nucleus is empty space on average.
  • Planetary model (illustrative): electrons orbit the positively charged nucleus like planets around the sun (Figure 8.6).
  • The nucleus is tiny relative to the overall size of the atom; the atom is mostly empty space.
  • Analogy: If the atom were the size of a football stadium, the nucleus would be the size of a marble in the center; the space between the nucleus and the edge is vast relative to the nucleus.
  • Important caveat: The planetary model is a simplified representation that helps study atoms; the actual behavior of electrons is more complex.
  • Take note from Textbooks: No one knows exactly what an atom looks like; planetary model is a simplified representation used for study; scientific models evolve with new evidence.

How the Number of Protons Identifies an Element

  • There are 118 known elements (discovered naturally or in labs).
  • The Periodic Table organizes elements by increasing proton number (atomic number, Z).
  • The proton number (atomic number) is the number of protons in an atom of the element.
  • Example: Helium has Z = 2, so a helium atom contains two protons.
  • No two elements share the same proton number; this unique proton count differentiates elements and their chemical properties.
  • A quick reference concept: the number above the symbol in the periodic table represents the proton number Z.
  • If needed, a mass number A = Z + Nn (where Nn is the number of neutrons) can distinguish isotopes of the same element.
  • Table reference example (8.4): Number of sub-atomic particles in hydrogen, helium and lithium atoms:
    • Hydrogen: Z=1; Np=1; Ne=1; N_n=0 (protium) or other isotopes with different neutron counts.
    • Helium: Z=2; Np=2; Ne=2; N_n=2 for the common helium-4 isotope.
    • Lithium: Z=3; Np=3; Ne=3; N_n=4 for the common lithium-7 isotope.

Representations of Atoms and Molecules

  • Atoms are represented using labelled symbols:
    • Hydrogen atom: labeled with the chemical symbol, e.g., H.
  • Molecules are formed when atoms chemically bond:
    • Hydrogen molecule: \text{H}_2, represented by two circles labeled H joined together.
    • Water molecule: H₂O, represented by two H circles and one O circle joined together.
  • Examples (from Figure 8.15):
    • Hydrogen molecule: two H atoms → total atoms in the molecule = 2.
    • Water molecule: two H atoms + one O atom → total atoms in the molecule = 3.
  • Visual representations often use colored circles to distinguish different elements; not all diagrams use explicit bonds, but the count of atoms and their identities is preserved.

Historical Models of the Atom and Why New Models Are Needed

  • Dalton’s billiard ball model (1800s): atoms as hard, solid spheres connected to illustrate chemical combinations.
  • Thomson’s plum pudding model (early 1900s): electrons embedded within a positively charged sphere (the “pudding”).
  • These early models are now considered inaccurate because they cannot explain all observed properties of atoms.
  • Modern atomic models have evolved through experiments and revisions of older ideas; science involves constant revision as new evidence emerges.
  • Key idea: The size, charge distribution, and behavior of sub-atomic particles guide how we model atoms; models are simplifications that aid understanding and prediction.

Sub-Atomic Particles: Relative Charge, Mass, and Position (Table 8.1 concept)

  • Proton
    • Relative charge: +1
    • Relative mass: m_p \approx 1\quad \text{amu}
    • Position: in the nucleus
  • Neutron
    • Relative charge: 0
    • Relative mass: m_n \approx 1\quad \text{amu}
    • Position: in the nucleus
  • Electron
    • Relative charge: -1
    • Relative mass: m_e \approx 1/1836\quad \text{amu} (much lighter than p or n)
    • Position: around the nucleus (in electron cloud/orbitals in more advanced models)
  • Summary: Protons and neutrons are in the nucleus forming the dense core; electrons occupy surrounding space and determine chemical behavior; the nucleus is positively charged due to protons; neutrons provide additional mass and stability.

Connections to Prior Concepts and Real-World Relevance

  • Elements and compounds: the identity of an element depends on its proton number; isotopes differ in neutron count and mass but often retain similar chemical properties.
  • Periodic table as a predictive tool: locating an element by its proton number helps predict behavior, bonding, and reactions.
  • Models and real-world science: models like the planetary model provide intuition but are refined over time to account for experimental findings (e.g., quantum models of electron behavior).
  • Practical implications: understanding isotopes is crucial in fields like medicine, archaeology (carbon dating), and nuclear science.

Quick Summary of Key Formulas and Facts

  • Charge of an atom:
    Q = (+e)Np + (-e)Ne = e\,(Np - Ne)
    Neutral atom: Np = Ne and Q = 0.
  • Atomic number: Z = N_p (number of protons).
  • Isotopes: same Z, different N_n (neutrons).
  • Mass approximation: $$m{\text{atom}} \approx mp Np + mn N_n,\