Comprehensive Study Guide on the History of the Atom and Subatomic Particle Theory

Atomic Theory (1808):
- All matter is constituted of atoms.
- Atoms are indivisible units that cannot be created or destroyed.
- All atoms of a specific given element are identical in both mass and properties.
- Atoms cannot be created, destroyed, or divided (reinforcing the indivisible nature).
- A chemical reaction is defined as a rearrangement of atoms.
- Compounds are formed by the combination of two or more different kinds of atoms (differing elements, differing atoms).
Model Identification: This is often referred to as the "Billiard Ball" Model (1803), proposing the atom as the indivisible unit of an element.

Key Discovery: Discovery of the electron ( $e^-$ ).
Experimental Methodology: Thomson performed the Cathode Ray Tube experiment.
The Plum Pudding Model (1904):
- Negative electrons are embedded within a sphere of uniform positively charged mass.
- The atom is held together by opposite electrical charges.
- Electrons reside within this sphere like plums in a pudding.

Key Discoveries:
- The nucleus.
- The proton ( $p^+$ ).
- The fact that the atom is mostly empty space.
The Gold Foil Experiment:
- Rutherford fired alpha particles at gold foil.
- Observations:
  - Most alpha particles passed through the foil undeflected, indicating the atom is mostly empty space.
  - Some alpha particles bounced off the gold foil.
- Conclusions:
  - The bouncing particles collided with something small, positively charged, and of large mass (the protons and the nucleus).
  - The mass of an atom is not spread evenly throughout but is concentrated in the center, which is the nucleus.
Remaining Paradox: Rutherford could not explain why electrons did not crash into the nucleus.

The Bohr Model:
- Bohr proposed that electrons move in fixed circular orbits around the nucleus.
- These orbits are determined by the energies of the electrons.
The Hydrogen Spectrum Experiment:
- Involves a hydrogen lamp, a slit, and a prism.
- The prism separates component wavelengths of light to produce a photographic film recording of the hydrogen spectrum.
Electron Configurations by Element (as per the Bohr Model chart):
- Lithium ( $Li$ , 3)
- Beryllium ( $Be$ , 4)
- Boron ( $B$ , 5)
- Sodium ( $Na$ , 11): $2, 8, 1$
- Magnesium ( $Mg$ , 12): $2, 8, 2$
- Aluminum ( $Al$ , 13): $2, 8, 3$
- Potassium ( $K$ , 19): $2, 8, 8, 1$
- Calcium ( $Ca$ , 20): $2, 8, 8, 2$
- Gallium ( $Ga$ , 31): $2, 8, 18, 3$
- Rubidium ( $Rb$ , 37): $2, 8, 18, 8, 1$
- Strontium ( $Sr$ , 38): $2, 8, 18, 8, 2$
- Indium ( $In$ , 49): $2, 8, 18, 18, 3$
- Cesium ( $Cs$ , 55): $2, 8, 18, 18, 8, 1$
- Barium ( $Ba$ , 56): $2, 8, 18, 18, 8, 2$
- Thallium ( $Tl$ , 81): $2, 8, 18, 32, 18, 3$
- Francium ( $Fr$ , 87): $2, 8, 18, 32, 18, 8, 1$
- Radium ( $Ra$ , 88): $2, 8, 18, 32, 18, 8, 2$

Modification of Bohr's Model: Heisenberg and Schrodinger (1926) transitioned the model away from fixed paths.
Electron Cloud/Quantum Mechanical Model:
- Electrons are NOT in circular orbits around the nucleus.
- Electrons occupy regions of space called atomic orbitals, forming an electron cloud.
- The Quantum Mechanical Model is a 3-D model describing the mathematical probability of where electrons can be found around the nucleus based on their energies.

Objective: To explain the discrepancy between atomic number and atomic mass.
Key Discovery: The neutron ( $n^0$ ).
Experimental Evidence: Proved the existence of the neutron using an alpha particle experiment.
- The setup involved alpha particles hitting Beryllium ( $Be$ ), producing unknown radiation that struck paraffin wax, causing the release of protons detectable by a sensor.
Properties discovered: The nucleus contains particles similar in mass to protons but possessing no electrical charge.

The Nucleus:
- Occupies a tiny fraction of the total volume of the atom.
- Contains almost all of the mass of the atom.
- Composed of protons and neutrons.
Proton ( $p^+$ ):
- Charge: Positive ( $1+$ ).
- Mass ( $\text{amu}$ ): $1.0073$ .
- Location: Nucleus.
- Determines the identity of the atom ( $\text{Number of protons} = \text{Atomic Number}$ ).
Neutron ( $n^0$ ):
- Charge: None (neutral).
- Mass ( $\text{amu}$ ): $1.0087$ .
- Location: Nucleus.
Electron ( $e^-$ ):
- Charge: Negative ( $1-$ ).
- Mass ( $\text{amu}$ ): $5.486 \times 10^{-4}$ .
- The mass is much smaller (~ $1/2000$ ) than that of a neutron/proton and is essentially ignored when calculating the mass of an atom.
- Location: Around the nucleus in electron clouds or orbitals.

Periodic Table Usage: Atomic mass should be rounded to the nearest whole number for general use (e.g., Silicon rounds to $28$ ).
Mass Number: The total number of protons and neutrons in an atom.
- $\text{Mass Number} = \#\,p^+ + \#\,n^0$
- Atomic mass is measured in atomic mass units ( $\text{amu}$ ) because grams are a unit of scale far too large.
Neutral Atoms: In an atom with no charge, the number of protons equals the number of electrons.
- $\text{Atomic \#} = \#\,p^+ = \#\,e^-$
Nuclear Symbol and Chemical Names:
- Notation: Element name – mass number (e.g., Helium - 4).
- $\text{Mass Number} = p^+ + n^0$

Changes in Identity: An atom can change its number of electrons or neutrons without changing which element it is.
Ions: Atoms with a positive or negative charge resulting from the loss or gain of electrons.
- Cation: A positively charged atom that has lost one or more electrons (p^+ > e^-).
  - $\text{Sodium (lost 1 electron)} = Na^{1+}$
  - $\text{Beryllium (lost 2 electrons)} = Be^{2+}$
  - $\text{Aluminum (lost 3 electrons)} = Al^{3+}$
- Anion: A negatively charged atom that has gained one or more electrons (e^- > p^+).
  - $\text{Chlorine (gained 1 electron)} = Cl^{1-}$
  - $\text{Oxygen (gained 2 electrons)} = O^{2-}$
  - $\text{Nitrogen (gained 3 electrons)} = N^{3-}$
- Charge Calculation Formula: $p^+ - e^- = \text{charge}$
Isotopes: Atoms of the same element with different numbers of neutrons.
- They remain the same element because the number of protons is identical.
- Naming convention: Element name followed by the atomic mass of that specific isotope.

Neutral Atoms Table

Element	Atomic #	Mass #	Protons ( $p^+$ )	Neutrons ( $n^0$ )	Electrons ( $e^-$ )
Magnesium-24	$12$	$24$	$12$	$12$	$12$
Fluorine-19	$9$	$19$	$9$	$10$	$9$

Complex Ion and Isotope Table

Nuclear Symbol	Atomic #	Mass #	Protons	Neutrons	Electrons
${}^{31}_{15}P$	$15$	$31$	$15$	$16$	$15$
${}^{25}_{12}Mg^{2+}$	$12$	$25$	$12$	$13$	$10$
${}^{18}_{9}F^{1-}$	$9$	$18$	$9$	$9$	$10$