General Chemistry (I) - Atomic Structure

Study Notes

Pure substances are classified as elements or compounds.
Substances have unique properties based on their atomic/ionic/molecular structure.
Dalton's atomic model (1803-1810):
- Elements are composed of atoms.
- Atoms of the same element are identical in mass and size.
- Atoms of different elements have different masses and sizes.
- Chemical compounds are formed by the union of atoms of different elements in simple whole-number ratios.
Law of definite composition: A particular compound always contains the same elements in the same proportions by mass.
Example: Water (H₂O) always contains 11.2% hydrogen and 88.8% oxygen by mass.

Faraday discovered that certain substances conduct electricity when dissolved in water.
Atoms are attracted to either positive or negative electrodes depending on their charge.
Arrhenius extended Faraday's work, defining an ion as an atom or group of atoms with a positive or negative charge.
Cations are positively charged ions.
Anions are negatively charged ions.
Ions are responsible for the conductivity of certain substances (e.g., sodium chloride).

Atoms are not indivisible but rather composed of subatomic particles: electrons, protons, and neutrons.
Electron:
- Negatively charged.
- Extremely small mass compared to protons and neutrons.
- Mass ~ 9.110 x 10⁻²⁸ g
- Charge ~ -1.602 x 10⁻¹⁹ C
- Found outside the nucleus.
- e/m = electron charge-to-mass ratio.
Proton:
- Positively charged.
- Mass ~ 1.673 x 10⁻²⁴ g
- Charge ~ +1.602 x 10⁻¹⁹ C
- Found in the nucleus.
Neutron:
- No charge.
- Mass ~ 1.675 x 10⁻²⁴ g.
- Found in the nucleus.
Atoms of each element have a characteristic number of protons (atomic number).
The mass of an atom is determined predominantly by the number of protons and neutrons.

Radioactivity experiments by Becquerel and others provided insights into atomic structure, including the existence of alpha, beta, and gamma rays, eventually leading to Rutherford's discovery.
Rutherford's gold foil experiment:
- Most alpha particles passed straight through the gold foil.
- Some alpha particles were deflected at large angles.
- A very small fraction of alpha particles bounced back.
Rutherford's model:
- Atom has a small, dense, positively charged nucleus at the center.
- Negatively charged electrons orbit the nucleus.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Isotopes of an element have the same atomic number but different mass numbers.
Mass number (A) = number of protons + number of neutrons.

Atomic mass is the average mass of the atoms of an element, taking into account the different isotopes and their relative abundance.
Atomic mass unit (amu) = 1/12 the mass of a carbon-12 atom.

Atoms consist entirely of protons, neutrons, and electrons.
Atoms are not solid spheres.
Wave-mechanical model of the atom:
- Electrons do not exist in fixed orbits.
- Electron density can be calculated.

Bohr's model:
- Electrons orbit the nucleus in specific energy levels (quantized).
- Electrons can gain or lose energy by jumping between energy levels.
Calculations of electron orbitals and energy levels.
Calculation of wavelengths corresponding to spectral lines in hydrogen's spectrum

Schrödinger's equation: A mathematical equation describing the behavior of electrons in an atom.
Electron's quantized energy levels.
Probability distribution of electrons.