Atomic Structure: From Dalton to the Quantum Model
Dalton’s Atomic Theory
In the nineteenth century, English chemist John Dalton, drawing on the ideas of Leucippus and Democritus, set out the atomic theory which states that:
- Chemical elements are composed of tiny indivisible particles called atoms.
- Atoms of the same element are equal in mass and other properties.
- Atoms of different elements differ in mass and other properties.
- Atoms of different elements combine to form compounds.
Discovery of the Electron
Michael Faraday applied electric shocks to containers with a partial vacuum, observing a faint luminescence. However, he couldn’t deduce much due to the low vacuum quality.
In 1857, Geissler invented a more efficient vacuum pump and connected a tube with two electrodes to an electric generator. He observed luminescence connecting the electrodes.
In 1897, J.J. Thomson, using a gas discharge tube with an improved vacuum and high voltage across its electrodes, observed a faint fluorescence. This was attributed to cathode rays emitted from the negative electrode.
Electrons are the constituent particles of cathode rays. They have mass and a negative charge.
The Proton
A proton is a particle with a positive charge equal in magnitude to the electron’s charge, but with a mass almost two thousand times greater.
Plum Pudding Model
In 1904, J.J. Thomson proposed the plum pudding model, envisioning the atom as a sphere of uniform positive charge with embedded electrons. The number of electrons ensured electrical neutrality. Due to the electrons’ tiny mass, the positive charge contributed almost all of the atom’s mass.
Concept of Ion
An atom is neutral when its positive and negative charges are equal. Losing electrons results in a positive ion (cation), while gaining electrons results in a negative ion (anion).
- Cation (positive ion): Formed when an atom or group of atoms loses one or more electrons.
- Anion (negative ion): Formed when an atom or group of atoms gains one or more electrons.
Rutherford Model
In 1911, Ernest Rutherford proposed a new atomic model based on his gold foil experiment. He bombarded a thin gold sheet with alpha particles (positively charged particles with large mass).
Rutherford’s atomic model described the atom as:
- A small, dense, positively charged nucleus containing protons.
- An outer shell containing electrons.
- Electrons orbiting the nucleus at high speed.
- A mostly empty space between the nucleus and the electron shell.
Atomic Number (Z)
The number of protons in an atom’s nucleus, unique to each element.
Mass Number (A)
The sum of the number of protons and neutrons in an atom’s nucleus.
Isotopes
Atoms of the same element with the same number of protons (Z) but different numbers of neutrons and different mass numbers (A).
Isotopes have various applications:
- Medicine (e.g., killing tumors)
- Archaeological research (e.g., determining age)
- Agriculture (e.g., food preservation)
- Industry (e.g., quality control)
Examples of hydrogen isotopes:
- Protium (Z=1, A=1)
- Deuterium (Z=1, A=2)
- Tritium (Z=1, A=3)
Model
A model is an approximation or representation of reality. A good model accurately reflects the observed phenomena.
Bohr Model
In 1913, Niels Bohr proposed a model explaining line spectra. He suggested that electrons orbit the nucleus in specific energy levels and emit or absorb energy when transitioning between levels.
Quantized Model
Bohr’s model introduced the concept of quantized energy levels. Electrons absorb energy to jump to higher levels and emit energy when returning to lower levels.
Charge Cloud or Orbital Model
The current atomic model describes electrons moving around the nucleus in a cloud-like region, not fixed orbits. The exact position of an electron cannot be determined, but the probability of finding it in a specific region can be calculated.
An orbital is the region of space where there’s a 99% probability of finding an electron.
In the hydrogen atom’s ground state, the electron has a 90% probability of being within a spherical region called the s orbital.
Each orbital can hold a maximum of two electrons. Different energy levels have different sets of orbitals and sublevels. Energy increases outwards from the nucleus.
The distribution of electrons in an atom’s energy levels and orbitals is called the electron configuration.
There are four types of orbitals: s, p, d, and f.
Characteristics of Atoms
- All atoms of an element have the same number of protons.
- Atoms of different elements have different numbers of protons.
Weighted Average
A weighted average considers the proportion of each component in a mixture to calculate the average value.