3.2 The Bohr Model

• Electrons are responsible for the formation of chemical bonds between atoms. 

• Electrons also play a crucial role in:

  • the format of the periodic table.

  • determining the electrical conductivity & other physical properties of materials.

https://javalab.org/en/ductile_and_malleable_properties_of_pure_metal_en/ 

The model describes the atom as consisting of a small, positively charged nucleus surrounded by electrons orbiting in specific energy levels or shells.

• • Niels Bohr's discovery of the atomic model is significant because it provided a fundamental understanding of the behavior of electrons within an atom.

  • Bohr's model also provided a basis for understanding chemical bonding, which occurs when atoms share or exchange electrons.

  • Bohr model failed to accurately describe atoms larger than Hydrogen.

What Bohr Got Right:

• • Quantized Energy Levels: Bohr correctly proposed that electrons occupy specific energy levels, or orbits, around the nucleus, where they do not emit energy as they move.

  • Explaining the Hydrogen Spectrum: His model accurately predicted the line emission spectrum of hydrogen by showing that electron transitions between energy levels release photons with specific frequencies.

  • Energy Quantization: Bohr introduced the idea that electron energy changes occur in discrete amounts, aligning with quantum theory.

What Bohr Got Wrong or Couldn't Explain:

• • Only Worked for Hydrogen: Bohr’s model was effective for hydrogen but couldn't accurately describe multi-electron atoms, where electron-electron interactions complicate energy levels.

  • Electron Orbits as Fixed Paths: Bohr imagined electrons traveling in fixed orbits around the nucleus, but later quantum mechanics replaced this with orbitals, regions where electrons are likely to be found, without a defined path.

Energy levels, electron orbits, and electron shells are related to the distribution of electrons in an atom, but they refer to slightly different concepts.

• Electron shells refer to the regions of space around the nucleus where electrons are most likely to be found. Each shell corresponds to a specific energy level and can hold a certain number of electrons. 

• Energy levels refer to the specific energies that electrons in an atom can have within the various shells. Electrons can occupy only certain energy levels, and they move between these levels by absorbing or emitting energy. The shells closest to the nucleus have the lowest energy, while those farther away have higher energy.

• Electron orbits refer to the circular paths that electrons can take around the nucleus. However, this model has been superseded by a more complex and accurate quantum mechanical model, in which electron behavior is described by wave functions rather than simple orbits.

A Bohr diagram is a simplified representation of an atom that shows the number of electrons in each energy level or shell.

Valence electrons are the electrons in the outermost energy level or shell of an atom that are involved in chemical bonding. These electrons are the ones that are most likely to interact with other atoms to form molecules, and their arrangement and number determine the chemical properties of an element.

Core/shielding electrons are the electrons that are not involved in the chemical bonding of an atom. They are the electrons that fill the inner energy levels or shells of the atom.

• These electrons shield the valence electrons from the attractive force of the nucleus. They reduce the net positive charge experienced by the valence electrons, making them less strongly attracted to the nucleus.

• This effect is called the shielding effect, and it explains why valence electrons are not as tightly bound to the nucleus as the core electrons.

To draw Bohr diagrams, follow these steps:

1. Determine the number of protons, neutrons, and electrons:

• Find the atomic number of the element; this is the number of protons.

• In a neutral atom, the number of electrons is equal to the number of protons.

• The atomic mass (rounded) minus the atomic number gives the number of neutrons.

2. Draw the nucleus:

• Represent the nucleus as a small circle in the center.

• Write the number of protons (p⁺) and neutrons (n⁰) inside the nucleus for clarity.

3. Add electron shells:

• Draw concentric circles around the nucleus for each electron shell.

• The first shell can hold up to 2 electrons, the second up to 8, the third up to 18, and so on (typically 8 for simpler diagrams).

4. Place electrons in shells:

• Begin with the innermost shell and fill outward.

• Place up to 2 electrons in the first shell.

• For the second shell, place up to 8 electrons.

• Continue this pattern, following the 2-8-18 rule for each shell.

5. Check your diagram:

• • • Ensure the total number of electrons equals the atomic number.

    • Verify that each shell follows the electron capacity rule.

This completes the Bohr diagram, giving a visual of the atom’s electron arrangement in shells around the nucleus.

Hydrogen Atom: Energy Levels

Atoms gain or lose electrons to achieve a more stable electron configuration, which typically involves filling their outermost energy level (valence shell) with electrons. 

• When an atom has a full valence shell, it is considered to be chemically stable and is less likely to undergo chemical reactions. 

• Atoms can gain or lose electrons in order to attain this stable configuration, which is often referred to as the octet rule.