4.3 Electrons, Electron Configuration, and Shells

Electrons, Electron Configuration, and Shells

• Influence of Electrons
    - Chemical reactions are fundamentally driven by the movement of electrons.
    - The behavior of matter is largely determined by the outermost electrons of an atom.

Shells of Electrons

• Bohr Model of the Atom
    - Proposed by Niels Bohr in 1913.
    - Atom consists of a positively charged nucleus surrounded by negatively charged electrons in concentric shells.
    - Each shell has a specific amount of energy, known as energy levels.
      - Shells closer to the nucleus have lower energy while those further have higher energy.
    - Evidence for this arrangement comes from atomic spectral analysis (discussed in section 4.3.4).

• Diagram Representation
    - Electrons cannot be precisely observed; they have a probabilistic 'cloud' nature.
    - An electron shell diagram simplifies the model by showing the nucleus at the center, with electrons arranged in shells, depicted as concentric circles.
    - Inner shell electrons are more strongly attracted to the nucleus than outer shell electrons.

Learning Intention

• At lesson's end, students should:
    - Describe electrons existing in energy levels called shells.
    - Appreciate the experimental evidence for this arrangement.

The Electron Shell Diagram

• Structure of Shells
    - Shells can be compared to drawers storing electrons.
    - The number of electrons per shell follows the formula:
      - $n = 2n^2$, where n is the shell number.
    - Example:
      - Fourth shell can hold a maximum of $2 imes 4^2 = 32$ electrons.
      - Fifth shell can hold a maximum of $2 imes 5^2 = 50$ electrons.

• Number of Electrons per Shell:
    - 1st Shell: 2
    - 2nd Shell: 8
    - 3rd Shell: 18
    - 4th Shell: 32

Electron Configuration

• Definition
    - Electron configuration is an ordered list of the number of electrons in each shell.

• Example: Sodium Atom
    - Sodium has an atomic number of 11 (11 protons, 11 electrons).
      1. Shell 1: 2 electrons (full).
      2. Remaining: 9 electrons.
      3. Shell 2: 8 electrons (full).
      4. Remaining: 1 electron to Shell 3.
    - Final Configuration: 2, 8, 1.

• Maximum Electrons Calculation
    - Maximum electrons can be calculated with the formula:
      - $ext{nth shell holds a maximum of } 2n^2 ext{ electrons.}$

Different Electron Configurations

Determining Electron Configuration

• Steps for elements with atomic number up to 18:
    1. Identify the number of electrons (same as atomic number).
    2. Fill shell 1 until full (2 electrons).
    3. Fill shell 2 until full (8 electrons).
    4. Assign remaining electrons to successive shells.
    5. Write configuration with commas separating shell numbers.

The Periodic Table Explained

• Historical Context
    - Mendeleev and Meyer grouped elements by similar chemical properties, unaware of electrons.
    - Electron shell arrangement explains many trends observed in the periodic table.

• Trends
    - Moving down groups adds shells.
    - Moving across periods increases outer shell electrons.

• Valence Electrons
    - The outermost shell contains valence electrons, crucial in chemical reactions.
    - Atoms in the same group share the same number of valence electrons, hence similar properties.

• Limitation
    - No more than 8 valence electrons can stabilize an atom.

Filling of Electron Shells

• Periodic Table Insights
    - First shell (2 electrons) allows 2 elements (H: 1 electron, He: 2 electrons).
    - Second shell (8 electrons) aligns with eight elements in the second period.
    - Third period has eight elements despite the third shell's capacity for 18 electrons.

• Example: Potassium Atom
    - Electron configuration: 2, 8, 9 (third shell unstable; begins filling fourth shell).
    - Stable configuration: 2, 8, 8, 1.

Quantum Mechanics

• Definition
    - Quantum mechanics studies behavior at the sub-atomic level, including electrons and shells.
    - Richard Feynman: “If you think you understand quantum mechanics, then you don’t understand quantum mechanics.”

Upwardly Mobile Electrons

• Energy Transfer
    - Atoms can gain enough energy to move electrons from one shell to a higher energy level.

• Flame Test: Demonstration of Electron Excitation
    1. Heat a substance in a flame to excite electrons.
    2. Electrons absorb energy and move to higher levels.
    3. Cooling causes electrons to fall, emitting visible light as they return to ground state.

• Applications
    - Flame colors serve as indicators of different elements.

Chasing Rainbows

• Significance of Emission Colors
    - Different atoms emit light of distinct colors, validating the existence of electrons in shells.
    - Emission spectra can identify elements by the light they release when energized.

Spectroscopy

• 
  

• Introduction to Spectroscopy
    - Sir Isaac Newton demonstrated white light breaks into a spectrum of colors.
    - The spectroscope analyzes light into specific frequencies and wavelengths.

• 
  

• Table of Frequencies and Wavelengths
  

  Color	Frequency (×10^12 Hz)	Wavelength (nm)
  Red	400–480	750–625
  Orange	480–510	625–590
  Yellow	510–530	590–570
  Green	530–600	570–500
  Blue	600–670	500–450
  Indigo	670–700	450–430
  Violet	700–750	430–400
  Emission vs. Absorption Spectra

  • Emission Spectrum
      - Observed as distinct lines on a colored background, characteristic for each element.

  • Absorption Spectrum
      - Formed by a continuous spectrum with dark lines corresponding to specific gases passed through.
      - Used similarly to identify elements, discovered patterns such as that of helium in sunlight.

  Quantum in the Cosmos

  • **Stellar Composition
      - Light from stars provides insights into their elemental composition and shows a predominance of hydrogen and helium.

  • Red Shift Phenomenon
      - Observations of spectra show a slight red shift, indicating that stars are moving away, helping calculate their speed.

  Electron Shell Transitions

  • Ground and Excited States
      - In ground states, only lowest energy shells are filled.
      - Excited states occur when electrons absorb energy and occupy higher levels.

  • Energy Transitions
      - Electrons can only occupy specific shells; transitions result in photon emission when returning to lower energy states.
      - The size of energy jumps correlates with the emitted light's color and energy.

  Extension: Sub-shells

  • Refinement in Shell Theory
      - Technology reveals shells subdivided into sub-shells with distinct energy levels.
      - Designations include s, p, d, f, and g, connected to main shell numbers (e.g., 2p, 3p).

  • Table of Shells and Sub-shells
    | Shell Number | Number of Sub-shells | Sub-shell Designations | Electron Capacity |
    |--------------|---------------------|---------------------|------------------|
    | 1 | 1 | 1s | 2 |
    | 2 | 2 | 2s, 2p | 8 |
    | 3 | 3 | 3s, 3p, 3d | 18 |
    | 4 | 4 | 4s, 4p, 4d, 4f | 32 |