Nucleus and Electron Shells (Video Notes)

Nucleus and Electron Count

• The nucleus contains protons (and neutrons).
• For a neutral atom, the number of electrons equals the number of protons.
• The element is defined by the number of protons (atomic number Z).

Electron Shells and Capacity

• Electrons fill shells from the innermost outward.
• Shell capacity follows the formula 2n^2 for shell number n.
  • n=1: 2 electrons
  • n=2: 8 electrons
  • n=3: 18 electrons
• The first shell is called the K shell.

Carbon (Z = 6) Example

• Electronic configuration: $1s^2 2s^2 2p^2$.
• Shell distribution: K shell = 2 electrons, L shell = 4 electrons.
• Carbon has 4 valence electrons (in the second shell outside the filled first shell).
• Unpaired electrons: there are 2 unpaired electrons in the 2p subshell (Hund's rule).

Quick recall

• Carbon: Z = 6; neutral atom has electrons = Z = 6.
• Number of unpaired electrons in carbon: 2.