Atomic Structure, Electron Shells, and Chemical Reactivity

Electron Energy Levels and Shells

• An electron's energy level correlates with its average distance from the nucleus.

• Shells closest to the nucleus have the lowest potential energy, while electrons in outer shells possess higher potential energy.

• Electrons move between shells by absorbing or losing energy equal to the difference in potential energy between shells:

  • Absorbing energy (such as light energy) moves an electron to a shell farther from the nucleus.

  • Losing energy causes an electron to fall back closer to the nucleus, releasing energy as visible light or ultraviolet radiation.

Electron Distribution and Shell Capacity

• An atom's chemical behavior is determined by the distribution of electrons in its electron shells.

• Elements are built sequentially by adding $1$ proton and $1$ electron at a time (along with neutrons) and are arranged in periods corresponding to their number of electron shells from hydrogen ($1\text{H}$) to argon ($18\,\text{Ar}$).

• Shell capacities and electron arrangements:

  • First shell: Holds a maximum of $2$ electrons and represents the lowest available state of potential energy.

  • Second shell: Holds a maximum of $8$ electrons.

  • Helium ($\text{He}$): Has atomic number $2$, atomic mass $4.003$, and $2$ electrons in its first shell.

  • Lithium: Has $3$ total electrons ($2$ in the first shell, $1$ in the second shell).

  • Neon: Has $10$ total electrons ($2$ in the first shell, $8$ in the second shell).

Valence Electrons and Chemical Reactivity

• Valence electrons are the electrons located in the outermost shell (valence shell), which mostly determine chemical behavior.

• Atoms with the same number of valence electrons exhibit similar chemical behavior:

  • Fluorine ($\text{F}$) and chlorine ($\text{Cl}$) both have $7$ valence electrons and form compounds with sodium ($\text{Na}$), creating sodium fluoride ($\text{NaF}$) and table salt ($\text{NaCl}$).

• Atoms with completed valence shells (helium, neon, and argon) are inert and chemically unreactive.

• Chemical reactivity arises from unpaired electrons in incomplete valence shells.

• The first $4$ electrons added to the second and third shells remain unpaired, and subsequent electrons form pairs.

Concept Check Questions

• Question 1: A nitrogen atom has $7$ protons, and the most common isotope of nitrogen has $7$ neutrons. A radioactive isotope of nitrogen has $8$ neutrons. Write the atomic number and mass number of this radioactive nitrogen as a chemical symbol with a subscript and superscript.

• Question 2: How many electrons does fluorine have? How many electron shells? How many electrons are needed to fill the valence shell?