Atomic Structure and Electronic Configuration

Atomic Structure

Sub-Atomic Particles

• Atoms are composed of sub-atomic particles.
• Ions are formed when atoms gain or lose electrons; the number of protons and neutrons remains constant.
• Charge numbers indicate the number of electrons gained or lost.
  • Example: Calcium (Ca) with a nuclide notation of $_{20}^{40}Ca$ forms $Ca^{2+}$, indicating 2 more protons than electrons. It has 20 protons, 20 neutrons (40 - 20), and 18 electrons (20 - 2).

Cations (Positive Ions)

• Formed when an atom (typically a metal) loses its valence electron(s) to achieve a stable noble gas electronic configuration.
• Have a net positive charge due to more protons than electrons.
• Example: Sodium (Na) loses an electron to form $Na^+$.
• The charge indicates the number of valence electrons lost.
• Sodium atom loses its valence electron to form $Na^+$. Sodium atom has one valence electron. To achieve stable noble configuration, sodium atom loses its valence electron. There are now more protons than electrons and the resulting ion has a 1+ charge.

Anions (Negative Ions)

• Formed when an atom (typically a non-metal) gains electron(s) to achieve a stable noble gas electronic configuration.
• Have a net negative charge due to more electrons than protons.
• Example: Fluorine (F) gains an electron to form $F^-$.
• The charge indicates the number of valence electrons gained.
• Fluorine gains an electron to form $F^−$. Fluorine atom has seven valence electrons. To achieve stable noble configuration, fluorine atom gains an electron. There are now more electrons than protons and the resulting ion has a 1− charge.
• Example: Oxygen (O) with a nuclide notation of $_{8}^{16}O$ forms $O^{2-}$, indicating 2 more electrons than protons. It has 8 protons, 8 neutrons (16 - 8), and 10 electrons (8 + 2).

Distribution of Sub-Atomic Particles

• Electrons move around the nucleus in electron shells (energy levels).
• Valence electrons in the outermost shell possess the greatest energy.
• Inner electron shells are filled first.
• The first electron shell holds up to 2 electrons.
• The second and third shells can typically hold up to 8 electrons each.

Duplet and Octet Electronic Configuration

• Helium (He) has a duplet electronic configuration with 2 electrons in its outermost shell.
• Group 18 elements (noble gases) have filled outermost shells, making them stable and unreactive (inert).
• Neon (Ne) and Argon (Ar) have octet electronic configurations with 8 electrons in their outermost shells.
• Atoms lose, gain, or share electrons to achieve a noble gas electronic configuration (duplet or octet) for stability.

Electronic Structure

Trends in the Periodic Table

• Elements in the same period have the same number of electron shells.
  • Example: Period 2 elements have 2 electron shells.
• Elements in the same group have the same number of valence electrons.
  • Example: Group 13 elements have 3 valence electrons (subtract 10 from group number for Groups 13-18).
• Chemical properties depend on the number of valence electrons.
• Elements in the same group have similar chemical properties.

Writing Electronic Configurations

1. Locate the element in the periodic table.
2. Identify the period to determine the number of electron shells.
3. Identify the group to determine the number of valence electrons.
   • For the first 18 elements, distribute electrons based on the 2, 8, 8 rule.
     • The first shell holds a maximum of 2 electrons.
     • The second and third shells usually hold a maximum of 8 electrons each.
   • Elements in Groups 1 and 2 have 1 and 2 valence electrons, respectively.
   • Elements in Groups 13 to 18 have 3 to 8 valence electrons, respectively.
   • Helium is an exception with only 2 valence electrons in Group 18.

• Example: Lithium (Li)
  • Found in Period 2 (2 electron shells).
  • Belongs to Group 1 (1 valence electron).
  • Electronic configuration: 2, 1
• Example: Beryllium (Be)
  • Found in Period 2 (2 electron shells).
  • Belongs to Group 2 (2 valence electrons).
  • Electronic configuration: 2, 2
• Example: Sodium (Na)
  • Found in Period 3 (3 electron shells).
  • Belongs to Group 1 (1 valence electron).
  • Electronic configuration: 2, 8, 1

Electronic Configuration of Ions

1. Write the electronic configuration of the atom.
2. For cations, remove outermost electrons to achieve noble gas configuration.
3. For anions, add electrons to the valence shell to achieve duplet (for He) or octet configuration.
4. Indicate gained electrons with a different symbol.

Metals vs. Non-metals

• Metals (Groups 1, 2, 13, etc.) lose electrons to form cations (positive ions).
• Non-metals (Groups 14, 15, 16, 17, etc.) gain electrons to form anions (negative ions).

Summary

• Electrons fill electron shells starting from the innermost shell.
• Atoms lose, gain, or share electrons to achieve noble gas configuration (duplet or octet).
• In the Periodic Table:
  • Group number corresponds to the number of valence electrons.
  • Period number corresponds to the number of electron shells.