Trends in the Periodic Table

Shell Model of the Atom

• Classifies electrons as "core" or "valence."
• Valence electrons increase across a row.

Trends Across a Row

• As you move across a row, the number of valence electrons increases. Also the atomic radius decreases. Example:
  • Lithium (Li):
    • 3 protons
    • 4 neutrons
    • Tends to lose 1 electron
    • Atomic radius = 145 pm
  • Beryllium (Be):
    • 4 protons
    • 5 neutrons
    • Tends to lose 2 electrons
    • Atomic radius = 105 pm
  • Boron (B):
    • 5 protons
    • 6 neutrons
    • Tends to lose 3 electrons
    • Atomic radius = 85 pm

Coulomb's Law

• Explains how electrons are held in place around the nucleus.
• Electrons orbiting the nucleus are held "in place" by Coulomb's Law.
  • Lithium:
    • 3 protons
    • 4 neutrons
    • Valence (outer) electron
    • Core electrons
    • Tends to lose 1 electron
    • Atomic radius = 145 pm

Coulomb's Law: Energy and Force

• Force: $F = k\frac{q<em>1q</em>2}{r^2}$
  • F is the force of attraction or repulsion between charges.
  • $q<em>1$ and $q</em>2$ are the charges on each particle.
  • r is the distance between charges.
• Energy: $E = k\frac{q<em>1q</em>2}{r}$
  • E is the energy of attraction or repulsion between charges.
• Note: We will evaluate qualitatively, but if you want to know:
  • For integer charge and distance in pm: $k = 2.3 \times 10^{-6} N \cdot pm^2$
  • The unit of energy is Joule = N·m
• Example: $F = 2.3 \times 10^6 N \cdot pm^2 \frac{(+2)(-1)}{(100 pm)^2} = -4.6 \times 10^{-10}$

Factors Influencing Ionic Bonding Strength

• Attraction increases as size decreases.
• Attraction increases as charge increases.

Trends in a Group (Column)

• Lithium (+3)
• Sodium (+11)
• Potassium (+19)
• The shell model provides an explanation for similarities in properties in a group of elements.

Periodic Table Trends

• Mass increases across a row due to the addition of neutrons and protons.
• Mass increases down a group due to the addition of shells.
• Atomic radius decreases across a period (row).
  • Even though we're adding more density, it gets smaller.
• Atomic radius increases greatly going down a group due to the addition of shells.

Atomic Radius and Ions

• Neutral atoms (PT)
• Cations (+, less electrons): smaller due to decreased electron-electron repulsion.
• Anions (-, more electrons): larger due to increased electron-electron repulsion.

Reactivity Trends

• Metals: like to lose electrons to become cations
  • Reactivity increases down a group (e.g., Li, Na, K, Rb, Cs).
  • Reactivity decreases across a period.
• Non-metals: gain electrons to become anions
  • Reactivity decreases down a group due to increased shielding.
  • Reactivity increases across a period.

Atomic Size

• Atoms or ions in order of decreasing size:
  *Cs, Rb, Rb+
  *Na, Mg, Mg+
  *Be, N, Ne

Trends Across Periods and Groups

• Atomic radius
• Reactivity
• Mass
• Valence electrons

New Concepts and Trends

• Ionization energy:
  • Energy required to remove the outermost electron from an atom.
• Electronegativity:
  • The tendency of an atom to attract electrons toward itself.
  • Non-metals tend to have higher electronegativity.

Ionization Energy

• Energy required for the complete removal of an electron from an atom or ion.
• Reactants → Products
  • Example: $Ca \rightarrow Ca^{2+} + 2e^-$
  • Example: $Fe^{3+} + e^- \rightarrow Fe^{2+} + 3e^-$
• Factors that impact IE:
  • Ionization energy increases across a period (Energy = force x distance).
  • Ionization energy decreases down a group.

Electronegativity

• The tendency of an atom to attract electrons toward itself.
• Electronegativity increases from bottom left to top right on the periodic table.
• Water molecule: Electronegativity in molecules creates dipole moments.

Noble Gases

• Noble gases are not mentioned in the electronegativity graph.
• This is likely due to their full valence shells, making them stable and not prone to attracting additional electrons (i.e., their electronegativity is effectively zero).