Chemistry 201 Chapter 7
Chapter 7: Periodic Properties of the Elements
7.1 Development of the Periodic Table
(1869-1870) Mendeleev and Meyer: independently proposed elements could be represented as periodic functions of their atomic weights.
7.2 Atomic Number and Modern Periodic Table
(1913) Mosley: introduced atomic numbers, arranging elements in order of increasing atomic number, leading to the modern periodic table.
7.3 Group Similarities
Elements in the same group share similar chemical properties due to similar electronic configurations.
Example: Oxygen (O) and Sulfur (S) are in Group 6 with the same valence configuration: [He] 2s2 2p4 (O) and [Ne] 3s2 3p4 (S).
Periodic Trends Covered in This Chapter
Effective Nuclear Charge (Zeff)
Sizes of Atoms and Ions
Ionization Energy
Electron Affinity
7.4 Effective Nuclear Charge (Zeff)
Interaction between charged particles: electrons attract the nucleus and repel each other. This dual interaction defines the Zeff experienced by electrons.
Example: Sodium (Na)
Electron Configuration: Na = [Ne] 3s1
Valence electron is shielded by 10 core electrons, resulting in Zeff of +1 for the outer electron.
7.5 Calculation of Zeff
Formula: Zeff = Z - S
Where Z = atomic number, S = number of shielding electrons.
Example: Lithium (Li) and Beryllium (Be) show shielding effects by their core electrons against the positive charge of the nucleus.
7.6 Trends in Zeff
Within a Group: Zeff remains constant.
Within a Period: Zeff increases from left to right due to minimal shielding from electrons in the same shell.
7.7 Atomic and Ionic Sizes
Bonding Atomic Radius: Half the distance between two covalently bonded nuclei.
Non-bonding Atomic Radius: Closest separation of isolated atoms.
Trends:
Within a group: Atomic size increases down the group (increasing principal quantum number).
Within a period: Atomic size decreases from left to right (increasing Zeff).
7.8 Ion Sizes
Ionic size varies based on charge, number of electrons, and electron orbitals.
Cations (e.g., Na+) are smaller than their parent atoms due to electron removal and reduced repulsion.
Anions (e.g., Cl-) are larger than their parent atoms due to added electrons increasing repulsion.
Trends in Ionic Sizes
Within a group: Ions increase in size down the group.
Within a period: Ion size decreases from left to right due to increasing Zeff.
Iso-electronic Ions: Same number of electrons, size decreases with increasing nuclear charge.
7.9 Ionization Energy (I)
Definition: Energy needed to remove an electron from a gaseous atom or ion.
Example: Na(g) → Na+(g) + e-
First Ionization Energy (I1): Energy to remove the first electron. E.g., I1 for Na = 495 kJ/mol.
Trend: I2 > I1 > I3 (greater energy required for successive ionizations).
Trends in Ionization Energy
Within a Group: Ionization energy decreases from top to bottom (valence electrons further from nucleus).
Within a Period: Ionization energy increases from left to right (increasing Zeff and decreasing size).
7.10 Electron Affinity (EA)
Definition: Energy change when an electron is added to a gaseous atom.
Example: Cl(g) + e− → Cl−(g)
General Trends: EA becomes more negative across a period except for noble gases.
7.11 Summary of Periodic Trends
Within a Period:
Ionization energy increases.
Electron affinity becomes more exothermic.
Size decreases.
Effective nuclear charge increases.
Metallic character decreases.
Within a Group:
Ionization energy decreases.
Electron affinity decreases (less exothermic).
Size increases.
Effective nuclear charge remains the same.
Metallic character increases.
Properties of Metals, Nonmetals, and Metalloids
Trends in Metallic Character:
Group 1A (Alkali Metals) are highly reactive and soft. Reactivity increases down the group, producing hydroxides with water.
Group 2A (Alkaline Earth Metals) exhibit moderate reactivity with water, increasing down the group.
Alkali Metals vs. Alkaline Earth Metals
Alkali Metals: Low densities, low ionization energies, only found in compounds.
Alkaline Earth Metals: Higher densities than alkali metals, moderate ionization energies.
Nonmetals and Their Properties
High electron affinities; typically form anions.
Compounds between metals and nonmetals are usually ionic, while those between nonmetals are molecular.
7.12 Group Trends in Selected Nonmetals
Group 6A (Chalcogenes): Oxygen and sulfur are nonmetals; allotropes like O2 and O3 exist.
Group 7A (Halogens): Nonmetals with large, negative electron affinities, leading to reactions with metals.
Group 8A (Noble Gases): High ionization energies, positive electron affinities, and limited reactivity.