Chapter 7: Periodic Properties of the Elements – Study Notes

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Chapter 7: Periodic Properties of the Elements

7.1 Periodic Table

The periodic table is a systematic arrangement of elements based on increasing atomic number. This arrangement reveals recurring trends, known as periodicity, in the physical and chemical properties of the elements.

Periodicity: The repetitive pattern of a property for elements as a function of atomic number.
Trends are observed both across periods (rows) and down groups (columns).

Example: Atomic radius, ionization energy, and electron affinity all show periodic trends.

7.2 Effective Nuclear Charge (Zeff)

Effective nuclear charge is the net positive charge experienced by valence electrons in an atom. It is less than the actual nuclear charge due to shielding by inner (core) electrons.

Valence electrons are attracted to the nucleus but repelled by other electrons.
Across a period (left to right): Zeff increases because the number of protons increases while shielding remains relatively constant.
Down a group: Both the number of protons and core electrons increase, but the increase in distance (energy levels) means Zeff increases less dramatically.

Formula:

where is the atomic number and is the number of shielding (core) electrons.

Example: Sodium (Na) has a higher effective nuclear charge than lithium (Li).

7.3 Sizes of Atoms and Ions

The atomic radius is the distance from the nucleus to the outermost electron shell. The size of ions depends on their charge and electron configuration.

Atomic size decreases across a period (left to right) and increases down a group (top to bottom).
Cations (positively charged ions) are smaller than their parent atoms because they lose outer electrons.
Anions (negatively charged ions) are larger than their parent atoms due to increased electron-electron repulsion.

Example:

Strontium (Sr) is larger than Indium (In).
Ca is larger than Ca2+.
Cl- is larger than Cl.
Cs is larger than S.

7.4 Ionization Energy

Ionization energy is the minimum energy required to remove an electron from a gaseous atom or ion in its ground state.

Higher ionization energy means it is more difficult to remove an electron.
Ionization energy increases across a period and decreases down a group.
Atoms closer to a full octet have higher ionization energies.
Successive ionization energies increase for each electron removed.

Example:

Ca has a higher ionization energy than Ba.
Se has a higher ionization energy than Fe.
O has a higher ionization energy than Zr.

7.5 Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a gaseous atom. A more negative value indicates a greater tendency to accept an electron.

Electron affinity generally increases (becomes more negative) across a period and decreases down a group.
The more energy released, the more stable the resulting anion.

Equation:

Example:

Carbon (C) has a higher electron affinity than Tin (Sn).
Zinc (Zn) has a higher electron affinity than Potassium (K).

Electronegativity

Electronegativity is the ability of an atom in a molecule to attract shared electrons to itself.

Electronegativity increases across a period and decreases down a group.
Noble gases generally do not form compounds and thus are not assigned electronegativity values.
Fluorine (F) is the most electronegative element.

Example: Phosphorus (P) is more electronegative than Barium (Ba).

Summary Table: Periodic Trends Examples

The following table summarizes the trends for selected sets of elements and ions:

Set	Smallest in Size	Lowest Ionization Energy	Lowest Electron Affinity	Lowest Electronegativity
Cu0, Cu+1, Cu+2	Cu+2	Cu0	Cu0	Cu0
K, Ni, Se	Se	K	K	K
Rb, I, Ru	I	Rb	Rb	Rb
Ba, Pt, S	S	Ba	Ba	Ba

Key Periodic Trends Summary

Effective Nuclear Charge (Zeff): Increases across a period, slightly increases down a group.
Atomic/Ionic Size: Decreases across a period, increases down a group. Cations are smaller, anions are larger than parent atoms.
Ionization Energy: Increases across a period, decreases down a group.
Electron Affinity: Generally increases (becomes more negative) across a period, decreases down a group.
Electronegativity: Increases across a period, decreases down a group. Fluorine is the highest.

Additional info: These trends are fundamental for predicting chemical reactivity, bonding, and the properties of elements and their compounds.