Introduction
The periodic table is a powerful tool in chemistry that organizes elements based on their atomic number, electron configurations, and recurring chemical properties. One of the key aspects of the periodic table is the periodic trends that can be observed across periods (rows) and groups (columns). These trends help predict and explain the chemical behavior of the elements. This study note will delve into the periodic trends, focusing on electron affinity, electronegativity, and chemical properties, as well as the specific trends seen in oxides, Group 1 (alkali metals), and Group 17 (halogens).
3.1.4 Periodic Trends: Physical - Electron Affinity
Definition
Electron affinity ($E_{\text{ea}}$) is the energy change that occurs when an electron is added to a neutral atom in the gas phase to form a negative ion. It can be represented as: $$ X(g) + e^- \rightarrow X^-(g) $$
Trend Across a Period
- Increase from left to right: As you move across a period, the electron affinity generally becomes more negative. This is because the nuclear charge increases, making the addition of an electron more energetically favorable.
For example, the electron affinity of chlorine is more negative than that of sodium: $$ \text{Na} + e^- \rightarrow \text{Na}^- \quad \Delta E = -53 \text{ kJ/mol} $$ $$ \text{Cl} + e^- \rightarrow \text{Cl}^- \quad \Delta E = -349 \text{ kJ/mol} $$
Trend Down a Group
- Decrease down a group: As you move down a group, the electron affinity becomes less negative. This is due to the increased distance between the nucleus and the added electron, which reduces the attraction.
Remember that noble gases have positive electron affinities because they have a stable electron configuration and adding an electron would require energy input.
3.1.5 Periodic Trends: Physical - Electronegativity
Definition
Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. It is a dimensionless quantity, often measured on the Pauling scale.
Trend Across a Period
- Increase from left to right: Electronegativity increases across a period due to the increasing nuclear charge, which attracts bonding electrons more strongly.
Fluorine has the highest electronegativity ($\chi = 3.98$ on the Pauling scale), while cesium has one of the lowest ($\chi = 0.79$).
Trend Down a Group
- Decrease down a group: Electronegativity decreases as you move down a group because the added electron shells increase the distance between the nucleus and the bonding electrons, reducing the attraction.
Electronegativity is crucial for understanding bond polarity and the behavior of molecules in chemical reactions.
3.1.6 Periodic Trends: Chemical
Reactivity of Metals
- Increase down a group: For alkali metals (Group 1), reactivity increases down the group because the outer electron is more easily lost due to increased atomic size and decreased ionization energy.
Lithium reacts less vigorously with water compared to cesium, which reacts explosively.
