What are the trends in the periodic table?

Trends in the Periodic Table

The periodic table organizes elements by increasing atomic number and groups elements with similar properties. Several key trends (periodic trends) can be observed as you move across periods (rows) or down groups (columns):

1. Atomic Radius

• Definition: The atomic radius is the distance from the nucleus to the outermost electron shell.
• Trend:
• Decreases across a period (left to right) due to increasing nuclear charge pulling electrons closer.
• Increases down a group as additional electron shells are added.

2. Ionization Energy

• Definition: The energy required to remove the outermost electron from a neutral atom in the gas phase.
• Trend:
• Increases across a period (left to right) because electrons are held more tightly.
• Decreases down a group as outer electrons are farther from the nucleus.

3. Electronegativity

• Definition: The ability of an atom to attract electrons in a chemical bond.
• Trend:
• Increases across a period.
• Decreases down a group.

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Worked Example: Atomic Radius Trend

Question: Compare the atomic radius of sodium (Na) and chlorine (Cl).

Step 1: Locate Na and Cl in Period 3.

• Na: Group 1, Period 3
• Cl: Group 17, Period 3

Step 2: Apply the trend.

• Atomic radius decreases across a period.

Step 3: Conclusion.

$$ \text{Atomic radius of Na} > \text{Atomic radius of Cl} $$

Numerically (approximate values):

$$ \text{Na: } 186, \text{\pm} \qquad \text{Cl: } 99, \text{\pm} $$

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Takeaways

• Atomic radius decreases across a period and increases down a group.
• Ionization energy and electronegativity increase across a period and decrease down a group.
• These trends are due to changes in nuclear charge and electron shielding.