Atomic Structure, Periodic Trends, and Ionic Compounds

Atomic Structure and Periodic Trends

Atomic Size and Periodic Trends

The periodic table organizes elements based on their atomic number and recurring chemical properties. Understanding periodic trends is essential for predicting element behavior.

• Atomic Size (Atomic Radius): The atomic radius is the distance from the nucleus to the outermost electron shell of an atom.

• Trend Down a Group: Atomic size increases as you move down a group (column) because additional electron shells are added, making the atom larger.

• Trend Across a Period: Atomic size decreases from left to right across a period (row) due to increasing nuclear charge, which pulls electrons closer to the nucleus.

• Example: Sodium (Na) is larger than chlorine (Cl) in the same period, but potassium (K) is larger than sodium (Na) because it is in the next period down.

Equation for Effective Nuclear Charge (Zeff):

$Z_{\text{eff}} = Z - S$ Where $Z$ is the atomic number and $S$ is the number of shielding (core) electrons.

Formation of Ions

Cations and Anions

Atoms can gain or lose electrons to form ions, which are charged particles. The process depends on the element's position in the periodic table.

• Cations: Formed when an atom loses one or more electrons, resulting in a positive charge. Typically formed by metals.

• Anions: Formed when an atom gains one or more electrons, resulting in a negative charge. Typically formed by nonmetals.

• Example: Sodium (Na) loses one electron to become $\text{Na}^+$; chlorine (Cl) gains one electron to become $\text{Cl}^-$.

Equation for Ion Formation:

$\text{Na} \rightarrow \text{Na}^+ + e^-$ $\text{Cl} + e^- \rightarrow \text{Cl}^-$

Ionic Compounds

Formation and Naming of Ionic Compounds

Ionic compounds are formed when cations and anions combine in ratios that result in a neutral compound. The electrostatic attraction between oppositely charged ions holds the compound together.

• Formation: Metals react with nonmetals to form ionic compounds. The metal loses electrons (forms cation), and the nonmetal gains electrons (forms anion).

• Naming: The cation is named first, followed by the anion. For simple binary compounds, the anion's name ends in "-ide." For compounds with polyatomic ions, use the ion's name.

• Example: Barium chloride is formed from barium ions ($\text{Ba}^{2+}$) and chloride ions ($\text{Cl}^-$). The formula is $\text{BaCl}_2$.

Equation for Formation of Barium Chloride:

$\text{Ba} + \text{Cl}_2 \rightarrow \text{BaCl}_2$

Common Ionic Compounds Table

Cation	Anion	Compound Name	Formula
Na+	Cl-	Sodium chloride	NaCl
Ba2+	Cl-	Barium chloride	BaCl2
K+	Br-	Potassium bromide	KBr
Ca2+	O2-	Calcium oxide	CaO

Additional info: Table entries inferred for completeness based on standard ionic compounds.

Summary

• Atomic size increases down a group and decreases across a period.

• Metals lose electrons to form cations; nonmetals gain electrons to form anions.

• Ionic compounds are named by listing the cation first, then the anion, with appropriate suffixes.

• Understanding periodic trends and ion formation is essential for predicting chemical behavior and compound formation.