Periodic Properties of Elements: Study Notes

Periodic Properties of Elements

Introduction

The periodic properties of elements are fundamental concepts in general chemistry, describing how atomic structure influences chemical behavior. These properties include trends in atomic and ionic size, ionization energy, electron affinity, and metallic character, all of which are governed by the arrangement of electrons and the structure of the periodic table.

Alkali Metals

Properties and Electron Configuration

• Alkali metals (Group 1A) have one more electron than the previous noble gas.

• In chemical reactions, alkali metals tend to lose one electron, resulting in the same electron configuration as a noble gas.

• They form cations with a 1+ charge (e.g., Na+, K+).

Example: Sodium (Na) loses one electron to form Na+, achieving the electron configuration of neon.

Halogens

Properties and Electron Configuration

• Halogens (Group 7A) have one fewer electron than the next noble gas.

• In reactions with metals, halogens gain one electron to attain the electron configuration of the next noble gas, forming anions with a 1- charge (e.g., Cl-, Br-).

• In reactions with nonmetals, halogens share electrons to achieve noble gas configurations.

Example: Chlorine (Cl) gains one electron to form Cl-, matching the electron configuration of argon.

Electron Configuration and Ion Charge

Predictable Ion Charges Based on Periodic Table Position

• Many metals and nonmetals form ions with predictable charges based on their group:

  • Group 1A: 1+

  • Group 2A: 2+

  • Group 7A: 1-

  • Group 6A: 2-

• Atoms form ions to achieve the electron configuration of the nearest noble gas.

Example: Magnesium (Mg) in Group 2A forms Mg2+ by losing two electrons.

Elements That Form Ions With Predictable Charges

Periodic Table Overview

The following table summarizes the predictable charges for main-group elements:

Group	Common Ion Charge	Example Elements
1A	1+	Li+, Na+, K+
2A	2+	Mg2+, Ca2+, Sr2+, Ba2+
3A	3+	Al3+
5A	3-	P3-
6A	2-	O2-, S2-, Se2-, Te2-
7A	1-	F-, Cl-, Br-, I-

Conceptual Connection: Predicting Ion Charge

Electron Configuration and Ion Formation

If a main-group element has an outer electron configuration of $ns^2np^4$, it is likely to form an ion with a 2- charge. This is because gaining two electrons will complete the p sublevel, resulting in a stable noble gas configuration.

• Key Point: Elements with $ns^2np^4$ (such as oxygen and sulfur) tend to form 2- anions.

Example: Sulfur (S) with configuration $3s^23p^4$ forms S2- by gaining two electrons.

Summary Table: Elements That Form Ions With Predictable Charges

Element	Group	Ion Charge	Electron Configuration After Ion Formation
Na	1A	1+	Same as Ne
Mg	2A	2+	Same as Ne
Cl	7A	1-	Same as Ar
S	6A	2-	Same as Ar

Additional info: The tendency of elements to form ions with predictable charges is a direct consequence of their desire to achieve a stable, noble gas electron configuration. This underlies many chemical reactions and the formation of ionic compounds.