Chapter 5: The Periodic Table – Structure, Trends, and Properties

Chapter 5: The Periodic Table

Introduction

The periodic table is a fundamental tool in chemistry, organizing all known elements according to their properties and atomic structure. This chapter explores the history, organization, and trends of the periodic table, providing a foundation for understanding chemical behavior.

5.1 Classification of the Elements

Early Attempts at Classification

• Triads: In 1829, J. W. Döbereiner grouped elements into triads, sets of three with similar chemical properties and orderly trends in physical properties.

• Law of Octaves: In 1865, J. A. R. Newlands arranged 62 known elements into groups of seven by increasing atomic mass, suggesting that every eighth element would repeat properties (the law of octaves).

Example: Döbereiner's triads included elements like lithium, sodium, and potassium, which all react vigorously with water.

Mendeleev's Periodic Table

Development and Predictions

• Mendeleev's Arrangement: Dmitri Mendeleev organized elements by increasing atomic mass and grouped them by similar chemical properties, using the formula of their oxides.

• Prediction of New Elements: Mendeleev left gaps for undiscovered elements and predicted their properties, such as ekasilicon (later discovered as germanium).

Property	Ekasilicon Predicted (1869)	Germanium Discovered (1886)
Color	Gray	Gray
Atomic Mass	72 amu	72.6 amu
Density	5.5 g/mL	5.32 g/mL
Melting Point	Very high	High
Formula of Oxide	EsO2	GeO2
Density of Oxide	4.7 g/mL	4.7 g/mL
Formula of Chloride	EsCl4	GeCl4
Boiling Point of Chloride	100°C	86°C

Additional info: Mendeleev also predicted gallium and scandium before their discovery.

The Noble Gases

Discovery and Properties

• Group 18 (far right of the table) was added after the discovery of argon in 1894, followed by helium, neon, krypton, xenon, and radon.

• Originally called inert gases, but now known as noble gases due to the discovery of compounds of xenon and krypton.

5.2 The Periodic Law Concept

Modern Organization

• Moseley's Contribution: H. G. J. Moseley discovered that nuclear charge (number of protons) increases by one for each element.

• Periodic Law: Properties of elements recur in a repeating pattern when arranged by increasing atomic number, not atomic mass.

• Atomic Number: The number of protons in the nucleus, which determines the element's identity.

Example: The periodic table's current arrangement is based on atomic number, explaining trends more accurately than atomic mass.

5.3 Groups and Periods of Elements

Structure of the Periodic Table

• Groups (Families): Vertical columns (18 total).

• Periods (Series): Horizontal rows (7 total).

• First period: H and He; Second and third: 8 elements each; Fourth and fifth: 18 elements each.

Example: Group 1 contains alkali metals; Period 2 runs from Li to Ne.

Hydrogen's Unique Position

• Hydrogen shares properties with both nonmetals and metals, so its placement varies in different tables.

Groupings of Elements

Main Types of Elements

• Representative (Main-Group) Elements: Groups 1A-8A (or 1, 2, 13-18).

• Transition Elements: Groups 3-12 (B groups).

• Inner Transition Elements: Lanthanide and actinide series, found below the main table.

Common Family Names:

• Group 1: Alkali metals

• Group 2: Alkaline earth metals

• Group 17: Halogens

• Group 18: Noble gases

IUPAC Group Numbers

• In 1985, IUPAC standardized group numbers from 1 to 18, replacing older A/B and Roman numeral systems.

5.4 Periodic Trends

Atomic Radius

• Definition: The atomic radius is the distance from the nucleus to the outermost electron shell.

• Trends:

  • Decreases as you go up a group (fewer energy levels).

  • Decreases as you go left to right across a period (more protons pull electrons closer).

Example: Lithium (Li) has a larger atomic radius than fluorine (F) in the same period.

Metallic Character

• Definition: The degree to which an element exhibits metallic properties (such as conductivity, malleability, and tendency to lose electrons).

• Trend: Decreases from left to right across a period and from bottom to top in a group.

5.5 Properties of Elements

Predicting Properties

• Elements in the same group have similar chemical and physical properties.

• Unknown properties can be predicted based on trends within a group.

Element	Atomic Radius (nm)	Density (g/mL)	Melting Point (°C)	Atomic Mass (amu)
Li	0.152	0.53	180.5	6.94
Na	0.186	0.97	97.8	22.99
K	0.227	0.86	63.7	39.10
Rb	0.248	1.53	38.9	85.47
Cs	0.266	1.87	28.4	132.91
Fr	>0.266	>1.87	<28.4	Additional info: Predicted

Additional info: Francium's properties are predicted based on trends in alkali metals.

Summary

• The periodic table arranges elements by increasing atomic number.

• Groups (columns) and periods (rows) organize elements with similar properties.

• Periodic trends such as atomic radius and metallic character help predict element behavior.

• Standardized group numbers (IUPAC) aid in global scientific communication.