4.3

Atoms and Elements

Introduction to Atoms

Atoms are the fundamental building blocks of matter. Every element on the periodic table is composed of atoms, which retain the unique properties of that element. Understanding the structure and properties of atoms is essential for studying chemistry at all levels.

• Atom: The smallest particle of an element that retains the chemical characteristics of that element.

• Example: Aluminum foil is made up of countless aluminum atoms.

• Atoms can be visualized using advanced microscopes, such as scanning tunneling microscopes, which can magnify atoms millions of times.

Dalton’s Atomic Theory

Historical Development

The concept of atoms dates back to ancient times, but it was not until 1808 that John Dalton formulated a scientific atomic theory. Dalton's theory provided a foundation for understanding chemical reactions and the composition of matter.

• All matter is made up of tiny particles called atoms.

• Atoms of a given element are identical to one another and different from atoms of other elements.

• Compounds are formed when atoms of two or more elements combine in fixed ratios.

• Chemical reactions involve the rearrangement, separation, or combination of atoms. Atoms are not created or destroyed in chemical reactions.

Example: Water (H2O) is always composed of two hydrogen atoms and one oxygen atom.

Subatomic Particles

Discovery and Types

By the late 1800s, experiments revealed that atoms are not indivisible, but are composed of even smaller particles known as subatomic particles. The three main subatomic particles are protons, neutrons, and electrons.

• Proton (p+): Positively charged particle found in the nucleus.

• Neutron (n0): Neutral particle (no charge) also found in the nucleus.

• Electron (e-): Negatively charged particle found in the space surrounding the nucleus.

Electrical Charges in Atoms

Nature of Charges

Subatomic particles carry electrical charges that determine how they interact with each other:

• Like charges repel (positive-positive or negative-negative).

• Unlike charges attract (positive-negative).

These interactions are fundamental to the structure and stability of atoms.

Atomic Models

Thomson’s Plum-Pudding Model

J. J. Thomson discovered the electron in 1897 through cathode ray experiments. He proposed the "plum-pudding" model, where electrons were embedded in a positively charged cloud, similar to plums in a pudding.

Rutherford’s Nuclear Model

Ernest Rutherford’s gold foil experiment (1911) demonstrated that atoms have a small, dense, positively charged nucleus at the center, with electrons occupying the surrounding space. This overturned the plum-pudding model and led to the modern nuclear model of the atom.

• Nucleus: Contains protons and neutrons.

• Electrons: Move in the large, mostly empty space around the nucleus.

Atomic Mass and Atomic Mass Unit (amu)

Definition and Values

The mass of an atom is primarily due to its protons and neutrons, as electrons have negligible mass. Chemists use the atomic mass unit (amu) to express atomic and subatomic masses.

• 1 amu is defined as one-twelfth the mass of a carbon-12 atom.

• Proton mass: Approximately 1.007 amu

• Neutron mass: Approximately 1.008 amu

• Electron mass: Approximately 0.00055 amu

Table: Subatomic Particles in the Atom

Particle	Symbol	Charge	Location	Mass (amu)
Proton	p+	+1	Nucleus	1.007
Neutron	n0	0	Nucleus	1.008
Electron	e-	-1	Outside nucleus	0.00055

Summary Table: Subatomic Particle Properties

Description	Subatomic Particle
Found outside the nucleus	Electron
Has a positive charge	Proton
Has mass but no charge	Neutron

Practice Questions

• True or False: The mass of an electron is greater than the mass of a proton. False

• True or False: Protons have a positive charge, and electrons have a negative charge. True

• True or False: The nucleus of an atom contains only protons and neutrons. True

Additional info: This guide covers the foundational concepts of atomic structure, subatomic particles, and early atomic theory, which are essential for further study in general, organic, and biological chemistry.