Atoms & Subatomic Particles

Introduction

Atoms are the fundamental building blocks of matter. Understanding their structure and the properties of their subatomic particles is essential for studying general, organic, and biological chemistry. This section covers the identity, mass, and charge of elements based on their atomic structure.

Learning Objectives

• Describe the identity, mass, and charge of an element based on the number of protons, neutrons, and electrons.

• Understand the structure of atoms and the role of subatomic particles.

Atomic Theory

Dalton's Atomic Theory

Dalton's atomic theory laid the foundation for modern chemistry by proposing that matter is composed of atoms. The main points of Dalton's theory are:

• All matter is composed of atoms.

• Atoms of a given element differ from the atoms of all other elements.

• Chemical compounds consist of atoms combined in specific ratios.

• Chemical reactions change only the way atoms are combined in compounds; atoms themselves are not changed.

Example: Water (H2O) is always composed of two hydrogen atoms and one oxygen atom combined in a fixed ratio.

Subatomic Particles

Types of Subatomic Particles

Atoms are composed of three main subatomic particles: protons, neutrons, and electrons.

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

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

• Electron (e-): Negatively charged particle found outside the nucleus in the electron cloud.

Comparison of Subatomic Particles

The following table summarizes the properties of protons, neutrons, and electrons:

Additional info: 1 atomic mass unit (amu) is defined as 1/12 the mass of a carbon-12 atom.

Structure of the Atom

• The nucleus contains protons and neutrons and is very small and dense.

• The electron cloud occupies most of the atom's volume and contains electrons.

• Most of the atom's mass is concentrated in the nucleus.

Example: The diameter of an atom is approximately meters, while the nucleus is about meters in diameter.

Atomic Number and Mass Number

Atomic Number (Z)

The atomic number () is the number of protons in the nucleus of an atom. It determines the identity of the element.

• Each element has a unique atomic number.

• In a neutral atom, the number of electrons equals the number of protons.

Example: Carbon has , meaning every carbon atom has 6 protons.

Mass Number (A)

The mass number () is the total number of protons and neutrons in the nucleus of an atom.

• Mass number is not found on the periodic table; it is specific to each isotope.

• Number of neutrons =

Example: An atom of carbon-14 has and , so it has 8 neutrons.

Isotopic Notation

Atoms are often represented using isotopic notation:

• = chemical symbol

• = mass number (protons + neutrons)

• = atomic number (number of protons)

Example: represents a carbon atom with 6 protons and 6 neutrons.

Practice Problems

• Given an atom with 92 protons, 92 electrons, and 143 neutrons, what is its mass number? Solution:

• Given an atom with 8 protons, 10 electrons, and 8 neutrons, what is its atomic number and mass number? Solution: ,

Summary Table: Atomic Structure