Atoms and Elements: Structure, Classification, and the Periodic Table

Atoms and Elements

Introduction

This chapter introduces the fundamental building blocks of matter: atoms and elements. It explores the structure of atoms, the nature of subatomic particles, and the organization of elements in the periodic table.

Subatomic Particles

Types of Subatomic Particles

All atoms are composed of three main subatomic particles:

• Protons

• Neutrons

• Electrons

Protons and neutrons have nearly identical masses, while electrons are much lighter.

• Mass of proton: $1.67262 \times 10^{-27}$ kg

• Mass of neutron: $1.67493 \times 10^{-27}$ kg

• Mass of electron: $0.00091 \times 10^{-27}$ kg

Charge of Subatomic Particles

The charge of the proton and the electron are equal in magnitude but opposite in sign. Neutrons have no charge.

Particle	Mass (kg)	Mass (amu)	Charge (relative)	Charge (C)
Proton	1.67262×10-27	1.00727	+1	+1.60218×10-19
Neutron	1.67493×10-27	1.00866	0	0
Electron	0.00091×10-27	0.00055	-1	-1.60218×10-19

Elements: Defined by Their Numbers of Protons

Atomic Number

The most important number for the identity of an atom is the number of protons in its nucleus. This number is called the atomic number and is given the symbol Z.

• The number of protons defines the element.

• For example, helium has 2 protons, carbon has 6 protons.

The Periodic Table

Organization and Identification

The periodic table arranges elements by increasing atomic number. Each element is identified by a unique atomic number and a chemical symbol (one- or two-letter abbreviation).

• Helium: He

• Carbon: C

• Nitrogen: N

Structure of the Periodic Table

The periodic table is divided into vertical columns called groups (or families) and horizontal rows called periods.

• There are 18 groups and 7 periods.

• Groups are numbered 1-18 (or with A and B notation).

Classification of Elements

Elements are classified as:

• Metals: Good conductors, malleable, ductile, shiny, tend to lose electrons.

• Nonmetals: Poor conductors, not ductile or malleable, tend to gain electrons.

• Metalloids: Exhibit mixed properties, often semiconductors.

Main-Group and Transition Elements

• Main-group elements: Properties are predictable based on position.

• Transition elements/metals: Properties are less predictable.

Isotopes

Definition and Notation

Atoms of a given element have the same number of protons but may have different numbers of neutrons. These variants are called isotopes.

• Mass number (A): Sum of protons and neutrons.

• Notation: $^{A}_{Z}X$ where X is the chemical symbol, A is the mass number, Z is the atomic number.

• Alternative notation: Chemical symbol or name followed by a dash and the mass number (e.g., Ne-20, neon-20).

Natural Abundance

The relative amount of each isotope in a naturally occurring sample is called its natural abundance.

Ions: Losing and Gaining Electrons

Formation of Ions

In a neutral atom, the number of electrons equals the number of protons. During chemical changes, atoms can lose or gain electrons, forming charged particles called ions.

• Cations: Positively charged ions (e.g., Na+).

• Anions: Negatively charged ions (e.g., F-).

Periodic Table and Ion Formation

• Main-group metals tend to lose electrons, forming cations with the same number of electrons as the nearest noble gas.

• Main-group nonmetals tend to gain electrons, forming anions with the same number of electrons as the nearest noble gas.

• Alkali metals (group 1A): Lose one electron, form 1+ ions.

• Alkaline earth metals (group 2A): Lose two electrons, form 2+ ions.

• Halogens (group 7A): Gain one electron, form 1- ions.

• Oxygen family nonmetals (group 6A): Gain two electrons, form 2- ions.

Atomic Mass

Definition and Calculation

The atomic mass (atomic weight) of an element is the average mass of its isotopes, weighted by their natural abundance.

• Atomic mass is listed below the element's symbol in the periodic table.

• Formula: $\text{Atomic mass} = (\text{fraction of isotope 1}) \times (\text{mass of isotope 1}) + (\text{fraction of isotope 2}) \times (\text{mass of isotope 2}) + \ldots$

Example: Chlorine

• Chlorine-35: 75.77% abundance, mass = 34.97 amu

• Chlorine-37: 24.23% abundance, mass = 36.97 amu

• Calculation: $\text{Atomic mass of Cl} = 0.7577 \times 34.97 + 0.2423 \times 36.97 = 35.45$ amu

The Mole and Avogadro's Number

Definition of the Mole

The mole (mol) is a counting unit used by chemists, analogous to a dozen. One mole contains Avogadro's number of particles:

• $1 \text{ mole} = 6.02214 \times 10^{23}$ particles

Relation to Mass

The value of the mole is defined as the number of atoms in exactly 12 grams of pure carbon-12.

• $12 \text{ g C} = 1 \text{ mol C atoms} = 6.022 \times 10^{23}$ atoms

Conversions: Moles, Mass, and Number of Atoms

Conversion Factors

• To convert between moles and number of atoms: $1 \text{ mol atoms} = 6.022 \times 10^{23}$ atoms

• To convert between mass and moles: The mass of 1 mol of atoms of an element is its molar mass (g/mol), numerically equal to the atomic mass in amu.

Example Conversion

• $27 \text{ g Al} = 1 \text{ mol Al} = 6.022 \times 10^{23}$ Al atoms

• $12.01 \text{ g C} = 1 \text{ mol C} = 6.022 \times 10^{23}$ C atoms

Conceptual Plan for Calculations

To count the number of atoms in a sample by weighing:

1. Obtain the mass of the sample.

2. Convert mass to moles using the element's molar mass.

3. Convert moles to number of atoms using Avogadro's number.

General plan: $\text{mass of element} \rightarrow \text{mol element} \rightarrow \text{number of atoms}$

Summary Table: Subatomic Particles

Particle	Symbol	Mass (kg)	Mass (amu)	Charge
Proton	p	1.67262×10-27	1.00727	+1
Neutron	n	1.67493×10-27	1.00866	0
Electron	e-	0.00091×10-27	0.00055	-1

Summary Table: Element Classification

Type	Properties	Examples
Metals	Conductive, malleable, ductile, shiny, lose electrons	Chromium, Copper, Strontium, Lead
Nonmetals	Poor conductors, brittle, gain electrons	Oxygen, Carbon, Sulfur, Bromine, Iodine
Metalloids	Mixed properties, semiconductors	Silicon, Germanium

Summary Table: Common Groups in the Periodic Table

Group	Name	Typical Ion Formed
1A	Alkali Metals	1+ cation
2A	Alkaline Earth Metals	2+ cation
7A	Halogens	1- anion
8A	Noble Gases	Generally do not form ions

Additional info: Some context and examples were expanded for clarity and completeness.