Atoms and Elements: Atomic Number, Mass Number, and Composition

Atoms and Elements

Introduction

This section introduces the fundamental concepts of atoms and elements, focusing on atomic number, mass number, and the composition of atoms. Understanding these concepts is essential for studying general, organic, and biological chemistry.

Atomic Number

Definition and Significance

• Atomic number is a whole number specific to each element.

• It is the same for all atoms of a given element.

• The atomic number is equal to the number of protons in an atom.

• On the periodic table, the atomic number appears above the symbol of the element.

Examples

• Hydrogen (H): Atomic number = 1; every H atom has 1 proton.

• Carbon (C): Atomic number = 6; every C atom has 6 protons.

• Copper (Cu): Atomic number = 29; every Cu atom has 29 protons.

• Gold (Au): Atomic number = 79; every Au atom has 79 protons.

Formula

• Atomic number ($Z$) = Number of protons

Atomic Number and Protons in an Atom

Relationship

• All atoms of a given element have the same number of protons, which is equal to the atomic number.

• For example, all lithium atoms contain three protons ($Z = 3$), and all carbon atoms contain six protons ($Z = 6$).

Determining the Number of Protons

Practice Examples

• Chlorine atom: 17 protons

• Potassium atom: 19 protons

• Barium atom: 56 protons

Neutrality of Atoms

Charge Balance

• An atom of any element is electrically neutral, meaning it has a net charge of zero.

• This neutrality is due to having an equal number of protons (positive charge) and electrons (negative charge).

• For example, a neutral atom of calcium (atomic number 20) contains 20 protons and 20 electrons.

Mass Number

Definition and Calculation

• Mass number represents the total number of particles in the nucleus of an atom.

• It is equal to the sum of the number of protons and neutrons.

• Mass number does not appear in the periodic table because it applies to individual atoms only.

Formula

• Mass number ($A$) = Number of protons + Number of neutrons

• Number of neutrons = Mass number - Atomic number

Example Calculation

• Potassium has a mass number of 39 and an atomic number of 19.

• Number of neutrons = $39 - 19 = 20$

Summary Table: Atomic Number, Mass Number, and Composition

Purpose

This table summarizes the relationships between atomic number, mass number, protons, neutrons, and electrons for selected elements.

Element	Atomic Number (Z)	Mass Number (A)	Number of Protons	Number of Neutrons	Number of Electrons
Hydrogen (H)	1	1	1	0	1
Carbon (C)	6	12	6	6	6
Potassium (K)	19	39	19	20	19
Lead (Pb)	82	207	82	125	82
Zinc (Zn)	30	65	30	35	30
Additional info: Barium (Ba)	56	137	56	81	56

Practice Problems and Solutions

Example 1: Lead Atom

• Mass number: 207

• Atomic number: 82

• Number of protons: 82

• Number of neutrons: $207 - 82 = 125$

• Number of electrons (neutral atom): 82

Example 2: Zinc Atom

• Mass number: 65

• Atomic number: 30

• Number of protons: 30

• Number of neutrons: $65 - 30 = 35$

• Number of electrons (neutral atom): 30

Example 3: Unknown Element

• Number of protons: 14

• Number of neutrons: 20

• Atomic number: 14

• Mass number: $14 + 20 = 34$

• Element: Silicon (Si)

Key Formulas

• Number of protons = Atomic number ($Z$)

• Number of neutrons = Mass number ($A$) - Atomic number ($Z$)

• Mass number ($A$) = Number of protons + Number of neutrons

Summary

Understanding atomic number and mass number is fundamental to identifying elements and their isotopes. The atomic number determines the identity of the element, while the mass number provides information about the specific isotope. Atoms are neutral when the number of protons equals the number of electrons.