Atomic Structure and Isotopes

20min Part 1 / Ch1 / Lesson 2

Prerequisites: 1-1-1

Objectives

Describe the structure of an atom (protons, neutrons, electrons)
Understand the relationship between atomic number and mass number
Define isotopes and describe their properties

Structure of the Atom

Atom = Nucleus (protons + neutrons) + electrons

An atom consists of a nucleus (protons $p^+$ and neutrons $n$ ) and electrons $e^-$ . Atomic number $Z$ = number of protons = number of electrons (in a neutral atom). Mass number $A$ = protons + neutrons.

Atomic Number and Mass Number

Standard notation:

{}^{A}_{Z}\mathrm{X}

$Z$ : Atomic number (number of protons)
$A$ : Mass number (protons + neutrons)
Number of neutrons = $A - Z$

Example: ${}^{12}_{6}\mathrm{C}$ has 6 protons, $12 - 6 = 6$ neutrons, and 6 electrons.

Isotopes

Isotopes = atoms of the same element (same number of protons) with different numbers of neutrons. Chemical properties are nearly identical; only mass-dependent physical properties differ.

Isotopes of hydrogen:

Name	Symbol	Protons	Neutrons	Mass number
Protium	${}^{1}_{1}\mathrm{H}$	1	0	1
Deuterium (D)	${}^{2}_{1}\mathrm{H}$	1	1	2
Tritium (T)	${}^{3}_{1}\mathrm{H}$	1	2	3

Relative atomic mass is the weighted average of isotopes:

\text{Atomic mass} = \sum (\text{isotope mass} \times \text{abundance})

Example: Chlorine $\mathrm{Cl}$ has ${}^{35}\mathrm{Cl}$ (75.77%) and ${}^{37}\mathrm{Cl}$ (24.23%)

\text{Atomic mass} = 35 \times 0.7577 + 37 \times 0.2423 = 35.5

Common Misconception

“Isotopes have different chemical properties” → Nearly identical. Chemical reactions depend on electron behavior. Since isotopes have the same number of electrons, their chemistry is essentially the same. What differs is mass-dependent physical properties.

Check Your Understanding

Q1 The atomic number equals the number of:

Q2 How many neutrons does sodium (mass number 23, atomic number 11) have?

Q3 Which statement about isotopes is correct?

Exercises

Q1. Find the number of protons, neutrons, and electrons in ${}^{56}_{26}\mathrm{Fe}$ .

Solution

Protons: $Z = 26$
Neutrons: $A - Z = 56 - 26 = 30$
Electrons: $26$ (neutral atom, so same as protons)

Q2. Copper has isotopes ${}^{63}\mathrm{Cu}$ (69.17%) and ${}^{65}\mathrm{Cu}$ (30.83%). Calculate the atomic mass to 4 significant figures.

Solution

$\text{Atomic mass} = 63 \times 0.6917 + 65 \times 0.3083$ $= 43.58 + 20.04 = 63.62$

The atomic mass of copper is 63.62.