Quick Notes

Quick Notes

Chemistry Chapter 2: Matter and Atomic Structure
2.3 Structure of the Atom
2.3 Atomic Structure - Pandai Chemistry Note
CHEMISTRY • Form 4 • Chapter 2: Matter and the Atomic Structure

2.3Atomic Structure

Every atom is composed of subatomic particles: protons, neutrons, and electrons. This topic covers the definitions of proton number and nucleon number, how to write the standard representation of an atom, how electrons are arranged in shells around the nucleus, and how to construct atomic structure diagrams and electron arrangement diagrams.

Learning Objectives

  • Define proton number and nucleon number.
  • Determine the nucleon number, proton number, and number of electrons in an atom.
  • Write the standard representation of an atom.
  • Construct an atomic structure diagram and electron arrangement.

Standard Representation, Electron Arrangement, and Atomic Structure Diagrams

Clarify the difference between nucleon number (A) and proton number (Z), and contrast the electron arrangement diagram with the atomic structure diagram.

Standard Representation

An atom is represented as \(^A_Z\text{X}\), where \(A\) is the nucleon number and \(Z\) is the proton number.

\(^{23}_{11}\text{Na}\)

Electron Arrangement vs Atomic Structure

Electron arrangement diagram: shows the element symbol in the centre and only the electrons in shells.

Atomic structure diagram: shows the number of protons and neutrons in the nucleus, plus electrons in shells.

Electron Arrangement Diagram

Electron arrangement diagram of aluminium Aluminium symbol in the centre with 2, 8 and 3 electrons in three concentric shells. Al

Aluminium: 2.8.3

Atomic Structure Diagram

Atomic structure diagram of aluminium Nucleus containing 13 protons and 14 neutrons, surrounded by 2, 8 and 3 electrons in shells. 13 p 14 n

Aluminium: 13 p, 14 n

Brief Explanation

Understand the key ideas about protons, neutrons, electrons and atomic notation.

An atom has a central nucleus consisting of protons and neutrons, orbited by electrons in designated shells. The identity of an element is governed by its proton number. In a neutral atom, the number of electrons equals the proton number. The nucleon number is the sum of protons and neutrons.

Proton Number and Nucleon Number

Proton number = number of protons in the nucleus.

Nucleon number = number of protons + number of neutrons.

In a neutral atom: number of electrons = number of protons.

Number of neutrons = nucleon number − number of protons.

Formation of Ions

During ion formation, the number of protons and neutrons remains unchanged; only the number of electrons changes.

Anion: formed when an atom accepts electrons.

Cation: formed when an atom donates electrons.

Element Protons Neutrons Proton number Nucleon number
Oxygen 8 8 8 16
Sodium 11 12 11 23
Chlorine 17 18 17 35

Standard Representation

An atom is represented as \(^A_Z\text{X}\), where:

  • \(\text{X}\) = symbol of the element
  • \(A\) = nucleon number (top left)
  • \(Z\) = proton number (bottom left)

Example: \(^{12}_{\phantom{1}6}\text{C}\) shows carbon with nucleon number 12 and proton number 6.

Electron Arrangement

Electrons fill the shell closest to the nucleus first.

Maximum electrons per shell for elements 1–20:

  • First shell: 2
  • Second shell: 8
  • Third shell: 8

Valence shell is the outermost occupied shell. Valence electrons determine chemical properties.

Check Understanding

Answer the questions, then check your answers.

1

An atom of element \(\text{Y}\) has 11 protons and 12 neutrons. What is the nucleon number and standard representation of atom \(\text{Y}\)?

Answer: Nucleon number = \(11 + 12 = 23\). Standard representation = \(^{23}_{11}\text{Y}\).
2

What is the difference between an electron arrangement diagram and an atomic structure diagram?

Answer: An electron arrangement diagram shows the symbol of the element in the nucleus, whereas an atomic structure diagram shows the actual number of protons and neutrons in the nucleus.
3

What happens to the number of protons, neutrons, and electrons when a neutral chlorine atom (\(17\text{ p}\), \(18\text{ n}\), \(17\text{ e}^-\)) forms a chloride ion (\(\text{Cl}^-\))?

Answer: The number of protons (17) and neutrons (18) remains unchanged, while the number of electrons increases by 1 to become 18.

Common Mistakes

Avoid these errors when working with atomic structure.

!
Inverting the positions in standard notation by writing the proton number on top and the nucleon number at the bottom. The nucleon number (\(A\)) must be on top and proton number (\(Z\)) below (\(^A_Z\text{X}\)).
!
Assuming protons or neutrons change during ion formation. Only electrons are gained or lost; the nucleus remains unaffected.
!
Confusing the electron arrangement diagram with the atomic structure diagram by omitting the number of protons and neutrons in the nucleus of an atomic structure diagram.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

Atoms of different elements can have the same proton number.

Correct Fact

Each element has a unique proton number.

Misconception

The third shell can never hold more than 8 electrons.

Correct Fact

For the first 20 elements, the third shell holds up to 8 electrons. For elements with proton numbers exceeding 20, the third shell can hold a maximum of 18 electrons.

Summary

Key facts to remember.

  • Proton number (\(Z\)) = number of protons in the nucleus.
  • Nucleon number (\(A\)) = number of protons + number of neutrons.
  • In a neutral atom: number of electrons = number of protons.
  • Standard notation: \(^A_Z\text{X}\).
  • Shell capacity (elements 1–20): Shell 1 = max 2, Shell 2 = max 8, Shell 3 = max 8.
  • Valence electrons are the electrons located in the outermost shell (valence shell) and determine chemical properties.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

1

An atom of element \(\text{E}\) has a proton number of 11 and a nucleon number of 23. How many neutrons and valence electrons does this atom have?

2

Which statement correctly describes an atomic structure diagram of an atom?

B. Fill in the Blanks

3

The total number of protons and neutrons in the nucleus of an atom is called the ______ number.

4

For elements with proton numbers 1 to 20, the first electron shell can hold a maximum of ______ electrons.

C. Match the Atomic Term with Its Description

Drag each term on desktop. On mobile, tap a term and then tap the matching box.

Terms

Proton number Valence electrons Anion Cation

Descriptions

1The number of protons in the nucleus of an atom
2Electrons located in the outermost occupied shell of an atom
3A negatively-charged ion formed when an atom accepts electrons
4A positively-charged ion formed when an atom donates electrons
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Proton number Nucleon number Valence shell Valence electrons Standard representation of an atom Atomic structure diagram Electron arrangement diagram