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Elements in Period 3

4.6 Elements in Period 3
CHEMISTRY • Form 4 • Chapter 4: The Periodic Table of Elements

4.6Elements in Period 3

Period 3 consists of eight elements: sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, and argon. Going across the period from left to right, elements exhibit systematic changes in atomic radius, electronegativity, physical state, and metallic character, as well as shifts in the chemical properties of their oxides from basic to amphoteric and acidic.

Learning Objectives

  • Describe the trends in physical properties of elements across Period 3 (atomic radius, electronegativity, and physical state).
  • Explain the changes in the metallic character and properties of elements across Period 3 (metal, semi-metal/metalloid, non-metal).
  • Describe the changes in the chemical properties of oxides across Period 3 through their reactions with water, acids, and alkalis.
  • Describe briefly the uses of semi-metals (metalloids) such as silicon in the microelectronic industry.

Trends Across Period 3 Elements and Their Oxides

Atomic radius decreases, electronegativity increases, and oxides change from basic to amphoteric to acidic.

Na Mg Al Si P S Cl Ar
Atomic radius
Decreases from 0.186 nm to 0.094 nm
Electronegativity
Increases from 0.9 to 3.0
Physical state
Solid → gas
Oxide type
Basic → amphoteric → acidic

Brief Explanation

Understand the key trends before exploring the oxides.

Across Period 3, every successive element gains one extra proton in its nucleus. Because the additional electrons enter the same principal quantum shell (the third shell), the screening effect remains constant while the nuclear charge increases. This stronger positive pull draws the valence electrons closer, making atomic radius decrease from \(0.186\text{ nm}\) in sodium to \(0.094\text{ nm}\) in argon. Concurrently, the atom's ability to attract electrons—electronegativity—increases from \(0.9\) in sodium to \(3.0\) in chlorine.

1. Atomic Radius Trend

The number of protons increases across Period 3, which increases the nuclear charge. This increases the nuclear attraction force towards the electrons, pulling the electron shells closer to the nucleus and decreasing atomic radius.

2. Electronegativity Trend

Electronegativity increases because the nuclear attraction force towards electrons increases. Argon has no electronegativity value because it has a stable octet configuration.

Element Atomic radius (nm) Electronegativity Physical state Classification
Na0.1860.9SolidMetal
Mg0.1601.2SolidMetal
Al0.1431.5SolidMetal
Si0.1181.8SolidSemi-metal / Metalloid
P0.1102.1SolidNon-metal
S0.1042.5SolidNon-metal
Cl0.1003.0GasNon-metal
Ar0.094–GasNon-metal

Try These Questions First

Answer mentally, then check the explanation.

1

Why does the atomic radius decrease from sodium to chlorine across Period 3?

Answer: The number of protons increases, which increases the nuclear charge. This creates a stronger nuclear attraction force towards the electrons, pulling them closer to the nucleus.
2

What is meant by an amphoteric oxide, and which Period 3 oxide exhibits this property?

Answer: An amphoteric oxide is an oxide that reacts with both acids and alkalis to form salt and water. In Period 3, aluminium oxide, \(\text{Al}_2\text{O}_3\), is an amphoteric oxide.
3

Why are metalloids such as silicon widely used in the manufacture of microchips?

Answer: Silicon is a semi-metal (metalloid) that acts as a semiconductor because it becomes a good electrical conductor at high temperatures.

Common Mistakes

Avoid these errors when studying Period 3 trends.

!
Assuming atomic radius increases across a period. Students often confuse groups with periods and mistakenly think that having more protons and electrons makes the atom larger across Period 3.
!
Classifying \(\text{Al}_2\text{O}_3\) as basic. Because aluminium is a metal, students frequently assume its oxide is basic like \(\text{Na}_2\text{O}\) and \(\text{MgO}\), forgetting that \(\text{Al}_2\text{O}_3\) is amphoteric.
!
Assigning an electronegativity value to Argon. Noble gases generally do not have electronegativity values because they have stable octet configurations and do not readily attract bonding electrons.
!
Thinking all non-metal elements in Period 3 are gases. Only chlorine and argon are gases; phosphorus and sulphur are solids at room temperature.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Adding more electrons across Period 3 adds new electron shells.

Correct Fact

All elements in Period 3 have their valence electrons filling the third shell; the number of occupied shells remains three, meaning shielding/screening remains relatively constant while nuclear charge increases.

Misconception

Basic oxides react with alkalis to form salt and water.

Correct Fact

Basic oxides react only with acids. Only acidic oxides react with alkalis, while amphoteric oxides can react with both acids and alkalis.

Misconception

Metalloids conduct electricity better at low temperatures like metals.

Correct Fact

Metalloids are poor/weak electrical conductors at room temperature, but their conductivity increases at high temperatures, which makes them useful as semiconductors.

Summary

Key facts to remember.

  • Period 3 contains eight elements: \(\text{Na}\), \(\text{Mg}\), \(\text{Al}\) (metals), \(\text{Si}\) (metalloid), \(\text{P}\), \(\text{S}\), \(\text{Cl}\), \(\text{Ar}\) (non-metals).
  • Across Period 3 from left to right: number of protons increases → nuclear charge increases → nuclear attraction force on electrons increases.
  • Atomic radius decreases from \(0.186\text{ nm}\) down to \(0.094\text{ nm}\).
  • Electronegativity increases from \(0.9\) up to \(3.0\).
  • Physical states shift from solid to gas.
  • Oxide properties transition: basic oxides \(\text{Na}_2\text{O}\), \(\text{MgO}\); amphoteric oxide \(\text{Al}_2\text{O}_3\); acidic oxides \(\text{SiO}_2\), \(\text{P}_4\text{O}_{10}\), \(\text{SO}_2\), \(\text{Cl}_2\text{O}_7\).
  • Silicon (\(\text{Si}\)) is a semi-metal used as a semiconductor in microchips because it conducts electricity well at high temperatures.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following correctly describes the trend in atomic radius across Period 3 from sodium to chlorine?

2

Which oxide in Period 3 reacts with both nitric acid, \(\text{HNO}_3\), and sodium hydroxide solution, \(\text{NaOH}\)?

B. Fill in the Blanks

3

Across Period 3 from left to right, electronegativity increases because the nuclear attraction force towards the ________ increases.

4

Silicon is classified as a semi-metal or metalloid, and is used as a ________ in the manufacture of electronic microchips.

C. Match the Period 3 Oxide with Its Property

Drag each oxide on desktop. On mobile, tap an oxide and then tap the matching box.

Period 3 Oxide

Magnesium oxide, \(\text{MgO}\) Aluminium oxide, \(\text{Al}_2\text{O}_3\) Silicon(IV) oxide, \(\text{SiO}_2\) Sulphur dioxide, \(\text{SO}_2\)

Property of Oxide

1Basic oxide that reacts with acids to form salt and water
2Amphoteric oxide that reacts with both acids and alkalis
3Acidic oxide that reacts with alkalis but does not react with acids
4Acidic oxide gas that dissolves in water to produce an acidic solution
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Period 3 Atomic radius Electronegativity Nuclear charge Basic oxide Amphoteric oxide Acidic oxide Metalloid / Semi-metal Semiconductor
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.

Elements in Period 3

4.6 Elements in Period 3
CHEMISTRY • Form 4 • Chapter 4: The Periodic Table of Elements

4.6Elements in Period 3

Period 3 consists of eight elements: sodium, magnesium, aluminium, silicon, phosphorus, sulphur, chlorine, and argon. Going across the period from left to right, elements exhibit systematic changes in atomic radius, electronegativity, physical state, and metallic character, as well as shifts in the chemical properties of their oxides from basic to amphoteric and acidic.

Learning Objectives

  • Describe the trends in physical properties of elements across Period 3 (atomic radius, electronegativity, and physical state).
  • Explain the changes in the metallic character and properties of elements across Period 3 (metal, semi-metal/metalloid, non-metal).
  • Describe the changes in the chemical properties of oxides across Period 3 through their reactions with water, acids, and alkalis.
  • Describe briefly the uses of semi-metals (metalloids) such as silicon in the microelectronic industry.

Trends Across Period 3 Elements and Their Oxides

Atomic radius decreases, electronegativity increases, and oxides change from basic to amphoteric to acidic.

Na Mg Al Si P S Cl Ar
Atomic radius
Decreases from 0.186 nm to 0.094 nm
Electronegativity
Increases from 0.9 to 3.0
Physical state
Solid → gas
Oxide type
Basic → amphoteric → acidic

Brief Explanation

Understand the key trends before exploring the oxides.

Across Period 3, every successive element gains one extra proton in its nucleus. Because the additional electrons enter the same principal quantum shell (the third shell), the screening effect remains constant while the nuclear charge increases. This stronger positive pull draws the valence electrons closer, making atomic radius decrease from \(0.186\text{ nm}\) in sodium to \(0.094\text{ nm}\) in argon. Concurrently, the atom's ability to attract electrons—electronegativity—increases from \(0.9\) in sodium to \(3.0\) in chlorine.

1. Atomic Radius Trend

The number of protons increases across Period 3, which increases the nuclear charge. This increases the nuclear attraction force towards the electrons, pulling the electron shells closer to the nucleus and decreasing atomic radius.

2. Electronegativity Trend

Electronegativity increases because the nuclear attraction force towards electrons increases. Argon has no electronegativity value because it has a stable octet configuration.

Element Atomic radius (nm) Electronegativity Physical state Classification
Na0.1860.9SolidMetal
Mg0.1601.2SolidMetal
Al0.1431.5SolidMetal
Si0.1181.8SolidSemi-metal / Metalloid
P0.1102.1SolidNon-metal
S0.1042.5SolidNon-metal
Cl0.1003.0GasNon-metal
Ar0.094–GasNon-metal

Try These Questions First

Answer mentally, then check the explanation.

1

Why does the atomic radius decrease from sodium to chlorine across Period 3?

Answer: The number of protons increases, which increases the nuclear charge. This creates a stronger nuclear attraction force towards the electrons, pulling them closer to the nucleus.
2

What is meant by an amphoteric oxide, and which Period 3 oxide exhibits this property?

Answer: An amphoteric oxide is an oxide that reacts with both acids and alkalis to form salt and water. In Period 3, aluminium oxide, \(\text{Al}_2\text{O}_3\), is an amphoteric oxide.
3

Why are metalloids such as silicon widely used in the manufacture of microchips?

Answer: Silicon is a semi-metal (metalloid) that acts as a semiconductor because it becomes a good electrical conductor at high temperatures.

Common Mistakes

Avoid these errors when studying Period 3 trends.

!
Assuming atomic radius increases across a period. Students often confuse groups with periods and mistakenly think that having more protons and electrons makes the atom larger across Period 3.
!
Classifying \(\text{Al}_2\text{O}_3\) as basic. Because aluminium is a metal, students frequently assume its oxide is basic like \(\text{Na}_2\text{O}\) and \(\text{MgO}\), forgetting that \(\text{Al}_2\text{O}_3\) is amphoteric.
!
Assigning an electronegativity value to Argon. Noble gases generally do not have electronegativity values because they have stable octet configurations and do not readily attract bonding electrons.
!
Thinking all non-metal elements in Period 3 are gases. Only chlorine and argon are gases; phosphorus and sulphur are solids at room temperature.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Adding more electrons across Period 3 adds new electron shells.

Correct Fact

All elements in Period 3 have their valence electrons filling the third shell; the number of occupied shells remains three, meaning shielding/screening remains relatively constant while nuclear charge increases.

Misconception

Basic oxides react with alkalis to form salt and water.

Correct Fact

Basic oxides react only with acids. Only acidic oxides react with alkalis, while amphoteric oxides can react with both acids and alkalis.

Misconception

Metalloids conduct electricity better at low temperatures like metals.

Correct Fact

Metalloids are poor/weak electrical conductors at room temperature, but their conductivity increases at high temperatures, which makes them useful as semiconductors.

Summary

Key facts to remember.

  • Period 3 contains eight elements: \(\text{Na}\), \(\text{Mg}\), \(\text{Al}\) (metals), \(\text{Si}\) (metalloid), \(\text{P}\), \(\text{S}\), \(\text{Cl}\), \(\text{Ar}\) (non-metals).
  • Across Period 3 from left to right: number of protons increases → nuclear charge increases → nuclear attraction force on electrons increases.
  • Atomic radius decreases from \(0.186\text{ nm}\) down to \(0.094\text{ nm}\).
  • Electronegativity increases from \(0.9\) up to \(3.0\).
  • Physical states shift from solid to gas.
  • Oxide properties transition: basic oxides \(\text{Na}_2\text{O}\), \(\text{MgO}\); amphoteric oxide \(\text{Al}_2\text{O}_3\); acidic oxides \(\text{SiO}_2\), \(\text{P}_4\text{O}_{10}\), \(\text{SO}_2\), \(\text{Cl}_2\text{O}_7\).
  • Silicon (\(\text{Si}\)) is a semi-metal used as a semiconductor in microchips because it conducts electricity well at high temperatures.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following correctly describes the trend in atomic radius across Period 3 from sodium to chlorine?

2

Which oxide in Period 3 reacts with both nitric acid, \(\text{HNO}_3\), and sodium hydroxide solution, \(\text{NaOH}\)?

B. Fill in the Blanks

3

Across Period 3 from left to right, electronegativity increases because the nuclear attraction force towards the ________ increases.

4

Silicon is classified as a semi-metal or metalloid, and is used as a ________ in the manufacture of electronic microchips.

C. Match the Period 3 Oxide with Its Property

Drag each oxide on desktop. On mobile, tap an oxide and then tap the matching box.

Period 3 Oxide

Magnesium oxide, \(\text{MgO}\) Aluminium oxide, \(\text{Al}_2\text{O}_3\) Silicon(IV) oxide, \(\text{SiO}_2\) Sulphur dioxide, \(\text{SO}_2\)

Property of Oxide

1Basic oxide that reacts with acids to form salt and water
2Amphoteric oxide that reacts with both acids and alkalis
3Acidic oxide that reacts with alkalis but does not react with acids
4Acidic oxide gas that dissolves in water to produce an acidic solution
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Period 3 Atomic radius Electronegativity Nuclear charge Basic oxide Amphoteric oxide Acidic oxide Metalloid / Semi-metal Semiconductor
The views expressed in the notes here are those of the contributor and don’t necessarily represent the views of Pandai.
Chapter : Periodic Table of Elements
Topic : Elements in Period 3
Form 4 Chemistry
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