4.4.3 Density

This topic explains how the density of elements changes as you move down a group and across a period in the periodic table.

Definition

Density is the mass per unit volume of a substance.

Density = MassVolume

Simple explanation: Density tells you how much matter is packed into a given space.

Main Idea

Density of elements generally shows two trends in the periodic table:

• Down a group: density increases.
• Across a period: density increases from left to right until it reaches a maximum around the middle of the period, and then it starts to decrease.

Simple explanation: Density increases down a group, and across a period it rises and then falls, peaking near the middle.

Core Concept

Trend 1: Density Increases Down a Group

Density of elements generally increases as you move down a group.

• The atomic mass increases (more protons and neutrons in the nucleus).
• The size of atoms also increases.
• But the increase in mass exceeds the increase in volume.
• So density, which is mass per unit volume, increases.

Simple explanation: Atoms get heavier faster than they get bigger, so density increases down a group.

Trend 2: Density Varies Across a Period

Density can vary across the period.

• Density tends to increase from left to right.
• It reaches a maximum around the middle of the period.
• Then it starts to decrease.

Simple explanation: Across a period, density rises to a peak in the middle, then falls.

Detailed Explanation

Why Density Increases Down a Group

• Atomic mass increases — more protons and neutrons are added.
• Atomic size increases — new shells are added.
• However, the mass increase is greater than the volume increase.
• Since density = mass / volume, density increases.

• Li is the least dense.
• Na is denser than Li.
• K is denser than Na.
• Rb and Cs are even denser.

Simple explanation: Heavier atoms packed into not-much-larger space → higher density.

Why Density Peaks in the Middle of a Period

• On the left side, elements are large, light metals (low density).
• Moving toward the middle, the elements become heavier (more protons and neutrons), but their size decreases.
• So density increases up to a maximum.
• Beyond the middle, the elements become non-metals and noble gases, which have lower density (gases, lighter structures).

Simple explanation: The densest elements are the heavy transition metals in the middle of the periodic table.

Density and the Periodic Table

• Tungsten (W): 19.3 g/cm³
• Gold (Au): 19.3 g/cm³
• Osmium (Os): 22.6 g/cm³ (densest naturally occurring element)

• Lithium (Li): 0.53 g/cm³
• Hydrogen (H₂): 0.00009 g/cm³

Relation to Other Properties

Density is related to several other periodic properties:

Simple explanation: Density depends on both mass and size, so it does not follow a single simple trend.

Intermediate Level

Why Density Increases Down a Group

• Atomic mass increases (more nucleons).
• Atomic size increases (more shells).
• But the mass increases more than the size.
• So density increases.

• Mass ∝ number of nucleons (A).
• Volume ∝ r³ (where r is atomic radius).
• Since r increases slowly compared to A, density increases.

Why Density Peaks in the Middle of a Period

• Left side: large, light metals → low density.
• Middle: heavy transition metals with relatively small size → high density.
• Right side: non-metals and noble gases → low density.

• Increases from left to middle.
• Decreases from middle to right.

Densest Elements

The densest naturally occurring elements are:

All of these are transition metals located in the middle of the periodic table.

Lightest Elements

The lightest elements are:

Simple explanation: Gases are the lightest; lithium is the lightest metal.

Density and the Book's Description

The book states:

> "Density of elements generally increases as you move down a group. This is due to the increasing atomic mass and the larger size of atoms. As the number of protons and neutrons in the nucleus increases, the atomic mass increases. This results in higher density."

> "Density can vary across the period. In general, density tends to increase from left to right until it reaches a maximum around the middle of the period, and then it starts to decrease."

Important Points

• - Density: mass per unit volume (mass/volume).
• - Down a group: density increases (mass increases more than volume).
• - Across a period: density increases to a peak in the middle, then decreases.
• - Highest density: transition metals in the middle (Os, Ir, Pt, Au, W).
• - Lowest density: gases (H₂, He) and light metals (Li, Na).
• - Densest naturally occurring element: Osmium (Os), 22.6 g/cm³.
• - Lightest element: Hydrogen (H₂), 0.00009 g/cm³.

Remember

• - Density = mass/volume.
• - Down a group → density increases.
• - Across a period → density peaks in the middle.
• - Densest elements: Os, Ir, Pt, Au, W (transition metals).
• - Lightest elements: H₂, He, Li, Na.
• - Osmium (Os) = densest naturally occurring element.

Common Mistakes

• 1. Thinking density always increases across a period. It peaks in the middle and then decreases.
• 2. Thinking density decreases down a group. It increases.
• 3. Confusing density with atomic mass. Density depends on both mass and volume, not just mass.
• 4. Thinking gases have high density. Gases have very low density.
• 5. Forgetting that transition metals have the highest densities. The densest elements are in the middle of the periodic table.
• 6. Thinking all metals have similar densities. They vary widely — Li is 0.53 g/cm³, Os is 22.6 g/cm³.

Quick Revision

• Density = mass per unit volume.
• Down a group: density increases (atomic mass increases more than volume).
• Across a period: density increases to a maximum around the middle, then decreases.
• Highest density elements: transition metals in the middle (Os, Ir, Pt, Au, W).
• Lowest density elements: gases (H₂, He) and light metals (Li, Na).
• Densest naturally occurring element: Osmium (Os), 22.6 g/cm³.
• Lightest element: Hydrogen (H₂).

60-Second Revision

• Density = mass/volume.
• Down a group → increases.
• Across a period → peaks in the middle.
• Densest: Os, Ir, Pt, Au, W.
• Lightest: H₂, He, Li, Na.
• Warning: density does not increase continuously across a period — it peaks and then falls.