Forces Between Particles

Introduction

If particles are constantly moving, why does a solid not simply fly apart? Why do water droplets form round beads, and why can some insects walk on water? The answer is that particles attract one another. The strength of these attractions varies enormously between substances, and it controls whether a substance is a solid, liquid or gas at room temperature.

Core explanation

Particles attract each other. All particles are attracted to their neighbours. These attractions hold solids together and keep liquids from spreading out like gases. Attractions act over short distances: they are significant when particles are close and very weak when particles are far apart, as in a gas.

Particles also repel when squeezed. If particles are pushed very close together, strong repulsion stops them overlapping. This is why solids and liquids are nearly incompressible: the particles sit at a distance where attraction and repulsion balance.

Strength of attraction decides the state. At a given temperature, the state of a substance depends on a contest between particle motion (kinetic energy), which tends to separate particles, and attractions, which pull them together: - strong attractions, compared with motion → solid ; - moderate → liquid ; - weak → gas .

Melting and boiling points measure attraction. To melt a solid, particles must gain enough energy to break free from fixed positions; to boil a liquid, they must separate completely. Substances with strong attractions between particles have high melting and boiling points (iron, salt). Substances whose particles attract only weakly have low melting and boiling points (oxygen, nitrogen).

Types of attraction (preview). Some substances are held together by very strong forces within giant structures — ionic bonds in salt, metallic bonding in iron, covalent bonds in diamond. Others consist of separate small molecules held together by much weaker intermolecular forces , as in water, oxygen and wax. You will study these in detail in the bonding units.

Surface tension. In a liquid, particles in the middle are pulled equally in all directions by their neighbours. Particles at the surface are pulled only sideways and inwards. This inward pull makes the surface behave like a stretched skin — surface tension . It pulls small drops into near-spherical shapes, allows pond skaters to stand on water and lets a carefully placed steel paper clip float on water even though steel is much denser.

Step-by-step reasoning

To explain why oxygen is a gas and iron is a solid at room temperature:

1. At room temperature, all particles have kinetic energy that tends to separate them. 2. The attractions between oxygen molecules are very weak — easily overcome by this motion. 3. So oxygen particles move apart freely: oxygen is a gas (bp −183 °C). 4. The attractions between iron particles are very strong — far stronger than room-temperature motion. 5. So iron particles stay locked in place: iron is a solid (mp 1538 °C).

Visual explanation

A diagram shows particles as spheres joined by springs of different thicknesses: thick springs for a solid, thinner springs for a liquid, and faint dashed lines for a gas. A second diagram shows a particle in the middle of a liquid with arrows pulling in all directions, and a particle at the surface with arrows pulling only sideways and down, producing surface tension.

Real-world analogy

Imagine children at a playground holding hands. If the grip is strong, they stay in a tight group even when excited (solid). If the grip is weaker, they stay together but keep swapping partners (liquid). If they barely hold hands at all, they scatter across the playground (gas). The children's excitement is like temperature; their grip is like the attraction between particles.

Real-world example

Detergents work partly by reducing the surface tension of water. Soapy water spreads out and wets greasy surfaces and fabrics instead of forming beads, so it can reach and lift dirt. Waterproof fabrics do the opposite: they are treated so that water keeps its high surface tension and forms beads that roll off.

Why?

Why does a water droplet form a sphere? Surface tension pulls surface particles inwards, minimising the surface area. For a given volume, a sphere has the smallest possible surface area, so small drops, where surface tension dominates over gravity, become nearly spherical.

Common misconception

"Particles in a gas have no attraction at all." Gas particles do attract one another weakly, but because they are far apart and moving fast, the effect is small. When a gas is cooled enough, the attractions win and the gas condenses to a liquid — which would be impossible if there were no attraction.

Worked example

Question: Substance A melts at −95 °C; substance B melts at 801 °C. Which has stronger attractions between its particles, and what are their states at 25 °C if A boils at 56 °C?

Reasoning: B's much higher melting point shows stronger attractions. At 25 °C, A is above its melting point and below its boiling point, so it is liquid; B is below its melting point, so it is solid.

Answer: B has stronger attractions; at 25 °C A is a liquid and B is a solid.

Quick check

1. What causes surface tension? Answer: Attractions between particles, which pull surface particles inwards, making the surface act like a stretched skin.

Exam focus

Link melting and boiling points to the strength of attractive forces between particles. Say that energy is needed to overcome these forces. In "explain the state" questions, describe the balance between particle motion and attraction.

Advanced insight

Even between neutral, non-polar molecules there are weak attractions called London dispersion forces, caused by momentary uneven distributions of electrons. They grow stronger for larger molecules with more electrons, which is why the boiling points of the noble gases and of hydrocarbons increase with molecular size.

Summary

Particles attract each other, and repel strongly if pushed too close. The balance between attraction and particle motion decides whether a substance is a solid, liquid or gas. Stronger attractions give higher melting and boiling points. Attractions at the surface of a liquid cause surface tension, which shapes drops and supports small objects on water.

Practice questions

1. Which substance has stronger attractions between its particles: water (bp 100 °C) or ethanol (bp 78 °C)? Answer: Water, because it has the higher boiling point. 2. Why are liquids difficult to compress? Answer: The particles are already close together, and strong repulsion stops them being pushed closer. 3. How can a steel paper clip float on water when steel is denser than water? Answer: Surface tension supports it: the surface acts like a stretched skin, provided the clip is placed gently. 4. Explain why cooling a gas enough makes it condense. Answer: Cooling slows the particles, so the weak attractions between them become strong enough to pull them together into a liquid.