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What is matter?

Matter is a substance made up of various types of particles that occupies physical space and has inertia. According to the principles of modern physics, the various types of particles each have a specific mass and size.

The most familiar examples of material particles are the electron, the proton and the neutron. Combinations of these particles form atoms.

Matter explained: Atoms, molecules, elements and compounds

Fundamentally, matter is composed of elementary particles called quarks and leptons, both of which are considered elementary particles in that they aren't made up of smaller units of matter. Quarks -- groups of subatomic particles that interact by means of a strong force -- combine into protons and neutrons. Leptons -- groups of subatomic particles that respond to weaker forces -- belong to a class of elementary particles that includes electrons.

Atoms are the building blocks of matter. A combination of atoms forms a molecule. Large groups of atoms and molecules form the bulk matter of day-to-day life in the physical world. There are more than 100 different kinds of atoms listed in the periodic table, with each kind constituting a unique chemical element.

Atoms and/or molecules in two or more elements can join together to form a compound. This compound, which is the basis of matter, may not resemble any of the original ingredients.

For example, sodium and chlorine, two highly poisonous elements that are unstable at room temperatures, combine to form one of the most common and harmless compounds known to man called common salt (sodium chloride, or NaCl). Unlike its constituent elements, salt is highly stable, harmless to humans and even edible.

Similarly, hydrogen and oxygen, which are both gaseous elements can combine to form water, which is a liquid compound, not a gas, at room temperature.

The process by which such combinations and transformations of elements into compounds take place is called a chemical reaction.

molecule image
Atoms combine to form molecules, while atoms and/or molecules in two or more elements can join together to form a compound.

Protons, electrons and neutrons in matter

All matter consists of atoms, which, in turn, consist of protons, neutrons and electrons. Both protons and neutrons are located in the nucleus, which is at the center of an atom. Protons are positively charged particles, while neutrons are neutrally charged. Electrons are negatively charged, and they exist in orbitals surrounding the nucleus.

In any atom, like charges repel one another, and opposite charges attract one another. This is why two protons repel each other and so do two electrons, but a proton and an electron attract each other.

In an element, the numbers of electrons and protons are equal. Moreover, since they have opposite charges, they cancel each other out and keep the atom neutral.

The total number of protons present in the atom of a substance is known as the atomic number. Atomic mass refers to a weighted average of the number of neutrons and protons in the atom. The number and mass for each type of atom is listed in the periodic table.

When a chemical reaction takes place to combine two or more electrons into one or more compounds, the electrons of the atoms of each element interact with each other. However, the reaction does not affect the atoms' nuclei.

States of matter

Depending on temperature and some other factors, matter can exist in several states. The three most common states are known as solid, liquid and gas. A single element or compound of matter might exist in more than one state, depending on the temperature and pressure conditions. One common example is water, which can exist in solid, liquid and gaseous forms and can be readily observed in each of these states.

The state of matter can be changed by heating or cooling it or by changing the pressure conditions on it. When a material changes state, its molecules behave differently but don't break apart. Since they remain essentially the same, they don't form a different material but simply change the state of the existing material.


In solid materials, particles are tightly packed, which means they have a high density. This curtails their movement. Moreover, the electrons in each atom are in constant motion, so the atom has a small vibration. Nonetheless, its position remains fixed, which is why solid particles have low kinetic energy.

All solids have a definite shape, mass and volume, which prevents them from conforming to the shape and volume of a container where they are kept. This is one of the properties that differentiates solid matter from liquid matter.


Unlike solids, particles in liquid matter are more loosely packed. This enables them to flow around each other, which gives the liquid an indefinite shape. It is this lack of a specific shape that enables liquids to conform to the shape of containers. Liquids are also less dense than solids. Both solids and liquids are difficult to compress.


In unconfined gaseous matter, particles are spread out indefinitely since they have a lot of space between them. This space is also why atoms in gases have large vibrations, and particles have high kinetic energy.

Gases can also be confined, in which case they adjust to the volume and shape of the container that confines them. Unlike solids and liquids, gases can be compressed by reducing the size of the container, which then reduces the space between particles.

Solid vs. liquid vs. gas types of matter
What molecules look like in the major forms of matter

Other states of matter

Less familiar states of matter include plasma and Bose-Einstein Condensate (BEC). These states occur under special conditions.


A plasma, first identified in 1879, consists of highly charged particles with high kinetic energy. Typically, plasmas are gases that are ionized at high temperatures. Examples of these gases include helium, neon, argon, krypton, xenon and radon -- all of which are noble gases and can be ionized into the plasma state.

Stars are a good example of plasmas in the real world. Fluorescent lights are also a type of plasma, even though they have different physical characteristics from stars.

Bose-Einstein Condensate

BEC was first predicted in the 1920s by Satyendra Bose and Albert Einstein. However, it was only in 1995 that two other scientists, Eric Cornell and Carl Wieman, finally created it. They named BEC after its original theorists and shared the Nobel Prize in physics for their work in 2001.

BEC was created artificially by using a combination of lasers and magnets at super low temperatures that are just a few degrees above absolute zero (zero Kelvin). At these temperatures, molecular motion almost stops, so there's almost no kinetic energy being transferred from one atom to another. As a result, the atoms start to clump together, so thousands of separate atoms form one super atom. Under the right conditions, BEC can be created with certain elements. Cornell and Wieman did it with rubidium.

BEC has several real-world applications:

  • to study the properties of superfluids, which are fluids that can flow without friction;
  • to study the particle/wave paradox, where light slows down as it passes through BEC; and
  • to simulate and understand the conditions that exist in the universe's black holes.

Conversion of matter into energy

In some situations, matter is converted into energy by atomic reactions, also known as nuclear reactions. Nuclear reactions involve changes in the nuclei of atoms. This makes them different from normal chemical reactions.

The most common example of an atomic reaction is the hydrogen fusion reaction that occurs on the sun. The immense pressure inside the sun -- and also inside other stars – forces atoms of hydrogen to fuse together (hence fusion) to form atoms of helium. During this process, some mass is converted into energy, according to the formula suggested by Albert Einstein as his theory of relativity:

E = mc2

E is the energy in joules; m is the mass in kilograms; and c is the speed of light, which is approximately 2.99792 x 108 meters per second in a vacuum.

The transformation of mass into energy also occurs during nuclear fission, in which the nucleus of a heavy element -- e.g., uranium -- splits into fragments of smaller total mass. The mass difference between the original element and its split constituents is released as energy. This phenomenon underpins all research into nuclear energy, which is an alternative to the energy generated by burning fossil fuels, like coal.

Together, matter and energy constitute the basis of all objective phenomena observed in the real world.

Changing states of matter

Matter can be changed from one state to another by changing it physically or chemically. The following are the important processes that facilitate changes in states of matter.


Melting occurs when heat is applied to a solid. The solid matter's particles start to vibrate rapidly and move apart from each other. This process increases the distance between them. Once specific temperature and pressure conditions are achieved, the solid transforms into a liquid. This specific point is known as the solid's melting point.

Different solids have different melting points. For instance, the melting point of ice (solid water) is above zero degrees Celsius (32 degrees Fahrenheit) at sea level. However, the melting point of solid oxygen is -218.4 degrees Celsius.


Freezing occurs when heat is removed from the liquid, causing its particles to slow down and settle in one location. When the liquid reaches a specific temperature known as its freezing point, it transforms into a solid. For instance, in most cases, fresh water freezes at zero degrees Celsius (32 degrees Fahrenheit). Sea water has a lower freezing point due to its salt content.


Sublimation is a process in which a solid is converted directly into a gas, without going through the liquid phase. It is achieved by either increasing the temperature of the substance beyond the boiling point or by freeze-drying it by cooling it under vacuum conditions.

One common example of a solid that converts into a gas via sublimation is carbon dioxide. At room temperate and pressure, solid carbon dioxide is converted into its gaseous form, known as dry ice.


Vaporization is the process of converting a liquid to a gas, either by evaporation or boiling. Since the liquid particles constantly collide with each other, energy is transferred to particles near the surface. When enough energy gets transferred, some particles are removed from the substance as free gas particles. The temperature and pressure conditions under which a liquid becomes a gas is known as its boiling point.


Condensation happens when a gas is transformed into a liquid. For instance, when water vapor -- a gas -- reaches its dew point, it condenses into liquid water called dew.


Deposition is a process where a gas gets transformed directly into a solid, without going through the liquid phase. In this sense, deposition is the opposite reaction of sublimation. Deposition usually occurs when the air touching the solid is cooler than the rest of the air. This is what happens when water vapor is transformed directly into ice as frost.

Antimatter and positrons

In recent years, scientists have confirmed the existence of a substance called antimatter. The electron has an antiparticle twin called a positron, with equal mass but opposite electric charge.

Similarly, the proton has an antimatter twin called an antiproton, and the neutron has an antimatter twin called an antineutron. Einstein's formula states that, if a particle of matter encounters its antiparticle, both are converted entirely to energy. In this case, m is the combined mass of the particle and the antiparticle.

Small amounts of antimatter have been isolated in laboratory conditions, but no one has yet succeeded in creating a controlled matter/antimatter reaction or even an uncontrolled reaction of significant size.

See also: table of physical constants.

This was last updated in June 2022

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