If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

Matter, elements, and atoms

Learn about the structure of the atom, and how atoms make up matter. An atom is the smallest unit of matter that retains all of the chemical properties of an element. 


What is your body made of? Your first thought might be that it is made up of different organs—such as your heart, lungs, and stomach—that work together to keep your body going. Or you might zoom in a level and say that your body is made up of many different types of cells. However, at the most basic level, your body—and, in fact, all of life, as well as the nonliving world—is made up of atoms, often organized into larger structures called molecules.
Atoms and molecules follow the rules of chemistry and physics, even when they're part of a complex, living, breathing being. If you learned in chemistry that some atoms tend to gain or lose electrons or form bonds with each other, those facts remain true even when the atoms or molecules are part of a living thing. In fact, simple interactions between atoms—played out many times and in many different combinations, in a single cell or a larger organism—are what make life possible. One could argue that everything you are, including your consciousness, is the byproduct of chemical and electrical interactions between a very, very large number of nonliving atoms!
So as an incredibly complex being made up of roughly 7,000,000,000,000,000,000,000,000,000 atoms, you'll probably want to know some basic chemistry as you begin to explore the world of biology, and the world in general.

Matter and elements

The term matter refers to anything that occupies space and has mass—in other words, the “stuff” that the universe is made of. All matter is made up of substances called elements, which have specific chemical and physical properties and cannot be broken down into other substances through ordinary chemical reactions. Gold, for instance, is an element, and so is carbon. There are 118 elements, but only 92 occur naturally. The remaining elements have only been made in laboratories and are unstable.
Each element is designated by its chemical symbol, which is a single capital letter or, when the first letter is already “taken” by another element, a combination of two letters. Some elements follow the English term for the element, such as C for carbon and Ca for calcium. Other elements’ chemical symbols come from their Latin names; for example, the symbol for sodium is Na, which is a short form of natrium, the Latin word for sodium.
The four elements common to all living organisms are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N), which together make up about 96% of the human body. In the nonliving world, elements are found in different proportions, and some elements common to living organisms are relatively rare on the earth as a whole. All elements and the chemical reactions between them obey the same chemical and physical laws, regardless of whether they are a part of the living or nonliving world.

The structure of the atom

An atom is the smallest unit of matter that retains all of the chemical properties of an element. For example, a gold coin is simply a very large number of gold atoms molded into the shape of a coin, with small amounts of other, contaminating elements. Gold atoms cannot be broken down into anything smaller while still retaining the properties of gold. A gold atom gets its properties from the tiny subatomic particles it's made up of.
An atom consists of two regions. The first is the tiny atomic nucleus, which is in the center of the atom and contains positively charged particles called protons and neutral, uncharged, particles called neutrons. The second, much larger, region of the atom is a “cloud” of electrons, negatively charged particles that orbit around the nucleus. The attraction between the positively charged protons and negatively charged electrons holds the atom together. Most atoms contain all three of these types of subatomic particles—protons, electrons, and neutrons. Hydrogen (H) is an exception because it typically has one proton and one electron, but no neutrons. The number of protons in the nucleus determines which element an atom is, while the number of electrons surrounding the nucleus determines which kind of reactions the atom will undergo. The three types of subatomic particles are illustrated below for an atom of helium—which, by definition, contains two protons.
Image credit: modified from OpenStax CNX Biology
Protons and neutrons do not have the same charge, but they do have approximately the same mass, about 1.67×1024 grams. Since grams are not a very convenient unit for measuring masses that tiny, scientists chose to define an alternative measure, the dalton or atomic mass unit (amu). A single neutron or proton has a weight very close to 1 amu. Electrons are much smaller in mass than protons, only about 1/1800 of an atomic mass unit, so they do not contribute much to an element’s overall atomic mass. On the other hand, electrons do greatly affect an atom’s charge, as each electron has a negative charge equal to the positive charge of a proton. In uncharged, neutral atoms, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus. The positive and negative charges cancel out, leading to an atom with no net charge.
Protons, neutrons, and electrons are very small, and most of the volume of an atom—greater than 99 percent—is actually empty space. With all this empty space, you might ask why so-called solid objects don’t just pass through one another. The answer is that the negatively charged electron clouds of the atoms will repel each other if they get too close together, resulting in our perception of solidity.

Want to join the conversation?

  • aqualine ultimate style avatar for user Sean Collin
    The first sentence of the section "The structure of an atom" reads: "An atom is the smallest unit of matter that retains all of the chemical properties of an element. For example, one gold atom has all of the properties of gold and is still a solid metal at room temperature."

    My question is, how can just one atom be a liquid or sold or gas, etc? won't it still look and act the exact same no matter what state it's in? It still has electrons and a nucleus. What would the change look like on the atomic level in these different states?
    (448 votes)
    Default Khan Academy avatar avatar for user
    • leafers tree style avatar for user emilyabrash
      Thanks very much to everyone who noticed this problem and upvoted or commented on it. You're absolutely right that there is no meaningful way to classify an individual atom as a solid, liquid, or gas, as these terms are based on interactions between atoms or molecules.

      I've corrected that paragraph to reflect that the gold atom is still considered gold because it has the same chemical properties as a larger quantity of gold (thanks to having the set of subatomic particles, specifically protons, that define gold at the atomic level). The correction should be live on the site later today.

      If that section is still unclear, or if you have any other comments or suggestions, please don't hesitate to ask here (or to report issues with the "Report a mistake" button).

      Thanks again for noticing this!
      (113 votes)
  • leafers seedling style avatar for user david.marlyngayle
    What is the specific difference between atoms and molecules?
    (88 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Nathan Shapiro
    In the fourth paragraph, it says "There are 118 elements, but only 92 occur naturally. The remaining elements have only been made in laboratories and are unstable." Why are the elements made in laboratories unstable?
    (98 votes)
    Default Khan Academy avatar avatar for user
    • boggle blue style avatar for user Davin V Jones
      There are several forces at work inside the nuclei of atoms. The electromagnetic force of the protons wants to push them away from each other. Fusion allows these protons to get close enough together for the stronger, yet short ranged, nuclear force to overpower the electromagnetic force. Neutrons will help add to this nuclear force, but they tend to become unstable when they get into too great of concentration and the weak force will cause them to decay into protons.

      With all of that to consider, as you get larger and larger nuclei with more protons, the electromagnetic forces increase, as well as the distances in the nuclei, putting more strain on the nuclear forces holding it all together. The ratio of neutrons to protons gradually increases to try to compensate, but this results in the neutrons getting more unstable themselves, resulting in more opportunity for weak forces to destabilize them. Hence, larger atoms become more unstable.

      There are believed to be certain numbers/ratios that are inherently more stable. These are called magic numbers. It's thought that another one of these magic numbers lies just beyond the range of currently known elements, which means we could potentially discover some new elements that are relatively stable and extremely massive.
      (172 votes)
  • aqualine sapling style avatar for user Lim Ern You
    The last paragraph said that the electrons will repel each other if they get too close and resulting in our perception of solidity...what does that mean..can someone explain to me thx a lot
    (54 votes)
    Default Khan Academy avatar avatar for user
  • piceratops tree style avatar for user lilajane2000
    The only thing that has always confused me about atoms, is, how can nonliving atoms make up living things? If you look at the world at the subatomic viewpoint, does that mean nothing is alive? I mean, I get how living things are living and the defining properties of a living organism make sense to me, but if you talk about atoms, doesn't that mean we are living, but only to a point?
    (16 votes)
    Default Khan Academy avatar avatar for user
  • female robot amelia style avatar for user Max Hinkle
    I'm very interested in the whole idea of our perception of solidity (last sentence). Does this mean that the reason I cannot punch through a wall is because the atoms making up the elements of matter in the wall are repelling the atoms making up my body (more specifically my fist)?
    (12 votes)
    Default Khan Academy avatar avatar for user
  • male robot donald style avatar for user Shay2012
    What's smaller: A proton or a neutron?
    (8 votes)
    Default Khan Academy avatar avatar for user
  • piceratops ultimate style avatar for user Tabrez Doulat
    in the first paragraph in The Structure of an Atom it says that Gold atoms cannot be broken down into anything smaller while still retaining the properties of gold. So what would happen if the gold atom was split? What kinds of properties would it have?
    (8 votes)
    Default Khan Academy avatar avatar for user
  • blobby green style avatar for user hunarmittal07
    What do you mean by perception of solidity and explain with an example?
    (9 votes)
    Default Khan Academy avatar avatar for user
    • scuttlebug yellow style avatar for user Amrita
      Imagine a stack of books, the atoms of the books aren't able to pass through each other because of the electron clouds that surround their nucleus. The electron clouds repel each other because the forces aren't attracted to one another.

      Since they don't pass through each other, this gives us our perception of solidity, and why solid objects won't pass through each other.
      (11 votes)
  • mr pink red style avatar for user cheryl ferrero
    So water is a main element in our body right?
    (8 votes)
    Default Khan Academy avatar avatar for user
    • old spice man green style avatar for user Matt B
      Water is not an element but a molecule but it is the most common molecule in our bodies.
      Oxygen is the most common element by mass (43% of all weight; carbon is 16% and hydrogen is 10%) in the body. The most common element by number is hydrogen (62% of all atoms; water is only 24% and carbon is 12%).
      (11 votes)