How do atoms form? A physicist explains where the atoms that make up everything around come from

Atoms are the building blocks of everything around us, and they’re made up of protons, neutrons, and electrons. The number of protons in an atom determines its chemical properties.
The universe’s hydrogen and helium atoms formed about 400,000 years after the Big Bang, when the universe was still very hot and small. These atoms were created through a process called recombination, where electrons fell into orbits around the nuclei.
More massive atoms are formed in stars through a process called fusion, which involves high-energy collisions that overcome the repulsive electric force between positive charges. This process creates elements with several protons and neutrons stuck together in the nucleus.
Astronomers believe that most of the elements from carbon up to iron were formed in stars heavier than our Sun, while elements heavier than iron are thought to be formed through supernovae explosions, which eject heavy elements into space.
Despite understanding how atoms are made, scientists are still trying to figure out what dark matter is and how it might form. Dark matter makes up about 90% of the universe’s mass-energy budget, but its nature remains a mystery.

Many heavy atoms form from a supernova explosion, the remnants of which are shown in this image. NASA/ESA/Hubble Heritage Team

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com.

How do atoms form? – Joshua, age 7, Shoreview, Minnesota

Richard Feynman, a famous theoretical physicist who won the Nobel Prize, said that if he could pass on only one piece of scientific information to future generations, it would be that all things are made of atoms.

Understanding how atoms form is a fundamental and important question, since they make up everything with mass.

The question of where atoms come from requires a lot of physics to be answered completely – and even then, physicists like me only have good guesses to explain how some atoms are formed.

What is an atom?

An atom consists of a heavy center, called the nucleus, made of particles called protons and neutrons. An atom has lighter particles called electrons that you can think of as orbiting around the nucleus.

The electrons each carry one unit of negative charge, the protons each carry one unit of positive charge, and the neutrons have no charge. An atom has the same number of protons as electrons, so it is neutral − it has no overall charge.

A diagram of an atom, with protons and neutrons clumped together in the center and ovals representing electron orbits surrounding them. — An atom consists of positively charged protons, neutrally charged neutrons and negatively charged electrons.
AG Caesar/Wikimedia Commons, CC BY-SA

Now, most of the atoms in the universe are the two simplest kinds: hydrogen, which has one proton, zero neutrons and one electron; and helium, which has two protons, two neutrons and two electrons. Of course, on Earth there are lots of atoms besides these that are just as common, such as carbon and oxygen, but I’ll talk about those soon.

An element is what scientists call a group of atoms that are all the same, because they all have the same number of protons.

When did the first atoms form?

Most of the universe’s hydrogen and helium atoms formed around 400,000 years after the Big Bang, which is the name for when scientists think the universe began, about 14 billion years ago.

Why did they form at that time? Astronomers know from observing distant exploding stars that the size of the universe has been getting bigger since the Big Bang. When the hydrogen and helium atoms first formed, the universe was about 1,000 times smaller than it is now.

And based on their understanding of physics, scientists believe that the universe was much hotter when it was smaller.

Before this time, the electrons had too much energy to settle into orbits around the hydrogen and helium nuclei. So, the hydrogen and helium atoms could form only once the universe cooled down to something like 5,000 degrees Fahrenheit (2,760 degrees Celsius). For historical reasons, this process is misleadingly called recombination − combination would be more descriptive.

The helium and deuterium − a heavier form of hydrogen − nuclei formed even earlier, just a few minutes after the Big Bang, when the temperature was above 1 billion F (556 million C). Protons and neutrons can collide and form nuclei like these only at very high temperatures.

Scientists believe that almost all the ordinary matter in the universe is made of about 90% hydrogen atoms and 8% helium atoms.

How do more massive atoms form?

So, the hydrogen and helium atoms formed during recombination, when the cooler temperature allowed electrons to fall into orbits. But you, I and almost everything on Earth is made of many more massive atoms than just hydrogen and helium. How were these atoms made?

The surprising answer is that more massive atoms are made in stars. To make atoms with several protons and neutrons stuck together in the nucleus requires the type of high-energy collisions that occur in very hot places. The energy needed to form a heavier nucleus needs to be large enough to overcome the repulsive electric force that positive charges, like two protons, feel with each other.

A close up of a bright star, which looks like a sphere with some flashes on its surface, against a dark background. — The immense heat and pressure in stars can form atoms through a process called fusion.
NASA/SDO

Protons and neutrons also have another property – kind of like a different type of charge – that is strong enough to bind them together once they are able to get very close together. This property is called the strong force, and the process that sticks these particles together is called fusion.

Scientists believe that most of the elements from carbon up to iron are fused in stars heavier than our Sun, where the temperature can exceed 1 billion F (556 million C) – the same temperature that the universe was when it was just a few minutes old.

The periodic table of elements, color-coded by how each element formed. Lighter elements formed by cosmic ray fission or exploding stars, while larger elements formed by merging neurtron stars or dying low mass stars. Hydrogen and Helium formed through Big Bang fusion. — This periodic table shows which astronomical processes scientists believe are responsible for forming each of the elements.
Cmglee/Wikimedia Commons (image) and Jennifer Johnson/OSU (data), CC BY-SA

But even in hot stars, elements heavier than iron and nickel won’t form. These require extra energy, because the heavier elements can more easily break into pieces.

In a dramatic event called a supernova, the inner core of a heavy star suddenly collapses after it runs out of fuel to burn. During the powerful explosion this collapse triggers, elements that are heavier than iron can form and get ejected out into the universe.

Astronomers are still figuring out the details of other fantastic stellar events that form larger atoms. For example, colliding neutron stars can release enormous amounts of energy – and elements such as gold – on their way to forming black holes.

Understanding how atoms are made just requires learning a little general relativity, plus some nuclear, particle and atomic physics. But to complicate matters, there is other stuff in the universe that doesn’t appear to be made from normal atoms at all, called dark matter. Scientists are investigating what dark matter is and how it might form.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Stephen L. Levy receives funding from the National Science Foundation and the National Institutes of Health. He is affiliated with CyteQuest, Inc.

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Q. What is an atom?

A. An atom consists of a heavy center called the nucleus, made of particles called protons and neutrons, surrounded by lighter particles called electrons that orbit around the nucleus.

Q. How did most of the universe’s hydrogen and helium atoms form?

A. Most of the universe’s hydrogen and helium atoms formed around 400,000 years after the Big Bang, when the universe was about 1,000 times smaller than it is now.

Q. Why did they form at that time?

A. The universe was much hotter when it was smaller, allowing electrons to fall into orbits around the hydrogen and helium nuclei, which enabled their formation.

Q. How do more massive atoms form?

A. More massive atoms are made in stars through a process called fusion, where high-energy collisions occur in very hot places, overcoming the repulsive electric force between positive charges.

Q. What is the strong force that binds protons and neutrons together?

A. The strong force is a property of particles that allows them to bind together once they get close enough, forming nuclei through a process called fusion.

Q. Can elements heavier than iron form in stars?

A. No, elements heavier than iron cannot form in stars because the energy needed to overcome their repulsive electric force is too great.

Q. How are elements heavier than iron formed?

A. Elements heavier than iron can be formed in supernovae explosions, where the inner core of a heavy star suddenly collapses, triggering an explosion that ejects these elements into space.

Q. What is dark matter and how does it relate to atoms?

A. Dark matter is a type of matter that doesn’t appear to be made from normal atoms and is still being investigated by scientists, who are trying to understand its composition and formation.

Q. Can you explain the process of recombination in simple terms?

A. Recombination refers to the process where electrons fall into orbits around hydrogen and helium nuclei as the universe cools down, allowing these elements to form.

Q. How do atoms make up everything with mass?

A. Atoms are the fundamental building blocks of matter, making up everything with mass, from the smallest particles to the largest objects in the universe.