An atom is the most basic form of matter. Atoms were believed to be the smallest particles that could exist when they were first discovered. The word "atom" is derived from the Greek word for "indivisible" because something that is already the smallest possible size cannot be divided.
We now know that there are particles smaller than atoms and that, despite the origin of their name, they can be divided. We know this because splitting attoms creates energy that we harvest through a process called nuclear fusion.
Every atom contains three kinds of subatomic particles: electrons, protons, and neutrons. Protons and neutrons form the nucleus in the center of an atom, while electrons are in constant motion around the nucleus. Subatomic particles are made up of even smaller particles known as quarks. Quarks are so small that they can't be seen. Scientists only know quarks exist by observing their effects on surrounding particles.
Most atoms are neutral with no electrical charge. Electrically charged atoms are called ions. The charge is determined by protons and electrons. Protons are positively charged, and electrons are negatively charged. Neutral atoms contain an equal number of protons and electrons. Cations are positively charged because they contain more protons. Anions contain more electrons, so they are negatively charged. Ions can be very different from neutral atoms. Sodium ions and chloride ions form salt, but neutral sodium atoms burst into flame when they contact water. Neutral chlorine atoms combine with each other and form such a dangerous compound that towns are evacuated when trucks or trains carrying chloride gas are involved in accidents.
The Big Bang that formed our universe occurred 13.7 billion years ago. The new universe expanded and doubled its size at least 90 times within the very first second of existence. Quarks and electrons formed and spread throughout the universe after the first ten-millionths of a second. Protons and neutrons combined into nuclei 3 minutes after the Big Bang. Researchers are trying to recreate the Big Bang in powerful particle colliders. They hope to learn more about matter, and investigate the possibility of alternate realities and dimensions.
The first atoms formed 380,000 years after the Big Bang. It took that long for the universe to cool enough for moving electrons to slow down. The slower electrons were captured by nuclei to form atoms. Hydrogen and helium are the lightest atoms, and they were the first elements formed.
The first atoms formed were very light, while heavier atoms formed within stars. Some types of stars become supernovas when they die. Supernovas produce so much energy that they briefly outshine galaxies. The energy also manifests as an immense, explosive force. The force generated by supernovas dispersed heavy atoms throughout the universe.
Protons and neutrons in an atom's nucleus are held together by a strong force. Nuclei in certain atoms are unstable because the binding force isn't very strong. Unstable atoms decay and lose neutrons or electrons in an effort to become stable. An unstable atom becomes an ion if it loses or gains electrons, but it becomes radioactive if it loses neutrons.
Isotopes are versions of an element that have different numbers of neutrons. Isotopes of the same element always contain the same number of protons. The word "isotope" comes from the ancient Greek root "isos" meaning "equal," and "topos" meaning "the same place." The name was chosen because no matter how many isotopes of an element exist, they all occupy the same spot on the periodic table.
The periodic table, also called the periodic table of elements, is a chart displaying each chemical element. The elements are arranged into seven rows, or periods, according to atomic number. All atoms have at least one proton. The number of protons is the atomic number. Changing the neutron number creates isotopes, but changing the proton number creates a different element entirely.
A radioactive atom tries to reach a stable state by throwing off protons and neutrons or trying to release energy in other forms. Radioactivity refers to the actions of unstable atoms that emit nuclear radiation. The radiation comes from radioactive decay in the nucleus. The decay results in a different isotope that may or may not be radioactive.
Uranium is the most common nuclear fuel because it exists in nature. Uranium-238 makes up the majority of natural uranium. It is not very radioactive on its own, but it forms plutonium-239 in a nuclear reactor. Uranium-235 is naturally radioactive. It is used in nuclear reactors and weapons, and it is desired by every country using nuclear energy or trying to build weapons because it can be used as it is without enrichment. Only 0.7% of naturally-occurring uranium is uranium-235, but at one time it made up 85% of all uranium. The decline is due to the isotope's unstable core, which makes it so desirable in the first place.