Atom

Atoms are the basic particles of the chemical elements and the fundamental building blocks of matter. An atom consists of a nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished from each other by the number of protons that are in their atoms. For example, any atom that contains 11 protons is sodium, and any atom that contains 29 protons is copper.

Atoms are extremely small, typically around 100 picometers across. A human hair is about a million carbon atoms wide. Atoms are smaller than the shortest wavelength of visible light, which means humans cannot see atoms with conventional microscopes.

More than 99.94% of an atom's mass is in the nucleus. Protons have a positive electric charge and neutrons have no charge, so the nucleus is positively charged. The electrons are negatively charged, and this opposing charge is what binds them to the nucleus. If the numbers of protons and electrons are equal, as they normally are, then the atom is electrically neutral as a whole.

Atoms can attach to one or more other atoms by chemical bonds to form chemical compounds such as molecules or crystals. The ability of atoms to attach and detach from each other is responsible for most of the physical changes observed in nature. Chemistry is the science that studies these changes.

History of atomic theory

Main article: History of atomic theory

The word atom comes from an ancient Greek word meaning "uncuttable." Long ago, people thought matter was made of tiny, indivisible particles, but this was just a philosophical idea, not science. In the early 1800s, scientist John Dalton showed that matter is really made of small units he called atoms.

Dalton studied how elements combine. He found that when elements form compounds, they do so in simple whole-number ratios. For example, there are different kinds of tin oxide. One type has one atom of oxygen for every atom of tin, while another has two atoms of oxygen for every atom of tin. This pattern helped scientists understand that atoms are the building blocks of matter.

Later, scientists discovered that atoms are not indivisible after all. In 1897, J. J. Thomson found a tiny particle called the electron, which is much smaller than an atom. Then, in experiments with alpha particles, Ernest Rutherford discovered that atoms have a small, dense nucleus at their center, surrounded by electrons. This changed our understanding of atoms forever.

Structure

Main article: Subatomic particle

Atoms are made up of tiny particles called subatomic particles. The three main particles in an atom are electrons, protons, and neutrons.

Electrons are very small and have a negative charge. They move around the nucleus of the atom. Protons are larger than electrons and have a positive charge. They are found in the nucleus along with neutrons, which have no charge but are heavy like protons. The number of protons in an atom tells us what element it is.

The nucleus is the very small, dense center of the atom where the protons and neutrons are found. It is much smaller than the whole atom. The number of protons and neutrons in the nucleus can change, but this needs a lot of energy. When atoms join together or split apart, it can release huge amounts of energy, which is why the Sun shines and why nuclear power works.

See also: Electronegativity

The electrons in an atom are arranged in areas around the nucleus called electron shells. These shells are where we are most likely to find the electrons. The electrons can move between these shells by gaining or losing energy. When they do this, they give off or absorb light, which is why each element has a unique pattern of colors when heated. This arrangement of electrons is very important because it determines how atoms bond together to form molecules and different kinds of materials.

Properties

Atoms are the tiny building blocks that make up everything around us. Each atom has a central part called the nucleus, made of protons and neutrons, surrounded by electrons. The number of protons determines what element the atom is — for example, all hydrogen atoms have one proton.

Atoms can have different versions called isotopes, which have the same number of protons but different numbers of neutrons. Some isotopes are stable and stay the same, while others are radioactive and change over time. The size of an atom is tiny — a human hair is about a million carbon atoms wide! Atoms can bond together to form molecules and different materials like solids, liquids, and gases.

Identification

Atoms are so tiny that we can't see them with regular tools, but special devices like the scanning tunneling microscope can show us atoms on surfaces. This microscope uses a amazing effect called quantum tunneling, which lets tiny particles pass through barriers they normally couldn't.

We can also tell atoms apart by their weight. When an atom loses an tiny part called an electron, it becomes charged and we can bend its path with a magnetic field. The way it bends tells us the atom's weight. Tools like the mass spectrometer help scientists measure these weights to figure out what atoms are present.

Origin and current state

Baryonic matter makes up about 4% of the energy in the universe, with a very low density of particles. Inside galaxies like the Milky Way, the density is higher, especially in areas called the interstellar medium. Stars form from these dense clouds and create new elements through processes that change lighter elements into heavier ones.

Most of the matter in the Milky Way is found inside stars, where it is too hot for atoms to stay together. Atoms became common about 380,000 years after the Big Bang when the universe cooled down enough for electrons to join with nuclei. Elements heavier than iron are made in events like explosions of stars or collisions between stars made of neutrons. On Earth, most atoms have been around since the planet formed, with some created later by natural processes or human activities.