History of atomic theory

Atomic theory is the scientific idea that everything around us is made of tiny particles called atoms. The word "atom" has changed meaning over time as scientists learned more. At first, people thought atoms were the smallest pieces of matter that could not be broken down any further. Later, scientists discovered that atoms are the basic building blocks of chemical elements and that they can combine in simple whole-number ratios.

This theory is one of the most important ideas in science because it helps us understand how all physical things are made and how they behave. It is essential for many areas of science, including chemistry and physics. At the very beginning of The Feynman Lectures on Physics, famous physicist and Nobel Prize winner Richard Feynman called the atomic hypothesis one of the most powerful ideas in all of science.

Philosophical atomism

Main article: Atomism

The idea that everything is made of tiny, tiny pieces called atoms is very old. It started in ancient times when people thought about how matter works. The word "atom" comes from an ancient Greek word meaning "indivisible." Back then, these were just ideas based on thinking, not on real experiments or science. Today’s science about atoms is very different from these old thoughts.

Pre-atomic chemistry

In the late 1600s, Robert Boyle introduced the idea of a chemical element as a basic substance that is different from a compound. Later, in the late 1700s, Antoine Lavoisier made important discoveries in chemistry. He showed that compounds are made of elements in fixed amounts and that water can be broken down into hydrogen and oxygen, which are elements because they cannot be broken down further. Lavoisier also introduced the law of conservation of mass, which tells us that in a chemical reaction, matter is neither created nor destroyed — the total mass stays the same. Around the same time, Joseph Proust discovered the law of definite proportions, explaining that compounds always break down into elements in the same fixed ratios, no matter how much of the compound there is.

Dalton's chemical atomism

In 1804, John Dalton studied data about how elements combine to form compounds. He noticed that when two elements form more than one compound, the amounts of one element that combine with a fixed amount of the other are in simple whole-number ratios. For example, he looked at three compounds made from nitrogen and oxygen and found the amounts of oxygen they contained were in the ratios of 1:2:4.

Dalton used these patterns to suggest that matter is made of tiny, indivisible particles called atoms. He believed that atoms of the same element always have the same weight, while atoms of different elements have different weights. His ideas helped explain how chemicals react and form new substances. Later scientists like Amedeo Avogadro improved on Dalton's work, helping to solve some of the problems in his theories.

Opposition to atomic theory

Dalton's atomic theory sparked a lot of interest but wasn’t accepted by everyone. One reason was confusion over words. The word “atom” suggested something that can’t be divided, but Dalton used it for the tiniest piece of any chemical substance, not just elements. This led to debates about terms like “molecule” and “atom.” Over time, scientists clarified these ideas, with important discussions happening at the Karlsruhe Congress in Germany in 1860.

Some scientists also had philosophical concerns. Since atoms couldn’t be seen directly, some believed scientists should only focus on what they could observe, not guess about hidden realities. These thinkers split into two groups: those who relied on “equivalent weights” for experiments and others who thought only energy mattered in physical models. Later discoveries, like the periodic table, helped settle these debates.

Prout's hypothesis

Main article: Prout's hypothesis

Early scientists thought each element was made of unique atoms that all weighed the same. In 1815, a scientist named William Prout suggested that all matter might be made of tiny pieces of one basic element, which he called a protyle and linked to hydrogen. Another scientist, Berzelius, disagreed because measurements didn’t support this idea at the time. Later, in 1912, Francis Aston found some evidence that supported part of Prout’s idea.

Vortex theory

Main article: Vortex theory of the atom

From the 1860s to around 1890, some scientists thought atoms might be like spinning rings of fluid. They imagined matter as stable swirls in an invisible fluid, similar to smoke rings we can see. This idea was inspired by math related to fluid movement but didn’t fully explain things like magnetism or gravity. Even its supporters later found the idea didn’t work well.

Isomerism

Scientists found that some substances can have the same chemical ingredients but behave differently. For example, in 1827, Friedrich Wöhler showed that silver fulminate and silver cyanate have identical amounts of silver, carbon, nitrogen, and oxygen, yet their properties differ. In 1830, Jöns Jacob Berzelius named this phenomenon isomerism.

Later, in 1874, Jacobus Henricus van 't Hoff suggested that atoms in molecules arrange themselves in specific shapes. His idea helped explain why some compounds can have different structures while using the same atoms. For instance, the molecule pentane (C₅H₁₂) has three possible arrangements, which scientists later confirmed. These are called n-pentane, isopentane, and neopentane.

Mendeleev's periodic table

Main article: Periodic table

In 1869, Dmitrii Mendeleev noticed that when elements were arranged by their atomic weights, they showed repeating patterns in their properties. For example, lithium, sodium, and potassium shared similar traits, appearing every seven elements. Using these patterns, Mendeleev predicted elements that had not yet been discovered, such as scandium, gallium, and germanium, and his predictions were later proven correct.

The original periodic table was arranged by atomic weight, but sometimes elements were swapped to group those with similar properties together. For example, tellurium is placed before iodine even though it is heavier, so iodine can be in the same column as other halogens. Today, the periodic table is ordered by atomic number, a system that emerged after the discovery of the nucleus. When Mendeleev created his table, one entire row was missing because the noble gases had not yet been found.

Kinetic theory of gases

Main article: Kinetic theory of gases

In 1738, Daniel Bernoulli suggested that the pressure of gases and heat were caused by the movement of tiny particles called molecules. He used this idea to explain a rule about gases, known as the ideal gas law. Later, in the 1860s, James Clerk Maxwell brought back this idea and added that particles in a gas move at different speeds around an average speed. Around the same time, Ludwig Boltzmann used these ideas to explain how heat and energy work together. In 1905, Albert Einstein used these ideas to explain why tiny particles in water move around randomly, a phenomenon observed by Robert Brown in 1827 and later tested by Jean Perrin in 1908.

Plum pudding model

Main articles: Electron and Plum pudding model

Before 1899, people thought atoms were the smallest pieces of matter. Then, J. J. Thomson discovered the electron using special glass tubes called Crookes tubes. These tubes have two electrodes with a vacuum between them. When voltage is applied, glowing rays called cathode rays appear. Thomson found these rays were made of tiny, negatively charged particles, which he called "corpuscles." He measured their mass and charge and found they were much smaller than atoms.

In 1904, Thomson suggested a new model for atoms. He thought the atom was like a sphere filled with positive charge, with tiny electrons scattered inside, like raisins in a pudding—hence the name "plum pudding model." This model tried to explain how atoms could be neutral overall, with both positive and negative charges balanced inside. Later, experiments measured the exact charge of an electron.

Planetary models

In the late 1800s, scientists thought about how atoms might be structured, imagining models where electrons orbited like planets. However, these early ideas had problems because, according to the laws of physics at the time, moving electrons would lose energy and spiral into the atom's center.

In 1903, a scientist named Hantaro Nagaoka proposed a different model where electrons circled a large, positively charged center, similar to how rings of moons orbit Saturn. This idea sparked discussion, but it was later found to have issues with explaining how atoms emit light, and Nagaoka moved on to other theories by 1908.

Haas atomic model

In 1910, Arthur Erich Haas suggested a new idea about how atoms might work. He thought that in a hydrogen atom, an electron moves around a sphere of positive charge. This was similar to an earlier idea, but Haas added something new: he linked the electron’s energy to how fast it moves.

Haas used math to show how the forces balance out, and his work helped later scientists understand more about atoms. Three years after Haas, another scientist named Bohr used similar ideas to find the size of the hydrogen atom, which is now known as the Bohr radius.

Nicholson atom theory

In 1911, John William Nicholson created a model of the atom where negative electrons orbited a positive nucleus, similar to J.J. Thomson's plum pudding model. Nicholson used the study of light from stars and planets to develop his ideas, linking the way electrons moved to a constant discovered by Planck.

Although Nicholson's model is not often talked about today, it was very important for later work, especially for Niels Bohr's model of the atom. Nicholson's ideas helped Bohr think about how atoms work, even though Bohr later showed that hydrogen atoms had only one electron, not a pair as Nicholson suggested.

Discovery of the nucleus

Main article: Rutherford scattering experiments

In 1911, scientist Ernest Rutherford challenged an older idea about atoms. He and his team noticed something strange when they shot tiny, positive particles called alpha particles at thin metal sheets. Some of these particles bounced back at big angles, which shouldn’t have happened if atoms were shaped the way people thought before.

Rutherford figured out that atoms must have a tiny, dense center where most of the weight and positive charge lives. This center is called the atomic nucleus. His discovery helped scientists understand how atoms are really built. Later, another scientist named Niels Bohr added ideas about how the outer parts of the atom, the electrons, move around this nucleus.

Discovery of isotopes

Main article: Isotope

At the same time that scientists like Rutherford were exploring atoms, a chemist named Frederick Soddy was studying radioactive materials. He discovered that some elements that looked the same in chemical reactions were actually different inside. He called these isotopes. Later, a scientist named Francis Aston created a tool to measure these tiny differences and found that each isotope had a whole number mass, except for hydrogen. His important work helped us understand the building blocks of matter even better.

Atomic number

Before 1913, scientists believed that the properties of elements were based on their atomic weight. However, this didn't always match what was seen in the periodic table, such as with cobalt and nickel. In 1913, Antonius van den Broek suggested that elements should be organized by their charge, called Z, instead of atomic mass. This idea helped explain some problems in the periodic table.

Soon after, Moseley tested this idea using X-ray spectroscopy. He discovered that a specific type of X-ray from each element, called the K-alpha line, was connected to the element's charge, or atomic number, Z. His work showed a clear relationship between the X-ray frequencies and the atomic numbers of many elements.

Bohr model

Main article: Bohr model

In 1913, Niels Bohr created a new way to understand how atoms work. He thought electrons could only jump between certain energy levels, releasing or absorbing light in the process. This helped explain patterns in light from atoms, especially for hydrogen. Although Bohr's model was simple and could only fully explain hydrogen, it was an important step in learning about atoms. Later, scientists improved on his ideas to understand more complex atoms.

Discovery of the proton

In 1815, William Prout noticed that the weights of atoms seemed to be multiples of hydrogen’s weight, suggesting all atoms might be made of hydrogen. However, this idea faced challenges until isotopes were discovered in 1913.

Later, in 1917, Rutherford shot alpha particles at nitrogen gas and saw hydrogen ions come off. This helped him discover the proton, a tiny part inside atoms with a positive charge. The number of protons in an atom’s nucleus became known as the atomic number, which helps us tell different chemical elements apart.

Quantum mechanical models

Main article: History of quantum mechanics

In 1924, Louis de Broglie suggested that particles like electrons behave like waves. Erwin Schrödinger used this idea to create a mathematical description of electrons as waves, called a wave function. This helped explain some problems that older models couldn’t. Max Born later explained that this wave function shows the chances of finding an electron in different places.

Schrödinger’s model showed electrons in areas called atomic orbitals, which have different shapes based on energy and angular momentum. These shapes help us understand how atoms bond together to form molecules. The way electrons fill these orbitals follows a rule called the Pauli principle, which limits the number of electrons in each orbital.

Discovery of the neutron

Main article: Discovery of the neutron

In the 1920s, scientists thought that atoms had tiny particles called "nuclear electrons" inside their nuclei. They believed these helped explain certain properties of elements and radiation. In 1928, scientist Walter Bothe found that when beryllium was hit with special particles, it gave off a mysterious kind of radiation. At first, people thought this was a type of energy called gamma radiation. But later, James Chadwick showed that this radiation was actually made of tiny, neutral particles with mass similar to protons. He named these particles "neutrons." This discovery changed how we understand the building blocks of atoms.