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Period: 300 BCE to 400 BCE
Democritus
Democritus was a smart guy from ancient Greece. He thought about how things were made of tiny bits called atoms. He said these atoms couldn't be broken into smaller pieces. He thought they were the smallest things ever. That's where we get the word "atom" from! -
Period: to
Antoine Lavoisier
Antoine-Laurent de Lavoisier was a clever French scientist born in 1743. He changed how people thought about chemistry in the 18th century and is often called the "Father of Modern Chemistry." One of his important discoveries was about oxygen and how it's involved in burning things. He also figured out the Law of Conservation of Mass, which basically says that matter can't just appear or disappear. So, he was a really important person in the history of science! -
Law of Conservation of Mass
The Law of Conservation of Mass, discovered by the French scientist Antoine Lavoisier, tells us that during a chemical reaction, the total mass of substances involved remains the same. This means that mass is neither created nor destroyed—it's conserved. It's a really important idea in chemistry because it helps us understand how matter behaves when it undergoes changes. -
John Dalton
In the 19th century, a schoolteacher named John Dalton from England made significant contributions to the development of modern atomic theory. He built on the ideas of Democritus and conducted his own scientific research to revise and expand upon them. His work resulted in what is now known as "Dalton's Atomic Theory." This theory laid down important principles about the nature of atoms and their behavior, shaping our understanding of the building blocks of matter. -
Dalton's Atomic Theory
Tiny Building Blocks: Everything is made of small particles called atoms. Same Element, Same Atom: All atoms of the same element are identical. Different Elements, Different Atoms: Atoms of different elements are different. Indivisible Atoms: Atoms can't be made smaller or destroyed. Mixing Up: Atoms join together in simple ways to make compounds. Chemical Changes: In reactions, atoms can separate, join, or rearrange. -
Dmitri Mendeleev
Dmitri Mendeleev went to school in St. Petersburg and finished in 1856. He later became a professor and eventually chaired the university. His biggest achievement was creating the Periodic Law and making the Periodic Table. -
Cathode Ray Tube
JJ Thomson did experiments with cathode ray tubes. He found that when electricity passed through a gas, the ray would move away from negative fields. By testing different metals and gases, he figured out that the particles in the ray were negatively charged. This discovery led to the identification of the electron. -
JJ Thomson
JJ Thomson, an English physicist, discovered the electron in the late 19th century. He used a cathode ray tube experiment to find that particles in the tube were negatively charged. This led to the creation of the Plum Pudding model, which suggested that electrons were scattered throughout a positively charged sphere, similar to raisins in a plum pudding. -
Plum Pudding Atomic Model
JJ Thomson's Plum Pudding model suggested that negatively charged electrons were spread throughout a positively charged sphere, like raisins in a plum pudding. -
Robert Millikan
Robert Millikan (1856-1953), an American physicist, determined the charge of an electron and also discovered its mass, which is approximately 9.1 x 10^-28 grams, or about 1/1840th the mass of a hydrogen atom. -
Rutherford Model
Rutherford's nuclear model of the atom explains the results of the gold foil experiment. Most of an atom consists of electrons moving rapidly through empty space. The electrons move through available spces surounding the nucleus and are held within the atom by their attraction to the positively charged nucleus. -
Henry Moseley
Henry Moseley (1887-1915) was a British chemist who studied under Rutherford. He made significant contributions to atomic structure by using X-ray spectra. Moseley's discoveries helped determine the accurate positioning of elements in the Periodic Table by determining atomic numbers more precisely. Sadly, Moseley's promising career was cut short when he was killed in action at Gallipoli in 1915, during World War I. -
Bohr Planetary
Bohr developed a theory building on Rutherford's earlier atomic model. Rutherford had revealed that atoms have a positively charged nucleus, orbited by negatively charged electrons. Bohr's contribution was to propose that electrons move in specific orbits, each larger than the last. He also suggested that outer orbits can hold more electrons than inner ones, and that these outer orbits dictate the atom's chemical behavior. -
Niels Bohr
Niels Bohr introduced a theory for the hydrogen atom using quantum theory, which suggests that energy is transferred only in specific amounts. According to Bohr, electrons orbit the nucleus in specific paths, or orbits. When electrons move between orbits, emitting or absorbing energy, they do so in precise amounts. This theory could explain why atoms emit light in fixed wavelengths, as observed in experiments. -
Gold Foil Experiment
In the gold foil experiment, Ernest Rutherford directed alpha particles at a thin gold foil. Surprisingly, most alpha particles passed through the foil with little deflection. However, a small fraction of particles were deflected, and even fewer bounced back. Rutherford inferred that atoms consist mostly of empty space, with a concentrated positive charge in a tiny nucleus at the center. He also proposed that electrons orbit the nucleus, and the nucleus contains most of the atom's mass. -
Erwin Schrodinger
In Niels Bohr's atomic theory, electrons move in fixed orbits around the nucleus and emit radiation with specific wavelengths when they transition between these orbits. While Bohr's theory explained hydrogen's spectrum well, it needed refinement for complex atoms and molecules. Erwin Schrödinger proposed a wave equation in 1926, treating matter like electrons as both particles and waves. This equation accurately predicted the energy levels of atoms,advancing our understanding of atomic structure -
Quantum Mechanical Model
The quantum-mechanical theory, created by smart scientists like Albert Einstein and Niels Bohr, changed how we see atoms. Instead of fixed paths, it says electrons move in fuzzy areas around a tiny nucleus. Even though these scientists weren't always sure about their ideas, they thought electrons have certain energy levels but uncertain positions. -
Ernest Rutherford
Ernest Rutherford (1871-1937) found out about protons, which are positively charged bits inside an atom. He did this using the gold foil experiment. He realized that atoms are mostly empty space where electrons move around. But there's a small, dense part called the nucleus where the protons stay, and it holds most of the atom's mass. -
James Chadwick
James Chadwick discovered the neutron, a particle with no charge, which is located in the nucleus of an atom. He found that during certain reactions, a particle with the same mass as a proton but no charge was released. Ernest Rutherford had suggested the existence of such a neutral particle before, and Chadwick confirmed it through his experiments. This neutral particle was named the neutron. -
Electron Cloud Model
Erwin Schrödinger, an Austrian physicist, proposed the electron cloud model of the atom. In this model, electrons are not shown as tiny particles orbiting the nucleus in set paths. Instead, their location around the nucleus is described as a cloud-like region where they are likely to be found. This model suggests that we cannot precisely determine the exact position of electrons at any given time, but rather describe their probable locations around the nucleus.