Early Ideas on the Atom
Origin of atomic concept: The idea that everything is made up of tiny building blocks called atoms started a long time ago in ancient Greece. Around 450 BC, a philosopher named Democritus came up with this idea. He imagined that if you kept cutting something into smaller and smaller pieces, you would eventually get to a part that could not be cut anymore. This smallest part, he called an “atomos,” which means “uncuttable” in Greek.
Democritus’ proposal: Democritus believed that all matter (everything around us) is made up of very small, indivisible pieces called ‘atomos.’ He said these pieces were so tiny that they couldn’t be divided any further. This idea was very simple, but it laid the foundation for our modern understanding of atoms.
Key Models of the Atom
John Dalton's Atomic Model (1805)
Indivisible particles: Dalton was a scientist who said that atoms are the smallest parts of matter and cannot be broken into anything smaller. He believed atoms were like solid balls that couldn’t be split apart.
Elemental uniformity: Dalton also said that all atoms of the same element are exactly the same—they have the same mass and behave in the same way.
Elemental diversity: But atoms of different elements are not the same. For example, an atom of oxygen is not the same as an atom of hydrogen—they have different weights and properties.
Conservation in reactions: Dalton explained that in chemical reactions, atoms are never created, destroyed, or split. They simply rearrange to make new substances.
Combination of atoms: According to Dalton, chemical reactions happen when atoms stick together or change partners to form new materials.
Model depiction: He imagined atoms as small, solid balls—like tiny marbles—that can’t be broken down.
J.J. Thomson's Atomic Model (1897)
Electron discovery: Thomson discovered that atoms are not solid after all. He used a special experiment with cathode rays and found out that atoms have tiny, negatively charged particles called electrons.
Plum pudding model: To explain this, Thomson came up with a new model. He said an atom is like a pudding with raisins in it. The pudding is a positively charged ball, and the electrons (raisins) are scattered inside it.
Atomic charge distribution: In his model, the positive charge is spread out all over the atom, and the small negative electrons are dotted throughout the positive background.
Challenge to Dalton: This was a big change from Dalton’s idea, because it showed that atoms are made of even smaller parts and are not indivisible after all.
Ernest Rutherford's Atomic Model (1911)
Gold foil experiment: Rutherford did a famous experiment where he shot tiny particles at a very thin sheet of gold foil. Most particles passed through, but a few bounced back.
Discovery of nucleus: From this, he figured out that atoms have a small center, called the nucleus, which has a positive charge and holds most of the atom’s mass.
Atomic emptiness: Rutherford found that atoms are mostly empty space, because most of the particles went straight through the foil.
Electron movement: He said electrons move around the nucleus, kind of like planets orbiting the sun, but not in fixed paths yet.
Niels Bohr's Atomic Model (1913)
Electron energy levels: Bohr improved Rutherford’s model by suggesting that electrons move in specific energy levels, also called shells.
Fixed electron orbits: He said electrons don’t just zoom around randomly. They travel in fixed paths or orbits around the nucleus.
Energy level restriction: Each electron can only stay in certain allowed orbits. They can jump between these orbits, but they can’t be in between them.
James Chadwick's Atomic Model (1932)
Neutron discovery: Chadwick discovered another important particle inside the atom—the neutron. Neutrons don’t have any charge.
Neutron mass: He found out that neutrons are about the same weight as protons, the positively charged particles in the nucleus.
Atomic mass contribution: Neutrons, along with protons, add to the weight of the atom.
Nuclear location: Neutrons live inside the nucleus with the protons, helping to hold it together.
Subatomic Particles and Atomic Structure
Atom’s components: Atoms are made of three smaller particles: protons, neutrons, and electrons.
Protons (p)
Symbol and charge: Protons are written as ‘p’ and have a positive charge of +1.
Mass and location: Each proton has a mass of 1 unit, and it is found in the nucleus at the center of the atom.
Neutrons (n)
Symbol and charge: Neutrons are written as ‘n’. They do not have any charge, which means they are neutral.
Mass and location: Neutrons also have a mass of 1 unit and are found in the nucleus together with the protons.
Electrons (e)
Symbol and charge: Electrons are shown as ‘e’ and carry a negative charge of -1.
Mass and location: Electrons are very, very light—only about 1/1840 of a proton’s mass. They move around the nucleus in specific orbits called shells.
Key Features of Atomic Structure
Nucleus composition: The nucleus is the tiny, heavy center of the atom and it contains protons and neutrons. These two particles are tightly packed together and make up almost all of the atom’s weight.
Positive nucleus: Because of the protons, the nucleus has an overall positive charge. Neutrons do not have a charge, so they don’t change this.
Electron arrangement: Electrons are found in shells or layers that go around the nucleus. These shells are also called energy levels, and they hold electrons in place due to electrical attraction.
Neutral atom balance: In an atom with no charge (a neutral atom), the number of electrons is the same as the number of protons. This equal number cancels out the charges and keeps the atom neutral.
Mass concentration: Most of the atom’s mass is found in the nucleus, not in the electrons. Electrons are very light, so they don’t add much to the atom’s overall mass.
Atomic number definition: The atomic number of an element is the number of protons in its atoms. It’s a unique number for each element and is used to organize the periodic table.
Nucleon number definition: The nucleon number, or mass number, is the total number of protons and neutrons in the nucleus. It tells us how heavy the nucleus is and helps in identifying isotopes of elements.
Electron Arrangement
Shell configuration: Electrons are arranged in shells or energy levels around the nucleus. The shells are filled in order, starting with the one closest to the nucleus.
First shell capacity: The first shell (closest to the nucleus) can hold up to 2 electrons. These electrons are held very tightly because they are close to the positive nucleus.
Second shell capacity: The second shell can hold up to 8 electrons. Once the first shell is full, electrons go into this next shell.
Third shell limit (up to Z=20): For elements with atomic numbers up to 20, the third shell also holds up to 8 electrons. This applies to elements like calcium and below on the periodic table.
Third shell for larger atoms: In elements with more than 20 electrons, the third shell can actually hold more than 8 electrons. This is because of the more complex structure of larger atoms.
Valence shell and electrons: The outermost shell of an atom is called the valence shell. The electrons in this shell are called valence electrons and they are important for chemical reactions. Atoms react with each other mainly through their valence electrons.
The Significance of These Models
Scientific evolution: The changes in atomic models over time show how scientists have learned more and more about atoms by doing experiments and using better tools.
Model refinement: Each scientist improved the model by fixing the mistakes or gaps in the earlier models, giving us a clearer picture of what atoms are like.
Dalton to Thomson: Dalton thought atoms were solid balls, but Thomson showed they have smaller parts inside—electrons.
Rutherford and Bohr: Rutherford discovered the nucleus, while Bohr explained that electrons move in fixed shells.
Chadwick’s contribution: Chadwick added the final piece by discovering neutrons, which also live in the nucleus and affect atomic mass
Impact on chemistry: Understanding atomic structure helps scientists explain how and why different materials behave the way they do in chemical reactions.