Evolution of the Atomic Theory

Democritus ( BC) all things are composed of minute, invisible, indestructible particles of pure matter, which move forever in empty space all things are composed of minute, invisible, indestructible particles of pure matter, which move forever in empty space stated that atoms are made of the same matter, yet they differ in shape, size, weight, sequence, and position stated that atoms are made of the same matter, yet they differ in shape, size, weight, sequence, and position Named “atomos” meaning indivisible Named “atomos” meaning indivisible

Aristotle (4BC) Claimed there were no such things as atoms. Claimed there were no such things as atoms. Believed everything was uniform Believed everything was uniform

John Dalton ( ) Theory: Theory: Atoms are small, spherical objects. Atoms are small, spherical objects. Each different kind of element is represented by a different kind of atom. Each different kind of element is represented by a different kind of atom. Each different kind of atom has its own distinct mass. Each different kind of atom has its own distinct mass. Atoms are indivisible. Atoms are indivisible.

Sir Joseph John Thomson ( ) Based on the evidence, Thomson came up with his "Plum Pudding" or what is more commonly known as the “Chocolate Chip Cookie Dough” model of the atom: Based on the evidence, Thomson came up with his "Plum Pudding" or what is more commonly known as the “Chocolate Chip Cookie Dough” model of the atom: "Plum Pudding" "Plum Pudding" Atoms are composed of smaller particles. Atoms are composed of smaller particles. These particles are the same for all different types of atoms. These particles are the same for all different types of atoms. These particles are negatively charged and are called electrons. These particles are negatively charged and are called electrons. Electrons are embedded throughout the uniform sphere of positive charge to make up a neutral atom. Electrons are embedded throughout the uniform sphere of positive charge to make up a neutral atom.

Ernest Rutherford ( ) Based on the gold foil experiment, Rutherford made the following statements about the atom: Based on the gold foil experiment, Rutherford made the following statements about the atom: Atoms contain a nucleus, or core, that contains most of the mass of the atom. Atoms contain a nucleus, or core, that contains most of the mass of the atom. The nucleus has a positive charge. The nucleus has a positive charge. Most of the atom is empty space Most of the atom is empty space empty space empty space

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Niels Henrik David Bohr ( ) Electrons are only allowed to move in certain paths, or orbits. Electrons are only allowed to move in certain paths, or orbits. Each of these orbits corresponds to a specific amount of energy, or energy level Each of these orbits corresponds to a specific amount of energy, or energy level Electrons can travel in one of the allowed orbits without losing energy. Electrons can travel in one of the allowed orbits without losing energy. The only way for an electron to change its energy level is for it to instantaneously change orbits. The only way for an electron to change its energy level is for it to instantaneously change orbits.

Niels Henrik David Bohr ( ) showed that an atom emits electromagnetic radiation only when an electron jumps from one quantum level to another showed that an atom emits electromagnetic radiation only when an electron jumps from one quantum level to another helped lead to the idea that electrons exist in shells and that the electrons in the outermost shell determine an atom's chemical properties helped lead to the idea that electrons exist in shells and that the electrons in the outermost shell determine an atom's chemical properties

Electron Cloud or Quantum Model it is impossible to know both the exact position and the exact momentum of an object at the same time it is impossible to know both the exact position and the exact momentum of an object at the same time

What we know so far… We now know that electrons do not travel in distinct orbits We now know that electrons do not travel in distinct orbits rather the model on the previous slide shows the most common probable location of an electron, much like the fan blades of a ceiling fan in rotation rather the model on the previous slide shows the most common probable location of an electron, much like the fan blades of a ceiling fan in rotation