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AIM: Models of the Atom DO NOW:

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1 AIM: Models of the Atom DO NOW:
HW: On a separate sheet of paper Make a timeline of the history and development of the atom. Include: Dalton’s Model, J. J. Thompson’s Model, Rutherford Model, Bohr Model, Electron Cloud Model Year and Short description Image or drawing of Model


3 Democritus’s Atomic Philosophy
Proposed an Atomic Theory which states that all atoms are small, hard, indivisible and indestructible particles made of a single material formed into different shapes and sizes. He named the smallest piece of matter “atomos,” meaning “not to be cut.”

4 Dalton’s Atomic Theory
In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. All matter is made of atoms. Atoms of an element are identical. Each element has different atoms. Atoms of different elements combine in constant ratios to form compounds. Atoms are rearranged in reactions.

5 Dalton’s Atomic Theory
Based on Dalton’s Atomic Theory (5 postulates), most scientists in the 1800s believed that the atom was like a tiny solid ball that could not be broken up into parts. Dalton was credited for the three Atomic Laws that were proven after his time.

6 Dalton’s Atomic Laws 1. Law of Conservation of Mass
Matter cannot be created or destroyed in any physical or chemical process, just transferred. 2.     Law of Constant Composition When atoms combine to form molecules, the ratio of atoms is constant. Example – H2O will always have 2 times as many Hydrogen atoms as Oxygen.

7 Dalton’s Atomic Laws Example: CO vs. CO2 Formula Ratio of N:O
3.     Law of Multiple Proportions – if two elements can combine to form more than one compound, then the ratio of the second element combined with a certain mass of the first element is always a ratio of small whole numbers. Example: CO vs. CO2 Formula Ratio of N:O

8 J. J. Thomson’s Plum Pudding Model
Used cathode rays to prove that Dalton’s Solid-ball model could be broken into smaller particles Thomson is credited with discovering electrons

9 J. J. Thomson’s Plum Pudding
A cathode ray is a tube that has a piece of metal, called an electrode, at each end. Each electrode is connected to a power source (battery). When the power is turned on, the electrodes become charged and produce a stream of charged particles. They travel from cathode, across the tube to the anode.

10 J. J. Thomson’s Plum Pudding Model
Stated that the atom is neutral In 1897, he proposed the Plum Pudding Model which states that atoms mostly consist of positively charged material with negatively charged particles (electrons) located throughout the positive material

11 Rutherford Model Rutherford’s experiment Involved firing a stream of tiny positively charged particles (alpha particles) at a thin sheet of gold foil (2000 atoms thick) Most of the positively charged particles passed right through the gold atoms in the sheet of gold foil without changing course at all. Some of the positively charged particles, however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.


13 Rutherford Model Gold Foil Experiment and suggested the following characteristics of the atom: It consists of a small core, or nucleus, that contains most of the mass of the atom This nucleus is made up of particles called protons, which have a positive charge The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space

14 Bohr Model In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level. According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus. Additionally, the electrons can jump from a path in one level to a path in another level (depending on their energy)

15 Bohr’s Model He proposed the following Protons and neutrons are in the
nucleus Electrons can only be certain distances from the nucleus The electrons orbit the nucleus at fixed energy levels The electrons must absorb or emit a fixed amount of energy to travel between these energy levels

16 Electron Cloud Model (Wave Mechanical Model)
In 1926, Erwin Schrodinger further explained the nature of electrons in an atom by stating that the exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds; electron clouds are places where the electrons are likely to be found

17 Electron Cloud Model (Wave Mechanical Model)
Depending on the electron’s energy they are locked into a certain area in the cloud. Electrons with the lowest energy are found in the energy level closest to the nucleus Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus. Today’s atomic model is based on the principles of wave mechanics.

18 Summary Who is the father of atomic theory? Dalton
What was the first model of the atom? Dalton’s Tiny Ball Model What are Dalton’s 3 Laws? Law of Conservation of Mass, Law of Constant Composition, Law of Multiple Porportion

19 Summary How were Thomson’s and Dalton’s model different?
Dalton’s model was 1 sphere that cannot be divided, Thomson had the plum pudding where electrons are randomly spread throughout a positively charged sphere. What did Thomson find out? Atoms have electrons, they have a negative charge

20 Summary What were Rutherford’s conclusions from the Gold Foil Experiment? Atom has a positively charged nucleus electrons are outside, atoms are mostly empty Nucleus contains most of the mass.

21 Summary Bohr Model of the Atom
Electrons go around the nucleus in concentric circular orbits Electron Cloud Model The electron does not move in a definite path. An electron is in an orbital which is the most probable location where the electron is The most dense part of the cloud is the most probable place to find the electron

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