Who is it? Have you ever tried to guess the identity of someone without looking? What are some things you know exist but can’t see? How do you know they exist? What kinds of things might scientists study that they can’t see?

Story of the Atom Objectives: I will be able to: describe how scientists discovered subatomic particles. identify how today’s model of the atom was developed. label the structures of the nuclear atom.

Story of the Atom Vocabulary: element: substance that cannot be broken down into simpler substances and whose atoms are exactly alike. atom: small particle that makes up most types of matter and is made up of smaller parts called protons, neutrons, and electrons. electron: tiny, negatively charged particle that is present in all atoms and that has almost no mass.

Story of the Atom Vocabulary con’t: proton: positively charged, heavy particle contained in the nucleus of all atoms. neutron: electrically neutral particle that is present in the nucleus of all atoms, has the same mass as a proton. nucleus: the positively charged center of an atom that consists of protons and neutrons and contains most of the mass of an atom. electron cloud: region surrounding the nucleus of an atom, where electrons are more likely to be found.

An Old Idea Our understanding of atoms took many centuries. The 4th century Greek philosopher, Democritus, suggested that the universe was made of invisible units called “atoms”. The word atom means “that which cannot be divided”. The Greek idea of atoms had to wait 2000 years before it became accepted.

Dalton’s Atomic Model In the 1700’s, chemists found out that new substances could be made by mixing some things together and by taking other things apart. When substances can no longer be broken apart into simpler substances what is left is an element.

Dalton’s Atomic Model In 1803, John Dalton proposed the idea that: elements are made up of atoms, and atoms are indestructible and cannot be divided into smaller particles, and all atoms of one element are exactly alike, but they are different from atoms of other elements. Dalton thought the atom was in the shape of a solid sphere that was the same inside and out.

Where’s the Proof? In 1870, William Crookes conducted an experiment that proved the existence of small particles with a cathode ray tube (CRT). When electric current was applied to the cathode, a glowing beam appeared in the tube. The glowing beam could be bent by a magnet, proving the glow wasn’t light but negatively charged particles. http://www.youtube.com/watch?v=Sikzu09q6cc&feature=related

 How did the English scientist William Crookes know that the greenish glowing beam in the CRT was not just a beam of light? What was the greenish glowing beam made up of?

Thomson’s Contribution In 1897, J.J. Thomson repeated Crookes experiment using different metals and different gases in the CRT. Thomson’s experiments showed charged particle beams occurred regardless of which metal or gas was used. Thomson concluded the charged particles were bits of matter that are part of every atom and were later named electrons. https://www.youtube.com/watch?v=oddjdB0qfMg

Thomson’s Contribution Thomson’s discovery led other scientists to wonder about the rest of the atom. If electrons are negatively charged and atoms have no charge, then the center of an atom must be positively charged. It wasn’t until 1920 that scientists identified protons. They found a proton is 2000 times heavier than an electron.

Thomson’s Contribution Thomson changed Dalton’s atom model from a solid sphere the same throughout to a sphere with electrons evenly distributed throughout the sphere (like chocolate chips in cookie dough).

Rutherford’s Atom In 1906, Ernest Rutherford shot alpha particles (particles that come off of unstable atoms) at a thin film of metal. He expected the alpha particles to go straight through but they didn’t, they went many different ways, even bouncing back, after hitting the thin metal. Rutherford was surprised!

Rutherford’s Atom What might be causing the deflections of the alpha particles?

The Nuclear Atom To make sense of his results, he hypothesized that instead of electrons and protons being evenly spaced in an atom, most of the mass and all of the positive charge is crammed into a small space in the center of an atom. This nucleus is surrounded by empty space where the electrons are found. http://www.youtube.com/watch?v=5pZj0u_XMbc&feature=related

The Nuclear Atom Rutherford’s model of the atom: The nucleus has a positive charge. The surrounding electrons have a negative charge

The Neutron Many scientists were very interested in Rutherford’s model of the atom but had questions. There was too much mass for the number of protons in many atoms. Where does the extra mass come from?

The Neutron Rutherford thought there must be another particle in the nucleus. This new particle (the neutron): has the same mass as a proton and is electrically neutral (no charge). Having no charge made it difficult to prove the existence of the neutron (James Chadwick finally proved its existence in 1932, more than 25 years later).

The Nuclear Atom After Rutherford’s experiments, the model of the atom was changed again to include neutrons. The nuclear atom has: a tiny nucleus with protons and neutrons tightly packed together and electrons in the space surrounding the nucleus.

The Neutron Rutherford’s revised model of the atom: https://www.youtube.com/watch?v=FSyAehMdpyI

Comparing Atom & Nucleus An atom is much larger than its nucleus. E.G. If the nucleus was the size of a poppy seed, the atom would be as large as a football stadium. An atom is mostly empty space.

Orbitals In 1913, Niels Bohr, a Danish scientist, revised the Rutherford’s model. Bohr showed that electrons are constantly moving. Using quantum theory, Bohr proposed that electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level (or orbit) to another. This model resembled planets orbiting the sun.

Orbitals Each orbital can only hold a certain number of electrons. The number of electrons in the outer orbital determines the atom’s physical properties. The negative charge and constant movement of an electron keeps it orbiting around the positively charged nucleus.

What About Electrons? In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr atom model one step further. Schrödinger showed that electrons do not have exact paths. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position.

What About Electrons? This atomic model is known as the quantum mechanical model or electron cloud model of the atom. The quantum mechanical model does not define the exact path of an electron, but predicts the odds of the location of the electron. This model can be portrayed as a nucleus surrounded by a fuzzy cloud of electrons.

What About Electrons? Where the cloud is most dense, the probability of finding the electron is greatest, and conversely, the electron is less likely to be in a less dense area of the cloud. We do not know where an electron is located, only where it is most likely to be. Electron Cloud Model

Atomic Model Summary John Dalton J.J. Thomson Ernest Rutherford Solid Sphere Plum Pudding Nuclear Atom Revised Nuclear Atom Planetary Model Electron Cloud John Dalton J.J. Thomson Ernest Rutherford Niels Bohr Erwin Schrödinger

Ernest Rutherford Erwin Schrödinger John Dalton J. J. Thomson Niels Bohr William Crookes

Review How did scientists discover subatomic particles? When electric current was applied to the cathode, a glowing beam appeared and the beam could be bent by a magnet. What was Dalton’s model of the atom like? a solid sphere that was the same inside and out. How was Thomson’s model of the atom different? a sphere with electrons evenly distributed throughout the sphere.

Review How did Rutherford’s findings change the model of the atom? most of the mass and all of the positive charge is crammed into a small space in the center of an atom surrounded by empty space where the electrons are found. What is the structure of the nuclear atom? tiny nucleus with protons and neutrons tightly packed together and electrons in the space surrounding the nucleus.

Web Links The Atom Song http://www.youtube.com/watch?v=vUzTQWn- wfE&feature=related Crash Course #37 https://www.youtube.com/watch?v=thnDxFdkzZs&t=4s http://www.youtube.com/watch?v=SmwlzwGMMwc&feat ure=bf_prev&list=PL9B3633FEE188F682 http://www.youtube.com/watch?v=zGM- wSKFBpo&feature=bf_prev&list=PL9B3633FEE188F682 https://atomictimeline.net/ http://www.abcte.org/files/previews/chemistry/s1_p6.html

Assignment On a separate sheet of paper: Put the title “The Story of the Atom” at the top center of the page. Put your name, date, and class period in the top left corner. Write out and answer questions 1-5 on the following slide.

Describe the three kinds of particles found in atoms Describe the three kinds of particles found in atoms. Where are they located in the atom and what are their charges? If an atom has 49 protons, how many electrons does it have? How does the nuclear atom differ from the uniform sphere model of the atom? In Rutherford’s experiment, why wouldn’t the electrons in the atoms of the gold foil affect the paths of the alpha particles? The mass of an electron is 9.11 x 10-28 g. The proton is 1836 times heavier. What is the mass of the proton in grams and in kilograms?