Introduction To Atoms Section 1: Development of The Atomic Theory

Atomic Theory Videos nDxFdkzZs nDxFdkzZs bWKF9uDF7w bWKF9uDF7w bWKF9uDF7w bWKF9uDF7w Cool Visual: story-of-the-atomic-model--36 Cool Visual: story-of-the-atomic-model story-of-the-atomic-model story-of-the-atomic-model--36

Atoms… …Are the smallest particles into which an element can be divided and will remain the same substance …Make up elements (which combine to form compounds) …often called the building blocks of matter

Atoms… …Were not seen prior to 1981 when the scanning tunneling microscope was invented …comes from the Greek word atomos, meaning indivisible However… The atomic theory has been around for more than 2000 years!

Refresh your memory… What’s a theory? A theory is an explanation for a broad range of hypotheses and observations that have been supported by testing

Let’s Tour The History of The Atom First Stop…Greece

Democritus proposed (around 440 BC)… …that if a person continued to cut a coin in half over and over again, they would eventually end up with an uncuttable piece …the name for the atom

Aristotle disagrees. He thought (around mid 300 BC, after Democritus)… …particles could be divided indefinitely It’s All Greek To Me!

So Who Thought Up The Atomic Theory? Let’s move to Britain!

…and about 2000 years have passed. …in the 1700’s, scientists have learned that elements combine in specific proportions based on mass What are combined elements with a constant mass ration called? (Hint: Ch 4) (Hint: Ch 4) Compounds!

Dalton’s Theory Stated… All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed All substances are made of atoms. Atoms are small particles that cannot be created, divided, or destroyed Atoms of the same element are exactly alike, and atoms of different elements are different Atoms of the same element are exactly alike, and atoms of different elements are different Atoms join with other atoms to make new substances Atoms join with other atoms to make new substances

So, is the atom really the smallest particle ? We thought so until 1897 (yes, just 110 years ago!) Another British scientist, J.J.Thomson, discovered that there were small particles inside the atom, therefore, the atom could be divided into smaller parts!

Thompson experimented with the cathode ray tube and discovered…

Thompson concluded, And he is credited with the discovery of the electron! He is known for the “plum pudding” or “chocolate chip cookie” model of the atom.

A Few Years Later… A student of Thompson’s named Ernest Rutherford decided to check out Thompson’s theory himself so he set up an atomic shooting gallery A student of Thompson’s named Ernest Rutherford decided to check out Thompson’s theory himself so he set up an atomic shooting gallery

Meet Rutherford

Bye-Bye Plum Pudding! Rutherford came up with his own model of the atom Rutherford came up with his own model of the atom He is credited with the discovery of the proton and nucleus He is credited with the discovery of the proton and nucleus Rutherford's model came to be known as the “solar system” model Rutherford's model came to be known as the “solar system” model

Jumping Electrons! Niels Bohr (who worked with Rutherford) suggested that electrons travel around the nucleus in definite paths

Jumping Electrons! Bohr also proposed that the electrons could jump between paths

Electron Clouds and Current Theory The exact movement of an electron cannot be predicted but, according to current theory, there are regions of the atom where the electrons are more likely to be found. These are called electron clouds. The exact movement of an electron cannot be predicted but, according to current theory, there are regions of the atom where the electrons are more likely to be found. These are called electron clouds.

Chadwick…1932 Credited with the discovery of the neutron which is located in the nucleus with the protons Credited with the discovery of the neutron which is located in the nucleus with the protons

The Atom Through Time…

History of the periodic table… Mendeleev – created the first periodic table, ordered by increasing atomic mass Mendeleev – created the first periodic table, ordered by increasing atomic mass Mosley – rearranged the periodic table in order of increasing atomic #, this is the way the current table is arranged Mosley – rearranged the periodic table in order of increasing atomic #, this is the way the current table is arranged

Let’s summarize In what two parts of an atom is most of its mass located? Most of the mass of an atom is located in its nucleus

Let’s summarize What are the two differences between the atomic theory described by Thompson and described by Rutherford? The differences in the theories are that Thompson’s model had the negatively charged particles in the positive material but Rutherford’s model had them moving around the positive material. Thompson’s model does not have a nucleus in the atom but Rutherford’s does.

Let’s summarize Identify the difference in how Bohr’s theory and the modern theory describe the location of electrons. Bohr’s theory was that electrons move in definite paths around the nucleus. The modern theory states that the path of an electron cannot be known. Only the areas of the atom where electrons are likely to be found can be described.

Draw an atom: Now that we know a little more about the atom lets draw one. Now that we know a little more about the atom lets draw one. –First, close your notebook and try from memory! –Draw one atom you remember from today and tape it to the correct cabinet around the room