1. All atoms share the same basic structure. Scientists have proposed many different models for the atom. Dalton’s Model. Early 1800s. Performed experiments with gases and his results convinced him that … M ODELS OF THE A TOM all matter is composed of tiny, invisible particles called atoms. atoms of each element are exactly alike. atoms of different elements have different masses. atoms of different elements can join to form compounds and the way they combine is always the same. pictured atoms as small, solid spheres. “Bouncy Ball”

2. Thompson’s Model. End of 1800s. Discovered that atoms were made of even smaller subatomic particles that were very small and negatively charged, called electrons. M ODELS OF THE A TOM Uncovered the electron while experimenting with metal electrodes in a vacuum. When the tube was connected to a high voltage electric source, glowing green rays appeared. In the presence of a charge the rays bent indicating they had to be made of charged particles. He knew atoms were electrically neutral, so he reasoned that there must be enough positive charge in the atom to balance the negative charge of the electrons. electrons are stuck into a positively charged sphere, like chocolate chips in cookie dough or plum pudding. “Chocolate Chip Cookie” electron Positive sphere

3. Rutherford’s Model. Early 1900s. Scientists knew that the positive charge of an atom was a subatomic particle called a proton (1.674x10 -27 kg), which had a much greater mass than the electron (9.11x10 -31 kg). He aimed a beam of positively charged particles at a sheet of gold foil and some of the particles bounced back. M ODELS OF THE A TOM protons are concentrated in a small area at the center of the atom, called the nucleus. an atom is almost all empty space. electrons are scattered about the nucleus. like a peach with its pit. “Peach Pit” nucleus

4. Bohr’s Model. 1913. Added to Rutherford’s model saying that each electron in an atom has a fixed amount of energy, which keeps it moving at a constant speed about the nucleus. M ODELS OF THE A TOM electrons travel in energy levels, or shells, that circle the nucleus. electron shells are like the layers of an onion or the planetary orbits around the sun. by absorbing or releasing a certain amount of energy, an electron can change energy levels. “Onion” electrons nucleus

5. Electron Cloud Model. The orbiting nature of electrons has been replaced by the concept of the electron cloud where the electrons dart about within an energy level in an ever-changing path M ODELS OF THE A TOM The fast moving electrons seem to fill the space around the nucleus just as the blades of a fan seem to fill the space around its axis. NOTE: We will study the atom using the Bohr model (aka the onion model) for simplicity sake. Various Electron Path Shapes called Orbitals

6. Inside the nucleus 1932, James Chadwick discovered the third subatomic particle, the neutron (1.675x10 -27 kg) which has about the same mass as the proton and has no charge. The neutron is located in the nucleus with the proton. M ODELS OF THE A TOM

7. L ET ’ S P RACTICE Let’s learn more about the atom from our good friend Bill Nye! How is the atom like a tiny solar system? How is the atom NOT like a tiny solar system?