2. Developing a Model of an Atom: Ernest Rutherford  In the early 1900s, Ernest Rutherford designed an experiment to test the Thomson model.

3. Developing a Model of an Atom: Ernest Rutherford  In the early 1900s, Ernest Rutherford designed an experiment to test the Thomson model.  To test the model, Rutherford aimed a beam of positively charged particles at a thin sheet of gold foil.

4. Developing a Model of an Atom: Ernest Rutherford  In the early 1900s, Ernest Rutherford designed an experiment to test the Thomson model.  To test the model, Rutherford aimed a beam of positively charged particles at a thin sheet of gold foil.  Rutherford placed a detector around the gold foil so that he could tell where a particle went after it passed through the gold foil.

5. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that if the Thomson model accurately described atoms, then all of the particles should pass straight through the foil.

6. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that if the Thomson model accurately described atoms, then all of the particles should pass straight through the foil.  However, he observed something unexpended.

7. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that if the Thomson model accurately described atoms, then all of the particles should pass straight through the foil.  However, he observed something unexpended.  Most of the particles did pass through the foil, just as predicted.

8. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that if the Thomson model accurately described atoms, then all of the particles should pass straight through the foil.  However, he observed something unexpended.  Most of the particles did pass through the foil, just as predicted.  But he was very surprised to observe that many particles were deflected, some of them even bouncing straight back at the source.

9. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that these particles struck compact particles in the gold foil that also had a positive charge.

10. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that these particles struck compact particles in the gold foil that also had a positive charge.  There fore, Rutherford concluded that not only was the mass of an atom found in its nucleus, but also that the nucleus contained positively charged particles.

11. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that these particles struck compact particles in the gold foil that also had a positive charge.  There fore, Rutherford concluded that not only was the mass of an atom found in its nucleus, but also that the nucleus contained positively charged particles.  Rutherford called these positive charged protons, after a Greek word meaning “first.”

12. Developing a Model of an Atom: Ernest Rutherford  Rutherford reasoned that these particles struck compact particles in the gold foil that also had a positive charge.  There fore, Rutherford concluded that not only was the mass of an atom found in its nucleus, but also that the nucleus contained positively charged particles.  Rutherford called these positive charged protons, after a Greek word meaning “first.”  A proton is a positively charged particle that is found in the nucleus of an atom.

13. Developing a Model of an Atom: Ernest Rutherford  Rutherford’s observations suggested that a new atomic model was needed.

14. Developing a Model of an Atom: Ernest Rutherford  Rutherford’s observations suggested that a new atomic model was needed.  In Rutherford’s model, almost all of the mass of the atom is contained in a tiny, positively charged region in the center of the atom, called the nucleus.

15. Developing a Model of an Atom: Ernest Rutherford  Rutherford’s observations suggested that a new atomic model was needed.  In Rutherford’s model, almost all of the mass of the atom is contained in a tiny, positively charged region in the center of the atom, called the nucleus.  According to Rutherford, the elements move in the vast space that makes up most of an atom’s size.

16. Developing a Model of an Atom: Ernest Rutherford  Rutherford’s observations suggested that a new atomic model was needed.  In Rutherford’s model, almost all of the mass of the atom is contained in a tiny, positively charged region in the center of the atom, called the nucleus.  According to Rutherford, the elements move in the vast space that makes up most of an atom’s size.  In fact, the nucleus is so small that if an atom were the size of an athletic stadium, then its nucleus would be only the size of a pea.

17. Developing a Model of an Atom: James Chadwick  A third subatomic particle was discovered a few years later by James Chadwick.

18. Developing a Model of an Atom: James Chadwick  A third subatomic particle was discovered a few years later by James Chadwick.  Chadwick’s experiments demonstrated that the nucleus had more mass than the of protons suggested that there should be.

19. Developing a Model of an Atom: James Chadwick  A third subatomic particle was discovered a few years later by James Chadwick.  Chadwick’s experiments demonstrated that the nucleus had more mass than the of protons suggested that there should be.  Chadwick made another observation – while the mass was greater than expected, the charge of the atom did not change.

20. Developing a Model of an Atom: James Chadwick  A third subatomic particle was discovered a few years later by James Chadwick.  Chadwick’s experiments demonstrated that the nucleus had more mass than the of protons suggested that there should be.  Chadwick made another observation – while the mass was greater than expected, the charge of the atom did not change.  This suggested to Chadwick that there was another subatomic particle in the nucleus.

21. Developing a Model of an Atom: James Chadwick  Chadwick described this particle as the same mass as proton but having no electrical charge.

22. Developing a Model of an Atom: James Chadwick  Chadwick described this particle as the same mass as proton but having no electrical charge.  He called the particle a neutron.

23. Developing a Model of an Atom: James Chadwick  Chadwick described this particle as the same mass as proton but having no electrical charge.  He called the particle a neutron.  Because each proton or neutron has a mass that is about 2000 times as large as the mass of an electron, almost all of the mass of the atoms is located in the nucleus.

24. Developing a Model of an Atom: James Chadwick  Chadwick described this particle as the same mass as proton but having no electrical charge.  He called the particle a neutron.  Because each proton or neutron has a mass that is about 2000 times as large as the mass of an electron, almost all of the mass of the atoms is located in the nucleus.  Although many other subatomic particles have been discovered, electrons, protons, and neutrons are the basic structures that make up the modern model of the atom.

25. Developing a Model of the Atom: Niels Bohr  Rutherford’s model placed the electrons in the area around the nucleus but did not describe any specific locations.

26. Developing a Model of the Atom: Niels Bohr  Rutherford’s model placed the electrons in the area around the nucleus but did not describe any specific locations.  As scientists made more observations about atoms and elements, they were able to add more information to the model.

27. Developing a Model of the Atom: Niels Bohr  Rutherford’s model placed the electrons in the area around the nucleus but did not describe any specific locations.  As scientists made more observations about atoms and elements, they were able to add more information to the model.  One important observation was that atoms absorb and release energy.

28. Developing a Model of the Atom: Niels Bohr  Rutherford’s model placed the electrons in the area around the nucleus but did not describe any specific locations.  As scientists made more observations about atoms and elements, they were able to add more information to the model.  One important observation was that atoms absorb and release energy.  Niels Bohr used this observation to develop the revised atomic mode.

29. Developing a Model of the Atom: Niels Bohr  Niels Bohr used this observation to develop the revised atomic model.

30. Developing a Model of the Atom: Niels Bohr  Niels Bohr used this observation to develop the revised atomic model.  According to the Bohr atomic model, also known as the planetary model, electrons move around the nucleus in specific orbits just as the planets move around the Sun in specific orbits.

31. Developing a Model of the Atom: Niels Bohr  Niels Bohr used this observation to develop the revised atomic model.  According to the Bohr atomic model, also known as the planetary model, electrons move around the nucleus in specific orbits just as the planets move around the Sun in specific orbits.  These orbits are called energy levels.

32. Developing a Model of the Atom: Niels Bohr  Niels Bohr used this observation to develop the revised atomic model.  According to the Bohr atomic model, also known as the planetary model, electrons move around the nucleus in specific orbits just as the planets move around the Sun in specific orbits.  These orbits are called energy levels.  Electrons can move from one energy level to another but only if energy is involved.

33. Developing a Model of the Atom: Niels Bohr  Niels Bohr used this observation to develop the reviesed atomic model.  According to the Bohr atomic model, also known as the planetary model, electrons move around the nucleus in specific orbits just as the planets move around the Sun in specific orbits.  These orbits are called energy levels.  Electrons can move from one energy level to another but only if energy is involved.  An electron moving form one orbit to another absorbs or releases energy.