Atomic Theory

1. 1803 1913 1926 Nucleus 2s orbital Atomic Theory 1904 2p orbitals 1s orbital 3s orbital 1911

2. Ancient Greek Theories of Matter Around 400 BC, the respected philosopher of the time, Aristotle, proposed that all matter was composed of 4 elements: earth, fire, air and water. This theory was NOT based on scientific evidence.

3. Ancient Greek Theories of Matter In contrast, a less popular Greek philosopher, Democritus, thought that matter was made of invisible atoms. He believed that atoms could not be divided, and were different sizes and shapes, depending on the material. Without scientific research to test the theories, this theory was ignored for approximately 2000 years.

4. Dalton’s Theory (The Billiard Ball) Circa 1800 • All elementsare composed of atoms. Atoms are indivisible and indestructible particles. • Atoms of the same element are exactly alike. • Atoms of different elements are different. • Compoundsare formed by the joining of atoms of two or more elements.

5. Thomson Model Thomson studied the passage of an electric current through a gas in a cathode ray tube. As the current passed through the gas, it gave off rays of negatively charged particles. From this important experiment, Thomson discovered the existence of the electron. Cathode ray tube

6. Thomson Model He proposed a model of the atom that is sometimes called the Plum Puddingmodel. Thomson thought atoms were made from a positively charged substancewithnegatively charged electrons scattered about, like raisins in a pudding.

7. Rutherford Model Rutherford designed a famous experiment called the gold foil experiment. His experiment revealed the existence of the positively charged nucleusof the atom.

8. Rutherford Gold Foil Experiment

9. Rutherford Model Based on the experimental results, Rutherford proposed that an atom contains a small, positively charged nucleus in the center which is surrounded by negatively charged electrons scattered randomly. +

10. Bohr Model According to the Bohr model, electrons move around the nucleus in definite orbits, like the planets orbit the sun. Each orbit has a different energy level and a fixed distance from the nucleus.

11. Bohr Model Bohr model of the Li atom with 3 electrons. The electrons orbit around the nucleus. The electrons are NOT randomly arranged.

12. Electron Cloud Model of the Atom Shows how probable it is for an electron to be found in a given location of the atom.

13. Quantum Mechanical Model • Most accurate model • Best explains what is observed through experiments

14. Important Contributions to Development of Atomic Structure Democritus: proposed the existence of atoms circa400 BC Dalton: beginning of modern atomic theory; Dalton’s work supported the Law of Conservation of Mass 1803 Thomson: discovered the electron, proving that atoms have subatomic particles 1897 Max Planck: used the idea of quanta (discrete units of energy) to explain hot glowing matter circa1900 Millikan: oil drop experiment determined the charge of an electron and allowed calculation of the mass of an electron circa1910 Rutherford: gold foil experiment determined that atoms contain a dense, positively charged nucleus in the center circa1915 Neils Bohr: theory of atomic structure relating electron arrangement in atoms to chemical properties and regularities of the periodic table; Bohr model suggests that electrons are found in successive orbitals around the nucleus 1913-1922 Chadwick: discovered the neutron, a neutral atomic particle with a mass close to a proton 1932

15. Timeline of Atomic Theory

16. The Atom • The smallest particle of an element that maintains the properties of that element • Composed of subatomic particles: protons, neutrons, and electrons Protons have a +1 charge Electrons have a -1 charge Neutrons are neutral no charge

17. Structure of the Atom The basic structure of an atom is represented as:

18. Structure of The Atom • Spherical and electrically neutral • Contain a positively charged, dense nucleus at the center composed of protons and neutrons. The nucleus is very small (less than 0.0001 times the size of the whole atom) but contains 99.9% of the mass. • Contain a negatively charged electron cloud which accounts for most of the volume of the atom. Most of the electron cloud is empty space, but the electrons move rapidly through this empty space at high speeds. • The electrons are drawn to the positively charged nucleus.

19. Relative Masses and Charges

20. The Atom • Identityof an atom is determined by the number of protons in the atom • Periodic table arranges elements based on the atomic number (Z) • Atomic number equals the number of protons in the atom

21. The Atom Atoms are electrically neutral (zero charge). proton = +1, electron = -1 (equal but opposite) atomic number = number of protons = number of electrons

22. Learning Check Complete the following:

23. The Atom How many neutrons are in an atom? We can calculate the number of neutrons as follows: mass number – number of protons = number of neutrons OR mass number– atomic number = number of neutrons

24. The Atom Mass number is always a whole number. Sometimes it is given in a problem. We can estimate the mass number by rounding the atomic mass to a whole number. We can calculate the mass number from the number of protons and neutrons: mass number = number of protons + number of neutrons

25. Isotopes of Atoms Atomic number (Z) = number of protons. Atoms are electrically neutral, so number of protons = number of electrons. Are all atoms of an element identical? No!

26. Isotopes of Atoms The number of neutrons in an atom of an element can vary. Atoms of a given element having different numbers of neutrons are known as isotopes. Isotopes are written as OR X-mass number C-12

27. Isotope An atom of an element with a different number of neutrons. Carbon has 3 isotopes: C-12, C-13, C-14 Protons Neutrons

28. Isotopes: Summary • Isotopes have different mass numbers due to their different numbers of neutrons • Isotopes have the same chemical behavior,since chemical behavior is determined by the electrons of the atom

29. Learning Check What is the atomic number of these isotopes? How many neutrons and protons does each have? Write the isotope symbols for Copper with a mass number of 65.

30. Isotopes Isotopes in nature have different percent abundances For instance, Cl-35 tends to be found with 75.78% abundance and Cl-37 has a percent abundance of 24.22 %. This means that ¾ of the Cl in nature is Cl-35. Since most of the Cl found in nature is Cl-35, the atomic mass of Cl is closer to 35 than to 37. Atomic mass of Cl = 35.45 g/mole

31. Isotopes Using the percent abundance of the isotopes, we can calculate the weighted average atomic mass of the element (this is why atomic mass is usually not a whole number!)

32. Isotopes Calculate the average atomic mass of H using the % abundances of its naturally occurring isotopes. (0.999885)(1.007825) = 1.007709 mass contribution of 1H (0.000115)(2.014102) = 0.00023162 mass contribution of 2H 1.007709 + 0.00023162 = 1.007941average atomic mass of H

33. Learning Check Calculate the weighted average atomic mass of the element with the given data. Predict: Will the actual weighted average atomic mass be closer to 6 or to 7?

34. Learning Check Calculate the contribution of each isotope: (6.015 amu)(0.0759) = 0.4565 amu (7.016 amu)(0.9241) = 6.483 amu Atomic mass = 6.483 amu + 0.4565 amu = 6.940 amu pay attention to significant figures! Identity of the atom is Li

35. Nuclear Changes In a chemical reaction,the nucleus NEVER changes. In a nuclear reaction, the nucleus DOES change… and the ELEMENT’S identity

36. Nuclear Decay 3 forms of nuclear decay: a decay b decay g decay emission of high energy photon (g ray)

37. Radioactive Nuclei are Unstable Why does this happen? Nuclei of radioactive elements are very unstable. Through radioactive decay, radioactive elements become more stable. LOTS of energy is given off

38. Nuclear Reactions and the Atom • Electrons are involved in chemical reactions. Gaining or losing electrons does not change the element. • The nucleus does not change in a chemical reaction. • The nucleus only changes in a nuclear reaction. When the nucleus changes, the element’s identity is usually changed.