Atoms are the smallest particles of elements that retain their chemical properties. Atoms consist of protons, neutrons, and electrons. John Dalton proposed his atomic theory in 1808, stating that all matter is made of indivisible atoms, atoms of the same element are identical, and atoms combine in simple whole number ratios. In the early 1900s, experiments showed that atoms have a small, dense nucleus surrounded by electrons, and electrons can exist in certain energy levels or orbits around the nucleus.

1. Atoms and Elements Chemical Foundations

2. Atoms and Elements Element - Substances that can not be broken down further by ordinary chemical means Atoms - The smallest particle of an element that retains the properties of the element

3. Postulates Elements consist of tiny particles called atoms Atoms retain their identity in reactions They do not change into other types of atoms Carbon stays carbon, Oxygen stays oxygen In a compound, atoms of two or more elements are combined

4. Atoms have parts Atoms are composed of protons, neutrons and electrons All of the parts of the atom stay together as that atom in chemical reactions

5. Elements Elements represent the different types of atoms that are known Come in solid, liquid and gas forms at earths normal temperatures

6. Top Ten

7. Most Abundant Elements on Earth

8. Most Common Elements

9. A collection of argon atoms.

10. Diatomic Elements

11. Nitrogen gas contains N 2 molecules.

12. Oxygen gas contains O 2 molecules.

13. Representation of NO, NO 2 , and N 2 O.

14. History of the discovery of the atom 300 BC in Greece School of thought that matter is made up of tiny indivisible, invisible, indestructable, fundamental units of matter called atmos Democritus of Abdera most well known atmostist. Did no experiments No proof philosopher

15. 1770’s Antoine Lavosier made observations and did experiments resulting in the Law of Conservation of Mass Law of Conservation of Mass Matter can not be created or destroyed Mass of materials before the chemical reaction is the same mass after the reaction

16. 1799 - Proust Joseph Proust showed that the proportion by mass of the elements in a compound is always the same Now called the Law of Definite Proportions Part of our definition of a chemical - any substance with a definite composition

17. John Dalton English School Teacher Alchemists were trying to change different metals into gold Wanted to learn in what ratios that different elements combine Performed experiments Formulated theories to explain his observations Proposed his theories in 1808 Based on the Greek idea of the atom

18. Key idea - Dalton’s Atomic Theory All matter is made of indivisible, invisible, and indestructible particles called atoms Atoms of the same element are identical Atoms of different elements differ in their physical and chemical properties

19. Dalton’s Atomic Theory Con’t Atoms of different elements combine in simple whole number ratios to form compounds Chemical Reactions occur when atoms are separated, joined or rearranged. Atoms of one element are NEVER changed into atoms of another element as a result of a reaction

20. 1870’s William Crooks developed the Crook’s Tube A tube with sealed gases and metal plates at the end Run a current through the metal, and a glowing beam forms

21. Schematic of a cathode ray tube.

22. 1897 - Sir J.J.Thompson Thompson proposed that the cathode ray formed in the tube is a stream of negatively charged particles Key idea - ELECTRONS Showed that all cathode rays are electrons regardless of the type of gas in the tube Atoms have parts that are negatively charged Since atoms are neutral, they must also have positively charged areas Gave rise to the plum pudding model of the atom

23. Plum Pudding model of an atom.

24. 1886 - Goldstein E. Goldstein discovered a positively charged particle. Called them PROTONS Work was largely ignored

25. 1909 - Ernst Rutherford Rutherford physicist who experimented with the positively charged particle He figured Atoms have no charge Electrical charges are properties of matter Electrical charges exist in single whole number units + or - (no fractions) Electrical Charges cancel each other, therefore a + and a - together = neutral atom

26. Rutherford’s experiment.

27. Results of foil experiment if Plum Pudding model had been correct.

28. Actual Results.

29. Rutherford’s model Key Idea -NUCLEAR ATOM All of the mass of the positive particles (protons) is at the center region of the atom Center region called the NUCLEUS Electrons surround the nucleus in a “cloud”

30. 1913 - Niels Bohr Danish Physicist Rutherford’s model did not work because the electrons are so small compared to the positive mass in the center of the atom

31. Bohr’s Atom There are certain energy levels in the space of the electron cloud that mathematically allow the electron to stay in the cloud (not crash into the nucleus) Key idea - There are certain ORBITS in which an electron can travel around the nucleus without losing energy

32. Bohr’s atom con’t Suggested circular orbits The farther away from the nucleus, the higher the energy level Under normal conditions, electrons are arranged in the GROUND STATE. In the ground state, the electrons occupy the lowest energy level closest to the nucleus

33. All models have problems All models are not exactly correct, but they lead to further science and discovery All have important key ideas

34. We now know… Atoms are made up of subparticles Atoms are divisible, but not by ordinary chemical means Atoms of elements can vary (isotopes) Electrons reside in ORBITALS, not orbits