Chapter 3

1. Chapter 3 The Atom!

2. What’s the big deal about the elements? Ancient Greeks through all matter was composed of 4 fundamental substances: Fire Earth Water Air This idea sadly was not supported by experimental evidence…

3. What’s the big deal about the elements? • Robert Boyle a Irish scientist insisted scientific ideas be backed by experimental evidence. • Determined that a substance was an element if it could not be broken down into two or more simpler substances . • Current Definition: • Element can refer to an individual atom, a sample large enough to mass, a molecule of a diatomic gas such as H2, or the presence of a certain amount of a type of atom.

4. Common Elements Found on Earth

5. Symbols for the elements • Written as • A single capitalized letter • Carbon C • Nitrogen N • Oxygen O • Two letters: the first is capitalized the second is not • Calcium Ca • Iron Fe • Chlorine Cl • Three letters: the first is capitalized the second two are not

6. Dalton’s Model of the Atom • Dalton’s Atomic Theory Published in 1808 • Elements are made up of tiny indivisible particles called atoms • All atoms of the same element are identical • Atoms of Different elements are different from each other. Carbon Atoms Oxygen Atoms

7. Dalton’s Model of the Atom • D. Atoms are combined in simple whole number ratios to form compounds (Law of Definite Proportions) • Carbon Monoxide CO • Carbon Dioxide CO2 • E. Atoms are not created or destroyed in chemical reactions (Law of conservation of Mass) 2 C + O2  2 CO

8. J.J. Thomson and William Thomson • J.J. discovers electrons in 1897 using cathode ray experiments • Cathode ray • Attracted to positive charge • Repelled by negative charge • Like charges repel • Unlike charges attract • Negatively charged particles • In cathode rays are electrons • Are a part of all atoms • But atoms also have positive charges

9. - - - - - - - - - - J.J. Thomson and William Thomson D. William and J.J. developed the Plum-Pudding model Atoms are spheres of positive charge Negative charges are evenly distributed throughout the sphere The amount of negative charge equals amount of positive charge. E. Diagram of the model

10.         Ernest Rutherford • Gold Foil Experiment • Fired alpha particles at gold foil • Most passed straight through • 1 in 8,000 bounced back • Discovers the atomic nucleus path of alpha particles  nuclei of gold atoms

11. electron  orbits  nucleus Gold Foil Conclusions • Atoms have a positively charged nucleus • The nucleus is tiny compared to the rest of the atom • Nucleus contains almost all of the mass of the atom • Atom is mostly empty space • Electrons orbit the nucleus

12. James Chadwick -1932 • Beryllium Foil Experiments • Discovers the neutron • Neutron has zero charge • Neutron has nearly the same mass as a proton • Together with the protons, accounts for the mass of the nucleus (# of protons + # of neutrons = atomic mass) • EG. Carbon • Atomic mass = 12amu • Number of protons = 6 • Number of neutrons = 6 • 6+6 = 12

13. Niels Bohr - 1914 • Bohr Atom • Dense positively charged nucleus • Electrons move in specific circular paths • Explains why atoms emit light • Explains hydrogen spectrum • Only works for hydrogen  electron orbits nucleus

14. Modern Model of the Atom! • Basic Structure • Nucleus • Contains protons and neutrons • Positively charged • Extremely small nucleus • Almost all the mass of an atom • Surrounded by electron cloud • Also called Quantum Mechanical Model

15. Modern Model of the Atom! • Basic Structure • Electron Cloud • Contains electrons • Negatively charged • Almost all volume of an atom • Essentially empty space • Exact motion of electron not known • Also called Quantum Mechanical Model

16. Quantum mechanical model electron cloud  contains electrons  nucleus Contains protons and neutrons

17. Identifying the Elements • About 114 elements have been isolated • About 90 elements are found in nature • Remaining elements are “artificial” and made in laboratories • Hydrogen is most abundant (90% of atoms in the universe are believed to be hydrogen) • 67 elements have been detected in the sun

18. Each element is identified by… • Atomic number: • Atomic number equals the number of protons in nucleus • All atoms of the same element have the same number of protons • Atomic Mass is the total number of protons and neutrons in an atom • If you know the atomic number and atomic mass of an atom of any element, you can determine the atom’s composition • The number of neutrons in an atom is the difference between the atomic mass and atomic number

19. Each element is identified by • Name • Many based on Latin, Greek, or German • Chemical Symbol • 1 or 2 letter symbol • 1st letter is capitalized • 2nd letter, if present, is lower case • Often first letter of name • Some based on Latin/Greek names

20. Each element is identified by The periodic table: Group Number gives the column Period Number give the row For Example: 11 -> Atomic Number Na -> Atomic Symbol 22.9898 -> Atomic Mass Sodium -> Name Period 3 Group 1A

21. Periodic Table of the Elements • Elements are arranged • In order of increasing atomic number • In horizontal rows called periods • In vertical columns called groups • Elements in the same period • Differ in chemical properties • Are identified by a period number • Elements in the same group • Share similar chemical properties • Are identified by a group number • Are also called families

22. Periodic table of the elements • 3 Categories of the elements • Metals: Solids, Shiny, Conduct Electricity, Malleable • Metalloids: Solids, Not shiny, can conduct electricity, not malleable - brittle • Nonmetals: Solids, Liquids or Gas, dull, do not conduct electricity, not malleable -brittle

23. The periodic table of elements • Groups with special names • Group 1A: Hydrogen, Alkali Metals • Group 2A: Alkaline Earth Metals • Group 7A: Halogens • Group8A: Noble Gases

24. Atoms • The smallest particle of an element that retains its identity in a chemical reaction • The smallest possible individual particle of an element that still has the properties unique to that element.

25. Atoms • Protons – particles in the nucleus, one unit of positive charge • Neutrons – neutral particles in the nucleus • Electrons – particles outside the nucleus, one unit of negative charge

26. Complete the following chart Lithium 3 3 3 6 6 24 12 Mg 12 16 8 8 17 Chlorine 17 18

27. Atoms… • Element • Symbol rAtoms of the same element have the same number of protons have the same number of electrons have an overall charge of zero number of protons equals number of electrons may have different numbers of neutrons

28. UHH OH… So what does that mean? • Isotopes atoms with the same number of protons, but different numbers of neutrons in the nucleus

29. Each Isotope is identified by: • Each isotope is identified by: chemical symbol, X = Symbol for element mass number, A = Number of protons + neutrons atomic number, Z = Number of Protons  isotope name = Element name - mass number

30. Lets try one together Lithium- 7 Mass number: 7 Atomic Number: 3 Number of protons: 3 Number of electrons: 3 Number of neutrons: 4 Lithium- 6 Mass number: 6 Atomic Number: 3 Number of Protons: 3 Number of Electrons: 3 Number of Neutrons: 3