Regents Chemistry

1. Regents Chemistry • Lecture Notes for Topic I The Atom

2. Elements • Elements are an extremely important part of our daily lives • Each element is different and has different properties and behaviors • Since ancient times, humans have used chemical changes to their advantage • Example: processing of ores to produce metals for ornaments and tools

3. Elements and Changes – early exp. • Greeks were the first to try to explain why chemical changes occur. • By 400 B.C., they had proposed that all matter was composed of four fundamental substances: fire, earth, water and air • The next 2000 years…chemical history was characterized by alchemy

4. Elements – early pioneers • Robert Boyle (1627 – 1691) – the first scientist to recognize the importance of careful measurements. • Defined the term element in terms of experimentation; • a substance was an element unless it could be broken down into two or more simpler substances

5. Elements – How many • Presently there are 110 different elements, of which 88 occur naturally (the rest have been made in laboratories) • The elements vary tremendously in abundance • For example, only 9 elements account for most of the compounds found in the Earth’s crust See pg. 87 tables 4.1 and 4.2

6. Elements • The term element can used in different ways • For example, element can mean a single atom of an element (microscopic view) • Element can also mean a sample large enough to mass on a balance; this sample contains many atoms (macroscopic view)

7. Elements - symbols • The names of chemical elements have many sources; often from Greek, Latin or German languages • Example; the elemental name bromine comes from a Greek word meaning “stench” • We use abbreviations (symbols) in place of the words

8. Your Assignment… • Read trace elements on pg. 88 and answer the following questions in complete sentences: • Name and describe (4) trace elements and their impacts on our bodies • What is Lithium used for? How does it affect our bodies? • What did William Walsh discover? How? • Are aluminum and fluoride good for us? Why or why not?

9. Regent Chemistry • Evolution of atomic structure

10. Early Atomic Theory of Matter Pre-Dalton – the atom is mostly empty space and void : The Greeks! Said matter was made up of particles from four elemental substances : Earth, water, air and fire Dalton’s Postulates (proposed 1808) 1. All elements are composed of indivisible atoms 2. All atoms of a given element are identical

11. Early Atomic Theory of Matter Dalton’s Postulates 3. Atoms of different atoms are different; that is, they have different masses 4. Compounds are formed by the combination of atoms of different elements. Compounds are formed in definite proportions!

12. What Dalton’s Law explains... • Law of constant composition • relatives numbers/kinds of atom are constant • Law of conservation of mass • total mass before = total mass after • Law of multiple proportions (video) • For example...

13. Same elements and mass before and after reaction

14. In the late 1890’s • J.J. Thomson – cathode ray tube with magnetic fields • Observed nature of rays is the same regardless of identity of cathode material • 1897 – concluded that cathode rays are not rays but are particles with mass • Generally accepted as the discovery of the electron -

15. High voltage produces radiation within the tube know as cathode rays • Movement in tube can be seen as cathode rays • cause certain materials to fluoresce • Can be deflected by magnetic fields J.J

16. Shortly after...William Thomson aka Lord Kelvin Concluded atom is a large positive sphere with electrons floating around like raisons in pudding

17. Ernest Rutherford..the famous exp. • Disproved Thomson’s Plum Pudding Model • How?? • Alpha and Beta particle scattering experiment (video) • The famous gold foil experiment (video) Coming up next..modern atomic structure

18. Your Assignment… • Write an essay using the information from this lecture and pg 90-96 in your textbook. Your essay should be approximately 500 words in length and compare/contrast the contributions of JJ Thomson, Lord Kelvin and Ernest Rutherford

19. Regents Chemistry • Modern Atomic Theory

20. Modern Atomic Theory The nucleus contains A positive charge.. Nucleus Electrons surround the nucleus in a “cloud” electrons

21. Modern Atomic Theory • Modern atomic theory is based on two models • The Bohr Model (planetary model) • The Wave Mechanical Model

22. The Bohr Model • Neils Bohr proposed a model of the atom with a dense, positive charge in the center and electrons surrounding the nucleus in specific orbits.. Ask me!

23. Location of electrons – energy levels • Electrons are in specific levels according to the amount of energy they contain • There are maximum numbers of electrons for each level • Electrons can jump from one level to another by absorbing or emitting photons of energy..

24. Location of electrons • We can identify an elements electrons distribution by looking at the periodic table • These number describe how many electrons are in each level

25. Jumping of Electrons • Check this out…what happens when we excite the electrons of specific metals? • Why do we see a color? • Why do different metals show different colors? • Can we get any other info from this demo?

26. Jumping levels • Electrons in a non excited state are said to be in the ground state • We just observed how to excite these electrons.. • But how does this change an elements electrons distribution • Check out your PT…

27. Spectral Lines • When electrons are emitted or absorbed from specific elements, we see the result in two ways.. • Emission Spectra • Absorption Spectra

28. Emission and Absorption Spectra

29. The Wave – Mechanical Model • Most current model of electron behavior • Proposes electrons are in orbitals, which are regions of high probability of finding an electrons • Also called the electron cloud model • Why this model?…because of wave behavior of electrons

30. Modern Atomic Theory • Classwork… • Read the attached article regarding protons, neutrons and electrons. Answer the questions on the handout using this information.

31. Regents Chemistry • The Structure of the Atom

32. The Nucleus • A dense centered nucleus contains protons and neutronsProtons are (+) • Neutrons have no charge • These have a mass of 1 amu and make up most of the mass of an atom • This mass is called an elements atomic mass

33. Subatomic Particles • We measure the mass of protons and neutrons in atomic mass units (amu) • The atomic mass unit is based on 1/12 the mass of the C – 12 isotope • The protons define an element • This is called an elements Atomic Number

34. Isotopes • WE CAN NEVER CHANGE AN ELEMENTS ATOMIC NUMBER (# p) • We does vary is an elements number of neutrons • Atoms of the same element that have different numbers of neutrons are called Isotopes C – 14 and C – 12 are isotopes

35. Isotopes cont… • An Isotope is an atom that looses or gains neutrons. It is thus lighter or heavier than the base atom of the element.

36. Average Atomic Mass • The atomic mass listed on the Periodic table is actually the average of all known atomic masses for that element • How do we do this…easy!

37. Atomic Weights and Isotopes Multiply the exact weight x % abundance for Each isotope and then add them up!

38. Regents Chemistry • Types of Matter

39. What is Matter? • Matter is the “stuff” of which the universe is composed • Anything that has mass and occupies space is considered matter!

40. Mixtures and Pure Substances • A mixture is something that has variable composition. • Example: soil, cereal, air • A pure substance will always have the same composition. Pure substances are elements or compounds. • Example: pure water, table salt, carbon

41. Mixtures • For Example: Elements, which are pure substances AIR Mixture of oxygen nitrogen, carbon dioxide Argon, water, others Compounds, which are pure substances

42. Elements and Compounds Pure substances have an invariable composition and are composed of either elements or compounds. Elements "Substances which cannot be decomposed into simpler substances by chemical means". Compounds Can be decomposed into two or more elements.

43. ElementsElements are the basic substances out of which all matter is composed. Everything in the world is made up from only 110 different elements. 90% of the human body is composed of only three elements: Oxygen, Carbon and Hydrogen Elements are known by common names as well as by their abbreviations (symbols).

44. CompoundsCompounds are substances of two or more elements united chemically in definite proportions by mass. The observation that the elemental composition of a pure compound is always the same is known as the law of constant composition (or the law of definite proportions).For Example...

45. Good Old H2O For example, pure water is composed of the elements hydrogen (H) and oxygen (O) at the defined ratio of 11 % hydrogen and 89 % oxygen by mass. Worksheet

46. Classification of Mixtures • Homogeneous Mixtures – are the same throughout (a single phase). ex: table salt and water, air, brass • Heterogeneous Mixtures – contain regions that have different properties from those of other regions (more than 1 phase). ex: sand in water, cereal Phase - area of uniform composition

47. Examples of Heterogeneous Mixtures • Sand on a beach • Cereal • sand in water • sugar in lemonade • Most of the time you can see the different substances, hence the mixtures are said to be not well mixed and can be separated physically

48. Examples of Homogeneous Mixtures, also called Solutions • Air • Table salt in water • Solution of Na2SO4 • You cannot see the different substances in the mixture (solution) - can be separated by chemical or physical means

49. Means of Separation • Use a magnet - physical • Ex: Iron and sand mixture • Filtration - physical • Ex: sand and water mixture • Distillation - physical • Ex: salt and water mixture • see distillation apparatus • Electrolysis - chemical • Ex: water - see video - worksheet