2. The Atom The History of Atomic Models

3.  Think with me…about sugar crystals, you can see that they are small crystals and every crystal is identical.  You may grind these particles into a very fine powder, but each tiny piece is still sugar.  If you dissolve the sugar in water, the sugar particles become virtually invisible. From Philosophy to Science

4.  You could even look at the sugar solution under a microscope and you’d still not be able to see the sugar.  However, you know it is still there because you can taste it.  These kind of observations and logic patterns led ancient philosophers to ponder the design of the universe From Philosophy to Science

5.  There were two schools of thought of the composition of the cosmos…  is everything in the universe continuous and infinitely divisible  Or, is there a limit to how small you can get?  Particle theory was not the most popular early opinion, but was supported as early as Democritus in ancient Greece. From Philosophy to Science

6.  Democritus proposed that all the matter is composed of tiny particles called “Atomos”  These “particles” were thought to be indivisible  Aristotle did not accept Democritus’ atom, he was of the “matter is continuous” philosophy  Because of Aristotle’s popularity his theory was adopted as the standard From Philosophy to Science

7.  By the 1700’s nearly all chemists had accepted the modern definition of an element as a particle that is indivisible  It was also understood at that time that elements combine to form compounds that are different in their properties than the elements that composed them  However, these understandings were based on observations not empirical evidence From Philosophy to Science

8.  There was controversy as to whether elements always combine in the same proportion when forming a particular compound.  In the 1790’s, chemistry was revolutionized by a new emphasis on quantitative analysis because of new and improved balances  This new technology led to the discovery of some new scientific understandings From Philosophy to Science

9.  The Law of Conservation of Mass:  Proposed by Antoine Lavoisier  States that mass is neither created nor destroyed during ordinary chemical rxns or physical changes.  Which means the total mass of the reactants must equal the total mass of the products. From Philosophy to Science

10. ++ Carbon, C Oxygen, O Carbon Monoxide, CO Mass x Mass y Mass x + Mass y  ++ Carbon, C Oxygen, O Mass x Mass y Carbon Monoxide, CO Mass x + Mass y

11.  The Law of Definite Proportions:  The fact that a chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or the source of the compound  NaCl is NaCl no matter if it is table salt (small crystals) or rock salt (large crystals) From Philosophy to Science

12.  The Law of Multiple Proportions:  If 2 or more different compounds are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers From Philosophy to Science

13. ++ == CarbonOxygen Carbon Monoxide, 1:1 == Carbon Dioxide, 1:2 ++ CarbonOxygen 11 12

14.  In 1808, John Dalton proposed an explanation for each of the proposed laws  He reasoned that elements were composed of atoms & that only whole #’s of atoms can combine to form compounds  His ideas are now called the Atomic Theory of Matter Atomic Theory

15. 1.All matter is composed of extremely small particles called atoms 2.Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, & other properties ELEMENT 2 ELEMENT 2 ELEMENT 3 ELEMENT 3 ELEMENT 1 ELEMENT 1 ELEMENT 4 ELEMENT 4 Atomic Theory

16. 3.Atoms cannot be subdivided, created, or destroyed 4.atoms of different elements combine in simple whole # ratios to form chem compds 5.in chemical rxns, atoms are combined, separated, or rearranged ++ ++ Atomic Theory

17.  Through these statements, evidence could be gathered to confirm or discount its claims  Not all of Dalton’s claims held up to the scrutiny of experimentation  Atoms CAN be divided into even smaller particles  Not every atom of an element has an identical mass Atomic Theory

18.  Dalton’s Atomic Theory of Matter has been modified.  What remains is… 1. All matter is composed of atoms 2. Atoms of any one element differ in properties from atoms of another element  One of the disputed statements of Dalton was that atoms are indivisible Atomic Theory

19.  In the 1800’s it was determined that atoms are actually composed of several basic types of smaller particles  it’s the number and arrangement of these particles that determine the atom’s chemical properties.  The def. of an atom that emerged was, the smallest particle of an element that retains the chemical properties of that original element. Atomic Theory

20.  All atoms consist of 2 regions that contain the subatomic particles  The nucleus  The electron cloud around the nucleus  The nucleus is a very small region located near the center of the atom  In every atom the nucleus contains at least 1 proton, which is positively charged particle and usually contains 1 or more neutral particles called neutrons Atomic Structure

21.  The electron cloud is the region that surrounds the nucleus  This region contains 1 or more elec-trons, which are negatively charged subatomic particles  The volume of the electron cloud is much larger than the nucleus Atomic Structure

22.  The discovery of the first subatomic particle took place in the late 1800’s.  A power source was attached to two metal ends of an evacuated glass tube, called a cathode ray tube.  A beam of “light” appears between the two electrodes called a cathode ray. Discovery of the Electron

23. Cathode Ray Electric Current

25. Steering Coils The electron beam is painting all 525 lines 30 times per sec, it paints a total of 15,750 lines per sec.

26.  Investigators began to study the ray and they observed that… 1. An object placed in the path of the ray cast a shadow on the glass 2. A paddle wheel placed the path of the cathode ray began to spin 3. Cathode rays were deflected by a magnetic field 4. The rays were deflected away from a negatively charged object Discovery of the Electron

27.  The first 2 observations support the idea that the ray is composed of tiny individual particles traveling through the vacuum tube  The second set of observations support the evidence that the ray is composed of a substance that is negatively charged.

28. Discovery of the Electron  J.J. Thomson studied the rays and proved that they were tiny negative particles being emitted from the metal atoms.  Dubbed these tiny particles “electrons”  Robert Millikan then used an ingenious investigation to calculate the mass to charge ratio of an atom  He determined that the electrons were not part of the mass of the atom.

29. First Atomic Model  What can their work help us conclude about the atom?  atoms are composed of smaller particles, and one of these compo-nents is negatively charged  atoms are neutral, so there must be an opposing (+) charge  because E’s are essentially mass-less, an opposing substance that makes up the mass of the atom

30. Negative particles embedded in a sphere of positive plasma-like matter. THINK… Chocolate Chip Cookie Negative particles embedded in a sphere of positive plasma-like matter. THINK… Chocolate Chip Cookie

31. Discovery of the Proton  In 1886, E. Goldstein observed in the cathode-ray tube a new set of rays traveling in the opposite direction than the cathode rays  The new rays were called canal rays and they proved to be positively charged  And the particles mass were about 2000 X’s that of the electron

32. Discovery of the Proton

33. Discovery of the Neutron  In 1932, the English physicist James Chadwick discovered yet another subatomic particle.  the neutron is electrically neutral  It’s mass is nearly equal to the proton  Therefore the subatomic particles are the electron, proton, and neutron.

34. electron e-e-e-e-0 9.11x10 -28 proton p+p+p+p++11 1.67x10 -24 neutron n0n0n0n001 Structure of the Atom

35. Atomic Structure  Scientists still didn’t really understand how the particles were put together in an atom.  This was a difficult question to resolve, given how tiny atoms are.  Most thought it likely that the atom resembled Thomson’s model

36. Rutherford Model  In 1911, Ernest Rutherford et al. provided a more detailed picture of the internal structure of the atom  In his experiment, Rutherford directed a narrow beam of alpha particles at a very thin sheet of gold foil.  Alpha particles (  ) are He atoms that have been stripped of their electrons

37. Rutherford Model  According to Thomson’s model, the heavy, positive alpha particles should pass easily through the gold, with only a slight deflection  And mostly that’s how it happened.  However, they found 1 in every 8000 particles had actually been deflected back toward the source.

40. Rutherford Model  Rutherford suggested a new structural model of the atom.  He stated that all the positive charge and the mass is concentrated in a small core in the center of the atom, AKA nucleus  And that the atom is mostly empty space with electrons surrounding the positively charged nucleus like planets around the sun.

41. Rutherford Model

42.  Rutherford’s planet system model was an improvement over earlier models, but it was still not complete.  Physics says that electrons can’t orbit the nucleus without losing energy,  Losing energy would cause the electron to spiral into the nucleus.  The attraction of the electron to the nucleus would cause it to spiral into the nucleus as well

43. Bohr Model  Niels Bohr proposed a new model that would allow the electrons to be outside the nucleus and in orbit around the nucleus.  His model coupled Rutherford’s model with a new concept of energy in Physics called quantum mechanics,  Bohr proposed that the electrons aren’t on any random orbit around the nucleus, they are on “special” orbits

44. Bohr Model  Bohr’s Model restricts the orbits on which an electron can be  The bases for what orbit an electron is allowed is entirely based on how much energy the electron has  If it has any more energy or any less energy it would be forced to be on a different path of different energy  The energy of the electron is quantized, which means it is of a very specific quantity

45. Bohr Model  Each path or level of energy that the electron is on is given a label of “n”  Such that n=1 is the closest energy level to the nucleus  n=2 is higher in energy and outside of, but adjacent to n=1, and so on…  Each energy level c  an only hold a certain number of electrons (2n 2 )  n=1 can hold 2 electrons  n=2 can only hold 8 electrons  n=3 can hold 18 electrons  Etc.

46. Bohr Model Bohr Model of the Atom Model describes the paths of electrons as energy levels. The electrons are only allowed to have a certain amount of energy which restricts their path around the nucleus.

47. Atomic Structure  With the exception of Hydrogen, every nucleus contains 2 kinds of particles protons and neutrons  they make up the mass of the atom (Mass Number = Protons + Neutrons)  Proton has a charge equal to but opposite of the charge of an elec.  Atoms are neutral because they contain equal #’s of protons & electrons

48. Structure of the Atom  The atoms of different elements differ in the # of protons in their nuclei and therefore in their positive charge  The # of protons the atom contains determines the atom’s identity  Only Oxygen contains 8 protons  Only Fluorine contains 9 protons  Only Neon contains 10 protons

49. Structure of the Atom  The nucleus is composed of a densely packed cluster of protons, which are all electrically positive  Don’t like charges repel?  Why don’t they fly apart?  When 2 protons are in very close proximity, there is a strong force of attraction between them.  similar attraction exists when neutrons are close

50. Structure of the Atom  These short-range p + -n 0, p + -p +, & n 0 - n 0 forces hold the nuclear particles together, A.K.A strong nuclear forces.  When these nuclear forces are strong enough the atom is stable  If the forces are not strong enough the atom (heavier atoms) the atom is unstable and becomes radioactive.

52. Counting Atoms  Basic Truth: All atoms contain the same basic parts, but atoms of different elements have different numbers of protons.  The PT lists atoms in consecutive order by their Atomic Number (Z)  The atomic number is directly related to the number of protons in the nucleus of each atom of that element

53. Counting Atoms  The atomic number is found above the elemental symbol on the PT and it defines the type of element  Atomic #47 can only be Ag and it also can only have 47 protons in each nucleus  Because atoms are neutral, we know from the atomic number the atom must also contain 47 electrons.

54. Counting Atoms  The total number of protons & neutrons determines the mass of the atom  Called the Mass Number  A Carbon atom, has 6 protons and 6 neutrons, so its mass number is 12  If you know the atomic number & mass number of an atom of any element, you can determine the atom’s composition

56. ATOMS OF THE 1 ST TEN ATOMS NAMESYMBOL ATOMIC # p+p+p+p+ n0n0n0n0 MASS # e-e-e-e- HydrogenH11011 HeliumHe22242 LithiumLi33473 BerylliumBe44594 BoronB556115 CarbonC666126 NitrogenN777147 OxygenO888168 FluorineF9910199 NeonNe1010102010

57. Counting Atoms  Every Cl atom has 17 protons, w/o exception, but not every Cl atom has 18 neutrons.  Atoms with the same # of protons but contain different #s of neutrons are called isotopes.  Since isotopes of an element have different #s of neutrons they have different masses

58. Counting Atoms  Isotopes are chemically alike because they have identical numbers of protons and electrons  It’s the electrons and protons that are responsible for chemical behavior  Isotopes can be noted using hyphen notation (Cl-35 vs Cl-37)  elemental symbol hyphen mass number

59. Isotope: one of two or more atoms having the same number of protons but different numbers of neutrons or Na-23 or Na-24