AP Chemistry The Atom 2015-08-25 www.njctl.org Slide 3 / 118 - PDF document

Slide 1 / 118 Slide 2 / 118 AP Chemistry The Atom 2015-08-25 www.njctl.org Slide 3 / 118 Table of Contents: The Atom (Pt. A) Click on the topic to go to that section · Subatomic Particles · Atomic Modeling

Slide 4 / 118 Subatomic Particles Return to Table of Contents Slide 5 / 118 Matter Atoms are classified as the basic unit of matter. Atoms contain the following: + positively charged particles neutrally charged particles - negatively charged particles What are these particles called? What is the magnitude of the charges? How big are these particles? Slide 6 / 118 Dalton's Atomic Theory English chemist John Dalton observed matter and drew some conclusions about atoms. According to Dalton: Matter is composed of atoms, which are indivisible. Each compound consists of a set ratio of atoms. Atoms of same element are C C C C identical Atoms of different elements are Si C different Atoms are not changed, created, Cl Cl H H or destroyed in a reaction, they H Cl are simply rearranged Cl H

Slide 7 / 118 1 The basic unit of matter is.... A an atom B an electron C a neutron D a proton Slide 8 / 118 2 The discovery of subatomic particles disproved which of Dalton's postulates? A Matter is composed of atom which are indivisible B Atoms of the same element are identical C Atoms of different elements are different D Atoms are not changed, created nor destroyed in a chemical reaction Slide 9 / 118 3 Which of Dalton's postulates holds true today? A Atoms are neither created and destroyed in chemical reactions. B All atoms of a given element are identical C Atoms are indivisible. D Atoms of different elements are different.

Slide 10 / 118 Dalton’s Postulates Dalton had the right idea with his postulates but he was not completely correct. He was limited by the equipment he had to observe reactions. Today we know that there are some forms of reactions in which mass does change, and atoms are changed from one type to another. They are called Nuclear Reactions. Also remember that today we know atoms can be broken down into smaller bits. We also know all atoms of an element are not identical - elements found in nature can vary in number of neutrons. However, for the purposes of general Chemistry, Dalton's Postulates are still a pretty reasonable approximation of what is actually happening in chemical reactions. Slide 11 / 118 . Cathode Rays In the late 1800's scientists were passing electricity through glass tubes containing a very small amount of a particular gas noticed that a beam of light was created. Scientists found that they could deflect this beam by subjecting it to an additional electrical field. + Why would the beam deflect toward the positive plate? + - - POWER ON Slide 12 / 118 . Charge to Mass Ratio Once these rays were understood to be particles, they were in search of their properties - like their mass and the size of their charge. A very weak electrical field could deflect the beam a great deal. The mass/charge ratio was determined to be 1.76 x10 11 C/kg. + deflection + - - POWER ON What does this tell us about the particle?

Slide 13 / 118 . Determining the Mass & Charge A scientist named Millikan squirted oil drops into a box and then passed high energy x-rays at the box hoping to knock electrons off the air molecules and onto the oil drops. Oil drops + X-rays - Click here to see an animation of the experiment By measuring the energy necessary to stop the drops from descending, he was able to determine the charge per drop. The more energy needed to prevent the drop from falling, the smaller the charge of the drop. Slide 14 / 118 . Using Charge to Mass Ratio Once the charge was known, it was easy to use Thomson's charge to mass ratio to find the mass. 1.6 x10 -19 C x 1 kg = 9.09 x 10 -31 kg 1.76 x10 11 C This is one tiny mass! Electrons are super super small. Slide 15 / 118 4 What characteristic about the cathode rays led them to believe they were negatively charged? A They were small B They were easily deflected C They were deflected towards a positive electrode D They moved quickly

Slide 16 / 118 5 Which one of the following is not true concerning cathode rays? A They originate from the negative electrode. B They travel in straight lines in the absence of electric or magnetic fields. C They move from the anode to the cathode. D They are made up of electrons. Slide 17 / 118 6 The magnitude of the charge on an electron was determined in the __________. A cathode ray tube, by J. J. Thomson B Millikan oil drop experiment C Dalton atomic theory D atomic theory of matter Slide 18 / 118 . Discovery of the Proton After the discovery of the electron, scientists believed that there must also be a positively charged particle in the atom. To look for these, they used an anode ray tube. - Power Positive anode rays + By placing holes in the cathode so particles could move through it, they found that particles were indeed moving from the anode to the cathode. Since they move towards a negative plate, they must be positive.

Slide 19 / 118 . Discovery of the Proton The anode rays were referred to as protons, which were found to be significantly heavier than electrons. 1 proton = 1840 x mass of electron Since the heaviest anode rays in oxygen were found to be 8 x heavier than those in hydrogen, it was assumed that oxygen had 8 protons compared to hydrogen's 1. The number of protons an atom has is different for each element on the periodic table. Slide 20 / 118 7 Which of the following is TRUE regarding protons? A They were originally called cathode rays B They move faster than cathode rays C They have a larger mass than electrons D They moved from the cathode to the anode Slide 21 / 118 8 Which of the following is NOT true regarding protons and electrons? A Both were found in all atoms B Their charges are equal in magnitude C Protons are significantly heavier than electrons D All elements have the same number of protons and electrons

Slide 22 / 118 9 The mass of an electron was found to be 9.1 x 10 -31 kg. What is the mass of a proton? A 1.67x10 -27 kg B 4.95x10 -34 kg C 9.1x10 -31 kg D 1.6x10 -19 kg Slide 23 / 118 Plum Pudding Model The prevailing theory about the make-up of an atom was the “plum pudding” model - proposed by J. J. Thomson around 1900. The model featured a positive sphere of matter with negative electrons embedded in it. It was based around the idea that positive and negative charges attract and like charges repel. Slide 24 / 118 Radioactivity Of course, models must be tested and the search was on to find evidence to support the "plum pudding" model. Ernest Rutherford used radioactivity was used to test this theory. What are the 3 types of radioactivity?

Slide 25 / 118 Rutherford's Gold Foil Experiment Physicists Geiger and Marsden under the direction of Ernest Rutherford shot a beam of alpha particles at a thin sheet of gold foil and observed the scatter pattern of the particles. Click here to see an animation of the experiment Slide 26 / 118 Discovery of the Nucleus While most particles went straight through some bounced back... totally unexpected! What did this indicate? Slide 27 / 118 10 The gold foil experiment performed in Rutherford's lab __________. A confirmed the plum-pudding model of the atom B led to the discovery of the atomic nucleus C was the basis for Thomson's model of the atom D utilized the deflection of beta particles by gold foil

Slide 28 / 118 11 In the Rutherford nuclear-atom model: A the heavy subatomic particles reside in the nucleus B the principal subatomic particles all have essentially the same mass C the light subatomic particles reside in the nucleus D mass is spread essentially uniformly throughout the atom Slide 29 / 118 . Discovery of the Neutron Since electrons were so much smaller than protons, Rutherford believed the mass of an atom would be simply related to the number of protons present. However, they found that atoms were heavier than predicted!! Example - Helium (He) Helium = 2 protons, 2 electrons Expected mass = 2 x (mass of proton) Actual mass = 4 x (mass of proton) The reason: the neutron. It's existence was suggested by Rutherford two years after his experiment and determined experimentally in 1932. Slide 30 / 118 . Neutrons, Protons, and Atomic Masses Since electrons have a much smaller mass than a proton or neutron, the mass of an atom (in amu) is generally considered to be equal to the sum of the protons and neutrons in an atom. (# of protons) + (# of neutrons) = atomic mass (A) in amu

Slide 31 / 118 12 What is the mass of an element that has 10 protons and 11 neutrons (in u)? Slide 32 / 118 13 How many neutrons are present in an oxygen atom with a mass of 18 u? Slide 33 / 118 14 How many protons are present in atom with a mass of 13 u if it has 7 neutrons?