Chemistry Chapter 4 The Structure of the Atom

1.)Who was John Dalton? An English schoolteacher (1766 – 1844) who reintroduced Democritus’ concept of atomos. He called the particles atoms.

2.)What are the main points of Dalton’s atomic theory? All matter is composed of extremely small particles called atoms. All atoms of a given element are identical Atoms cannot be created, divided or destroyed Different atoms combine in simple whole ratios to form compounds In a chemical reaction, atoms are separated, combined and rearranged.

3.) What two laws helped Dalton to develop his atomic theory? Law of conservation of mass and law of multiple proportions.

4.) How does Dalton’s atomic theory explain the Law of Conservation of Mass? Dalton’s atomic theory explains the conservation of mass in chemical reactions as being the result of the separation, combination and rearrangement of atoms.

5.) How does Dalton’s atomic theory explain the Law of Multiple Proportions? Dalton’s theory states that atoms combine in simple whole number ratios to form compounds. This explains why two compounds made of the same compounds have combinations of atoms in simple whole number ratios. Atoms are not divisible in Dalton’s theory

Defining the atom

6.) What is the current definition of an atom? The smallest particle of an element that retains all of the properties of that element

7.) Is there an instrument that actually allows us to “see” atoms? The scanning tunneling microscope.

Nanotechnology. look at page 107 8.) Scientists are now able to manipulate individual atoms. What field has this lead to? Nanotechnology. look at page 107

9.) Approximately what year was it when scientists began to understand the composition of the atom? The greatest increase in understanding was around the early 1900’s

Discovering the Electron

10.) What is the cathode ray tube? A partially evacuated glass tube filled with low pressure gas. On one side of the tube is a positively charged cathode and on the other side is a negatively charged anode. (continued next slide)

10.) What is the cathode ray tube? When an electric current is passed through the tube a colored beam can be seen where the electricity is. The cathode ray tube allowed scientists to “see” electricity.

That the cathode ray was a stream of charged particles with mass. 11.) What were scientists convinced of by the late 1800’s concerning cathode rays? That the cathode ray was a stream of charged particles with mass.

12.) How did they make this conclusion? They knew the stream was charged because the cathode ray could be moved with a charged electric plates and magnets. They knew it had mass because the stream could spin a paddle wheel.

13.) Who was J.J. Thomson? An English physicist (1856– 1940) who performed many experiments with the cathode ray tube.

14.) What did J.J. Thomson determine after his experiments with the cathode ray tube? He determined the ratio of charge to mass. He found the charge in coulombs of a gram of electrons. He also concluded that the mass of the individual charged particle was much less than the mass of the smallest atom (hydrogen)

15.) Why is this significant? Because it means that atoms are divisible. Up until this point atoms were considered indivisible.

16.) How did J.J. Thomson address the problem that matter for the most part is neutral, and yet there existed negatively charged particles he called electrons?

He proposed the Plum Pudding model of the atom He proposed the Plum Pudding model of the atom. The model he used to describe the atom had a positively charged background and electrons scattered throughout. This way there could be negatively charged particles with the entire atom being neutral.

17.) What development did Robert Millikan add to atomic theory in 1909?

17.) What development did Robert Millikan add to atomic theory in 1909? Robert Millikan discovered the charge on a single electron. His experiment was called the oil droplet experiment.

18. ) J. J. Thomson knew the charge on a gram of electrons 18.) J.J. Thomson knew the charge on a gram of electrons. Once Millikan determined the charge on a single electron, he could calculate the mass of a single electron. What did Millikan determine the mass of a single electron to be? 9.1 x 10-28 gram. This is 1/1840 the mass of a hydrogen atom

The Nuclear Atom

Our understanding of the nucleus came from another accidental discovery. By understanding radioactivity, the nucleus of the atom was discovered.

( Not in your book) What did Henri Bequerel discover in 1896?

19.)( Not in your book) What did Henri Bequerel discover in 1896? Henri Bequerel placed a piece of uranium on top of an envelope of undeveloped film. Because the exact image of the rock was exposed on the film, Bequerel knew that some sort of radiation was coming from the uranium. Radioactivity was discovered.

For the next 20 to 30 years many scientists worked with radioactive substances in order to have a better understanding of radioactivity. Pierre and Marie Curie were well known for their work with radioactive substances. They discovered both Radium and Polonium.

Ernest Rutherford is well known for his work with radioactivity Ernest Rutherford is well known for his work with radioactivity. There are two experiments that he performed that are especially significant. We will learn about them both.

20.) Who was Ernest Rutherford?

20.) Who was Ernest Rutherford? A New Zealand Chemist/Physicist (1871-1937)

24.) Describe the experiment in which Rutherford discovered the three types of radiation. Rutherford placed a chunk of radioactive substance in a lead block. He directed the radiation through one side of the block. He then used charged plates to determine the charge of radiation. (continued next slide)

24.) Describe the experiment in which Rutherford discovered the three types of radiation. He realized that there were three types of radiation. The negatively charged radiation was attracted to the postive plate. He called these beta particles (β) Continued next slide

24.) Describe the experiment in which Rutherford discovered the three types of radiation. The positively charged radiation was attracted to the negative plate. He called these alpha particles (⍺) The nuetral particles were unaffected by the charged plates. He called these gamma particles (ɣ)

21. ) In 1911 Rutherford designed the alpha scattering experiment 21.) In 1911 Rutherford designed the alpha scattering experiment. Describe this experiment. How did what actually happened in this experiment compare to what Rutherford expected to happen? What did this tell Rutherford about the structure of the atom? http://www.tutorvista.com/content/physics/physics-iv/atoms-and-nuclei/rutherfords-experiment-animation.php

21.) Slide 1 of 4 Rutherford wanted to know how alpha particles interacted with a solid. He placed an alpha emitter in a lead block and aimed it at a piece of gold foil. He either expected all of the particles to travel through with a small amount of deflection or all of them to bounce back.

21.) Slide 2 of 4 What happened was that most of the particles went straight through. Some were slightly deflected and VERY RARELY a particle completely reversed direction.

21.) Slide 3 of 4 When most of the particles went straight through it meant that an atom is mainly empty space. When rarely, an alpha particle was completely reversed, it meant that the concentration of positive charge was in a very small area.

21.) Slide 4 of 4 Rutherford called this area the nucleus. He proposes that almost all of the mass in the nucleus and that electrons orbit the nucleus like planets orbit the sun.

22.) What was the name of Rutherford’s model? Draw an example. The planetary model

23.) Most of an atom is __empty ___ space.

24.) Who discovered the proton? Henry Mosley, a worker in Rutherford’s lab. Because of Moseley's work, the modern periodic table is based on the atomic numbers of the elements.

25.) What are protons? The positively charged particle in the nucleus of an atom. The number of protons determines which element an atom is. The charge on the proton is exactly equal to, but opposite the charge of the electron.

26.) Who discovered the neutron? An English physicist James Chadwich (1891-1974) Chadwich was a co-worker of Rutherford.

27.) What is a neutron? The neutral particle in the nucleus of an atom. The nuetrons add stability to the nucleus.(strong nuclear force)

Fill in the table Particle Symbol Location Relative charge Relative mass Electron e- Outside nucleus 1- (1/1840) Proton P+ Inside nucleus 1+ 1 amu Nuetron n0 Inside Nucleus

29.) What is our concept of the atom today? Our currently accepted model is the electron cloud model. Electrons are in regions around the nucleus called the electron cloud. Where the cloud is most dense the probability of finding an electron is greatest. The nucleus contains the protons, neutrons and most of the mass. All atoms are nuetral. The number of protons is equal to the number of electrons.

Section 3: How Atoms Differ

30.) What is atomic number? It is the number of protons in the nucleus of an atom. All atoms of a given element have the same atomic number.

How many protons are in an atom of : Carbon? Lead? Iodine? Mendelevium? Calcium?

How many protons are in an atom of : Carbon- 6 Lead- 82 Iodine- 53 Mendelevium-101 Calcium- 20

31.) In an atom, how does the atomic number relate to the number of electrons? In an atom the number of protons is equal to number of electrons

In the following atoms how many electrons are there? Carbon? Lead? Iodine? Mendelevium? Calcium?

In the following atoms how many electrons are there? Carbon- 6 Lead- 82 Iodine- 53 Mendelevium-101 Calcium- 20

32.) What is an isotope? Atoms of the same element with different numbers of neutrons. All atoms of any element have the same number of protons. Isotopes have the same number of protons but different numbers of neutrons.

33.) What is the mass number? The mass number is the sum of the number of protons and neutrons. The number of neutrons can be determined by subtracting the number of protons (atomic number) from the mass number.

34.) What is the atomic mass unit (amu)? 1 atomic mass unit is the mass of 1/12 the mass of a single carbon 12 atom. Carbon-12 has 6 protons and 6 neutrons.

Scientists could have chosen any isotope to use to define the atomic mass unit. There is nothing special about the carbon 12 isotope. It is just the isotope they chose.

35.) On the periodic table you will notice that a lot of the atomic masses listed have decimals. If the mass is determined by the number of neutrons and protons, which are 1 amu each, how can there be decimal values?

Because the masses listed are a weighted average of all of the isotopes that exist. Also, a proton and neutron are exactly equal in mass.

For example Chlorine exists as about 75% Cl-35 and 25% Cl -37. (35) (.75) + (37)(.25) = 35.5 (Mass #)(% abundance) + (mass #)(%abundance) The weighted average is always closer to the isotope with the greatest percent abundance.

36.)How do you calculate the weighted atomic mass of different isotopes of the same element? Multiply the mass of each isotope by its % abundance and add the results.

37.)What can the reported atomic mass tell us about an element? It tells you which isotope is most common.

Section 4: Unstable Nuclei and Radioactive Decay

38.)What are nuclear reactions? Reactions that involve a change to the atom’s nucleus. Atoms of one element change into another type of element. Radioactive substances emit radiation because the nucleus is unstable.

39.)What is considered to be the half-life of a radioactive substance? The amount of time it takes for ½ of the substance to change or decay into another substance.

40.)What are the three types of radiation? Alpha (⍺) Beta (β) Gamma (Ɣ)

41.)Complete the following table Radiation type Symbol Mass (amu) charge Alpha ⍺ 4 2+ Beta β 1/1840 1- gamma Ɣ

42.)Describe alpha radiation An alpha particle is ejected from the nucleus. Since the alpha particle is made up of 2 protons and 2 neutrons the mass # is decreased by 4 amu. Continued next slide

42.)Describe alpha radiation Because the type of element is determined by the number of protons, the element changes to an element with two less protons(atomic # decreases by 2) Continued next slide

42.)Alpha decay example Write the nuclear reaction for the alpha decay of Europium - 152

43.)Describe beta radiation. With beta radiation, an electron is ejected from the nucleus and a nuetron becomes a proton. The mass number stays the same, but the atomic number increases by one.

43.) Beta Decay example What is the nuclear equation for the beta decay of carbon 14?

43.)Beta Decay example What is the nuclear equation for the beta decay of Hydrogen-3?

44.)Describe gamma radiation This is the most dangerous type of radiation. Gamma radiation usually accompanies alpha and beta radiation. Gamma radiation accounts for most of the energy lost during radioactive decay. Cont’d next slide

44.) Describe gamma radiation The emission of gamma rays by themselves do not create new atoms. We will not have nuclear equation for gamma decay. Cont’d next slide

48.)What is the reason that radiation occurs? The primary factor is the stability of the atom which is the result of the proton to neutron ratio Unstable nuclei lose energy through radioactive decay to produce a more stable nucleus.