Phys Sc 416/30

Pretest 1.2 Solutions

 

1.       True? Or False?

 

a.                   Democritus’ model can be used to explain physical changes._TRUE_______

b.                   Rutherford realized that the atom is mostly empty space. The mass of an atom is mostly concentrated in a tiny nucleus. TRUE_______

c.                   Bohr’s model has electrons moving in orbit-like paths TRUE_______

d.                   Excited electrons sometimes emit light after returning to “orbits” that are closer to the nucleus_ TRUE_______

e.                   Electrons are heavier than neutrons_ FALSE_______

 

2.            Explain Rutherford’s experiment. Use a diagram and distinguish between those alpha particles that come right back and those that are deflected at slight   angles.(430)

 

 

Rutherford and his team beamed alpha particles through gold foil and detected them as flashes of light or scintillations on a screen. The gold foil was only 0.00004 centimeter thick. Most of the alpha particles went straight through the foil, but some were deflected by the foil and hit a spot on a screen placed off to one side. Geiger and Marsden found that about

one in 20,000 alpha particles had been deflected 45o or more. Rutherford asked why so many alpha particles passed through the gold foil while a few were deflected so greatly. "It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper, and it came back to hit you," Rutherford said later. "On consideration, I realized that this scattering backwards must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive center carrying a charge."

 

 

 

3.         How do electron ”orbits” help explain hydrogen’s spectrum?

 

            Each time an electron gets excited to a higher energy level it emits a colour of a very specific wavelength because it can only fall back to one specific energy level. In all there are a very limited number of energy levels, so only a few coloured lines appear in an element's spectrum.

 

4.         List two differences between the Bohr model and the modern model.

 

Bohr

Modern

No neutrons

Neutrons exist in nucleus

Energy levels

Energy levels (shells) with specific number of electrons per shell

 

 

5.         Complete the following table:

 

Name

Isotope

Notation

Atomic Number

Mass Number

Number of Protons

Number of Neutrons

Number of Electrons

neutral oxygen

 

16O

 

8

16

8

16-8 = 8

8 - 0 = 8

sulfide

(-2)

 

24S

 

16

16 +8 = 24

16

8

16-(-2) = 18

Aluminum (+3)

 

27Al

 

13

27

13

14

10

silver....(+1)..................

47

 

 

108

47

61

47 - 1 = 46

Chloride (-1)

17

 

 

35

17

18

17 - (-1) = 18

 

 

6.         Give the charge for each of the following subatomic particles:

 

a.                   proton_____+1_

b.                   electron____-1__

c.                   neutron_____0__

 

7.         Arrange the following in order, from the lightest(1) to the heaviest(3).

proton___2___

electron__1____

neutron__3_(slightly heavier than the neutron)____

 

8.         A certain atom has two more protons than electrons. Its charge is _+2___.

 

9.         Will the chemical properties of helium change if it loses a(n)________?

 

a.                   electron___Yes. Ions and neutral atoms have different chemical properties_

b.                   neutron____No. Isotopes have the same chemical properties.

c.                   proton_____Yes. You would be creating a different element.

 

10.        If it was possible to remove 3 protons from a neutral aluminum:27 atom, what

would you end up with? Show the full isotope notation of the newly created atom.

           

Before

After

27Al

13 protons

13 electrons

14 neutrons

 

10 protons

13 electrons

14 neutrons

 

24Ne-3 ( no such thing exists in nature, by the way)

 

 

 

11.        What are isotopes? Give an example.

 

Isotopes are different versions of the same elements. They have different mass numbers.

 

12.         Show through example how an ion differs from its neutral counterpart.

 

Mg+2 does not burn; Mg does.

H+1 reacts with base; neutral hydrogen does not

 

13.        Draw a shell diagram for each of the following:

 

a.                   Oval: 9p 10n

2e) 8e)

 

 

Oval: 13p 14n
 


b.                                            2e) 8e)

 

 

 

c.                   Oval: 7p 8n                             

2e)5e)

 

 

 

 

                                    For (c) the valence # (last shell electrons) is___5_____.

 

 

 

 

14.        Draw a neutron, including its quarks and show how they add up to the neutron’s charge.

 

 

 


            Each d = -1/3

            u =2/3

            2(-1/3) +2/3 = 0

           

 

                                                                                   

15.       Which particle or form of energy will be emitted when the radioactive strontium-90 breaks down according to the following?

 

90Sr à 90Y + __?___

 

 

            A.        alpha                B.         beta                  C.        gamma             D.        neutron

 

            90 =90 + x

x = 0 mass

Find atomic numbers:

38 = 39 + y

Y = -1

Answer beta = electron

 

16.        Find the density of the following liquid:

Mass(g)
 

Volume(mL)
 
 

 

 

 


density = slope = (y2-y1)/(x2-x1) = (2.5 - 0) / (5.5 - 0) = 0.45 g/ml

 

            Don't forget that there will be other flashback questions on characteristic properties, chemical changes, physical changes, etc!

 

Sample Flashback Questions

 

 

 

1

Which of the following involves a chemical change?

 

1.  Boiling water

2.  Putting sugar in coffee

3.  Toasting two slices of bread

4.  Spreading jam on toast

 

 

A)

 

1

 

C)

 

3

 

B)

 

2

 

D)

 

4

 

 

2

Which of the following is a characteristic property of mercury (Hg)?

 

 

A)

 

Is has a metallic luster.

 

C)

 

It is gray.

 

B)

 

It evaporates slowly.

 

D)

 

Its melting point is -39°C

 

 

3

A pure gray substance was placed in a open crucible and heated in the presence of air.  The following observations were made.

 

 

 

 

Before Heating

 

After Heating

 

State

 

Liquid

 

Solid

 

Colour

 

Gray

 

Red

 

Mass

 

12.0 g

 

13.2 g

 

Which of the following statements IS DEFINITELY FALSE?

 

 

A)

 

The substance before heating was an element.

 

B)

 

The substance before heating was a compound.

 

C)

 

The substance after heating is an element.

 

D)

 

The substance after heating is a compound.

 

 

4

While identifying an unknown substance in the laboratory, you note that it has the following properties:

 

1.             Its melting point is 0°C;

2.             It is colourless;

3.             It does not change the colour of neutral litmus paper;

4.             It does not conduct electricity.

 

 

Which of these properties most clearly indicates that the unknown substance is pure water?

 

 

A)

 

1

 

B)

 

2

 

C)

 

3

 

D)

 

4

 

 

Answers

 

1.C         

2.D         

3.C ( It's possible for a compound to react with a gas and give a heavier compound, but it is not possible for an element to weigh more than the original substance.       

4.A