Name: ____________________________________ Date: ________________________ Period: ______

HISTORY OF THE ATOM

Sometimes scientists are required to make a conclusions about things which they really

cannot see. Scientists have done this to describe the INSIDE of an atom without being able

to see inside. They have come so far! Through experimentation and collaboration, scientists

have developed and modified several models of the atom over the years. Follow the

directions below as you complete this activity.

In the box below, draw a MODEL of the solar system.

In the box below, draw a MODEL of an animal cell.

What is a model? ________________________________________________________________________ 1

Below, you will find descriptions of six different models of the atom that have been

developed over the years (some date back to 400 BC!). Read the descriptions of each of

the models and draw a representation of each in the spaces provided.

For the GREEK PHILOSOPHERS, an atom was the smallest particle of a

m material that still kept the properties of that material. This smallest

piece results when a sample of the material is cut in half and each

half is cut in half again (and so forth) until the material can no longer

be subdivided.

For JOHN DALTON, atoms were the smallest piece of a substance that

s still kept the properties of that substance. Dalton imagined that

atoms were solid spheres that could not be further subdivided.

Compounds were formed when these spheres joined together.

Elements each were made up of the same kind of atom with different

elements having different atoms.

JJ THOMSON carried out some very important experiments through

which he discovered that all atoms contained negative particles

(electrons). Since atoms are electrically neutral, they also had to

have enough positive charge to balance the negative electrons.

Thomson visualized the atom as being like chocolate chip cookies;

the cookie was positive and the chocolate chips were like the

negative electrons, universally distributed throughout the atom.

ERNEST RUTHERFORD’S experiments showed more detail about the

atom than Thomson’s experiments. Rutherford’s model of the atom

showed that all atoms contained a very dense center called a

nucleus that was positively charged. The electrons were shown to be

located in the space around the nucleus. When asked to describe

how the electrons were arranged, Rutherford suggested that they

were arranged much like the planets are arranged around the sun.

NIELS BOHR recognized that there were some shortcomings about

both Thomson’s and Rutherford’s models of the atom. Through the

experiments he did, Bohr’s view of the atom borrowed the ideas of

Rutherford but expanded them to say that the electrons that were in

the space outside the nucleus were orbiting the nucleus in definite

orbits or circular paths. These orbits are called energy levels or shells.

Further experiments showed that the Bohr model also had to be

modified. These modifications produced the PRESENT DAY model

known as the “charge cloud model.” The charge cloud model does

not show the paths or orbits of the electrons. Instead, electrons are

represented as being clouds of negative charge around the

nucleus. Electrons are still in specific energy levels or shells, but

electrons are no longer represented as traveling in orbits.

Greeks

Thomson

Dalton

Rutherford

Present

Bohr

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I. Give the proper symbol used to represent each of the following elements.

1. Silver ______ 14. Aluminum ______

2. Arsenic ______ 15. Gold ______

3. Barium ______ 16. Bismuth ______

4. Bromine ______ 17. Carbon ______

5. Calcium ______ 18. Chlorine ______

6. Cobalt ______ 19. Chromium ______

7. Copper ______ 20. Fluorine ______

8. Iron ______ 21. Hydrogen ______

9. Mercury ______ 22. Iodine ______

10. Potassium ______ 23. Magnesium ______

11. Manganese ______ 24. Nitrogen ______

12. Sodium ______ 25. Nickel ______

13. Oxygen ______ 26. Phosphorous ______

II. Correctly spell the element represented by each of the following symbols.

1. Pb __________________ 14. Pt __________________

2. S __________________ 15. Sb __________________

3. Sn __________________ 16. Sr __________________

4. Ti __________________ 17. U __________________

5. W __________________ 18. Zn __________________

6. Ar __________________ 19. B __________________

7. Cd __________________ 20. Ce __________________

8. Cs __________________ 21. He __________________

9. Li __________________ 22. Mo __________________

10. Ne __________________ 23. Rb __________________

11. Se __________________ 24. Si __________________

12. Te __________________ 25. Tl __________________

13. V __________________ 26. Hf __________________

CHEMISTRY ELEMENT SYMBOL PRACTICE I

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ATOMIC NUMBERS & AVG ATOMIC MASSES

The atomic number of an element represents the number of protons in the nucleus of atoms of that specific element. No other element has that specific number of protons. Atomic numbers can be found on most Periodic Charts/Tables. The atomic number will always be a whole number value without decimals. Look on a periodic chart at the elements listed below. Do you see how to find an elements atomic number?

ELEMENT ATOMIC NUMBER

Hydrogen (H) 1

Krypton (Kr) 36

Potassium (K) 19

Gold (Au) 79

The average atomic mass of an element represents the relative mass of average atoms of that element compared to atoms of carbon-12 (C-12) which has a mass of exactly 12.

The atomic masses of the elements are known accurately to several decimal places, but for our purposes, we will usually round the atomic masses to one decimal place.

Look on a periodic chart at the elements listed below. Do you see how to find an elements atomic mass?

ELEMENT ATOMIC MASS

Hydrogen (H) 1.0

Krypton (Kr) 83.8

Potassium (K) 39.1

Gold (Au) 197.0

Practice Problems

1. Use your periodic table to find the atomic numberand atomic mass rounded to one decimal place of each of the following elements:

CHEMISTRY ATOMIC NUMBERS & ATOMIC MASSES PRACTICE

Sn

Cl

Pb

Ar

Ag

Co

Na

Rb

Mn

Cu

Ca

Fe

Avg.Atomic Atomic

NumberNameElement Symbol Mass

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MASS NUMBERS

The atomic weights of the elements that you find on your periodic table are said to be “relative” atomic weights. They are relative because all of the atomic weights are related to atoms of what we call C-l 2. But do you know what C-12 means?

When you see a chemical symbol written like the “C-12” that you see here, the number following the symbol of the element is called the mass number. The mass number of an atom is simply the sum of theprotons and neutrons in the nucleus (the center region) of that atom.

The mass of the proton and neutron are both very close to 1 a.m.u. (atomic mass unit) and since the mass of the electron is so small that it is not considered in chemical reactions, we simply add up the number of protons and the number of neutrons in an atom to give us the mass number for that atom. You can determine the mass number by rounding the average atomic mass from the periodic table.

EXAMPLE 1

How many protons and neutrons are present in the nucleus of an atom of F-l 9?

SOLUTION 2

First look up the atomic number of F on your periodic table. It is 9... do you see? Thus all Fluorine atoms have 9 protons. For a Fluorine atom to have a mass of 19 as in the F-19, the sum of the protons and neutrons must be 19. Since all Fluorine atoms have 9 protons, an atom of F-19 must have 10 neutrons. (9 + 10 =19)

EXAMPLE 2

Write the symbol for an atom that contains 17 protons and 18 neutrons.

SOLUTION 2

With 17 protons the atom must have an atomic number of 17. Element 17 on your periodic table is Chlorine... do you see? The mass number is the sum of the protons and neutrons (17 + 18) = 35. Thus thesymbol for this atom is... Cl-35.

STUDENT PRACTICE

1. Give the number of protons and neutrons thatare found in each of the following...

P-31 p = n =

Au-200 p = n =

C-14 p = n =

I-127 p = n =

Ra-226 p = n =

U-235 p = n =

2. Write the symbol for atoms which have...

30 protons & 35 neutrons

82 protons & 125 neutrons

1 proton & 1 neutron

CHEMISTRY MASS NUMBERS PRACTICE

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Mass Number and Atomic Number Worksheet

NameofElement

Symbol AtomicNumber Number

Mass

Protons Neutrons Electrons

Copper 29 35 29Tin Sn 69 50 I 53 127 Uranium 238 92 K 19 20 Lithium 7 3

O 8 8 Gold 79 197 16 32 16 Silver 47 108 47

Chromium 28 24 Co 59 32 27 Ni 28 Zinc 30 35

Al 14 13 Hg 80 201 Platinum 195 Fe 56 30

H 1 1 He 2 4 4 4 4 Mg 12 12 12 C 6 6 6 Silicon 14 14 Cl 17 18

Bi 209 83Boron 5 11 Ca 20 20 25 55 25 Lead 207 82Sodium Na Fluorine 9 10 9 P 15 31

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4.3 - Bohr Model Diagrams Ewald 8/08

Name Period Date

Bohr Model Diagrams

Use the information provided for each element to draw Bohr Model diagrams. Rather than drawing individual protons and neutrons, you may simply label how many of each there are in the nucleus (e.g. He: 2p, 2n). Then, draw the individual electrons on the appropriate energy levels (keep in mind the maximum number of electrons allowed on each level).

1. Beryllium – atomic #: 4, # of n: 5 2. Sodium – atomic #: 11, # of n: 12

3. Sulfur – atomic #: 16, # of n: 16 4. Fluorine – atomic #: 9, # of n: 10

5. Calcium – atomic #: 20, # of n: 20 6. Argon – Atomic #: 18, # of n: 22

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4.3 - Bohr Model Diagrams Ewald 8/08

7. Beryllium – atomic #: 4, # of n: 7 8. Nitrogen – atomic #: 7, # of n: 8

9. Aluminum – atomic #: 13, # of n: 15 10. Nickel – atomic #: 28, # of n: 30

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Isotopes Worksheet

Part I: Isotopes

1. Define an isotope.

2. What would happen if the number of protons were to change in an atom?

3. Another way to write isotopes is to write the name of the element then add the mass number after a dash,

for example, 14

6C is carbon-14. Why isn’t the atomic number needed for this notation?

4. Here are three isotopes of an element: 612

C 613

C 614

C

a. The element is: __________________

b. The number 6 refers to the _________________________

c. The numbers 12, 13, and 14 refer to the ________________________

d. How many protons and neutrons are in the first isotope? _________________

e. How many protons and neutrons are in the second isotope? _________________

f. How many protons and neutrons are in the third isotope? _________________

Complete the following chart:

Isotope name atomic # mass # # of protons # of neutrons # of electrons

92 uranium-235

92 uranium-238

5 boron-10

5 boron-11

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© John Erickson, 2004 WS4-2IsotopeNotation

Any given element can have more than one isotope. To distinguish between the different isotopes of an atom, the element is named with its mass number, for example lithium-7. Remember that the mass number is the number of protons and neutrons. When symbols are used to represent an isotope the mass number is written next to the symbol on the top left. The atomic number is written on the bottom left. Recall that the atomic number is the number of protons.

Answer the following questions about atoms.

1. The identity of an atom is determined by the number of ____________________.

2. The particle(s) found inside the nucleus are called: ________________________.

3. The number of protons and neutrons combined is called the _________________.

4. In large atoms the number of protons is ___________ than the number of neutrons.

5. The number of protons is also called the _________________.

6. Isotopes have the same number of __________, but different numbers of __________.

7. The number of protons found in a sulfur atom is _________.

8. The number of neutrons found in an aluminum-27 atom is _________.

9. The number of electrons found in a zinc atom is _________.

10. What is the name of the element with 82 protons? _____________.

Give the symbols for the nuclides described by the following particles. Include the atomic number and the mass number.

11. 92 protons, 145 neutrons 15. 20 protons, 20 neutrons

12. 8 protons, 10 neutrons 16. 22 protons, 23 neutrons

13. 82 protons, 125 neutrons 17. 18 protons, 22 neutrons

14. 80 protons, 119 neutrons 18. 25 protons, 32 neutrons

Determine the number of protons and neutrons from the following symbols.

19. B105

20. N157

21. Se7934

22. Sn11950

23. Dy16566

24. Fe56

25. Sm151

26. Pt195

27. Te126

28. Cl35

29. Ag107

30. ?9341

Isotope Notation Chem Worksheet 4-2

Mass number

Atomic number XAZ

X = element symbolA = mass number [# of protons (p) + # neutrons (n)] Z = atomic number [# of protons]N = # of neutronsA - Z = N

7 Li3

Lithium-7

3 protons 4 neutrons

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NAME______________________________

Average Atomic Mass Worksheet: show all work.

1) Rubidium is a soft, silvery-white metal that has two common isotopes, 85Rb and 87Rb. Ifthe abundance of 85Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?

2) Uranium is used in nuclear reactors and is a rare element on earth. Uranium has threecommon isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of uranium?

3) Titanium has five common isotopes: 46Ti (8.0%), 47Ti (7.8%), 48Ti (73.4%),49Ti (5.5%), 50 Ti (5.3%). What is the average atomic mass of titanium?

4) Why is the mass in amu of a carbon-12 atom reported as 12.011 in the periodic table ofthe elements?

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5) Naturally occurring chlorine that is put in pools is 75.53 percent 35Cl (mass = 34.969 amu)and 24.47 percent 37Cl (mass = 36.966 amu). Calculate the average atomic mass.

6) Copper used in electric wires comes in two flavors (isotopes): 63Cu and 65Cu. 63Cu has anatomic mass of 62.9298 amu and an abundance of 69.09%. The other isotope, 65Cu, has an abundance of 30.91%. The average atomic mass between these two isotopes is 63.546 amu. Calculate the actual atomic mass of 65Cu.

7) Magnesium consists of three naturally occurring isotopes. The percent abundance ofthese isotopes is as follows: 24Mg (78.70%), 25Mg (10.13%), and 26Mg (11.7%). The average atomic mass of the three isotopes is 24.3050 amu. If the atomic mass of 25Mg is 24.98584 amu, and 26Mg is 25.98259 amu, calculate the actual atomic mass of 24Mg.

8) Complete the table

Isotope Mass (amu) Relative Abundance (%) Neon-20 19.992 90.51Neon-21 20.994Neon-22 9.22

Avg. Atomic Mass = Total %:

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