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Chemistry Honors Key

Unit 2 Review Worksheet - Atomic structure1) What is the atomic mass of magnesium? The element consists of 78.70% of24Mg atoms (mass 23.99 amu),

10.13% of25 Mg atoms (mass 24.99 amu) and 11.17% 26 Mg atoms (mass 25.98 amu).

Answer 1:

(Abundance24Mg) (Mass24Mg) + (Abundance25 Mg) (Mass25 Mg) + (Abundance26 Mg) (Mass2 Mg) = atomicweight of Mg

24Mg (0.7870) (23.99 amu) = 18.88 amu25 Mg (0.1013) (24.99 amu = 2.53 amu26 Mg (0.1117) (25.98 amu) = 2.90 amu

18.88 + 2.53 + 2.90 = 24.31 amu

2) Carbon occurs in nature as a mixture of 126 C and13

6 C. The atomic mass of12

6 C is exactly 12 amu and theatomic mass of136 C is 13 amu. The atomic weight of carbon is 12.011 amu. What is the percentage of

126 C in

natural carbon? (14C also occurs but less than 1 part per trillion! Or 0.000000001%)

Answer 2

The equation to determine the atomic weight of carbon is

(Abundance12

6 C) (Mass12

6 C) + (abundance13

6 C) (mass13

6 C) = atomic weight of C

If x is the abundance 126 C then (1-x) is the abundance of13

6 C

Therefore, (x) (12.000) + (1-x) (13.003) = 12.011

12.000x + 13.003 13.003x = 12.011

-1. 003x = - 0.992

X = 0.989

The atomic weight of Mg is 24.31 amu.

126 C constitute 98.9% of the mixtures of the carbon isotope


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3) There are 2 naturally occurring isotopes of Rb (Rubidium) 85 Rb and 87 Rb. The mass of 85 Rb is 84.91 amu

and mass of 87 Rb is 86.92 amu. The atomic mass of Rb is 85.47 amu. What is the % abundance of each of the

isotopes?

Answer 3

The mass of 85 Rb is 84.91 amuThe mass of 87 Rb is 86.92 amu.

The atomic mass of Rb is 85.47

Let the abundance of85 Rb be X

So the abundance of87 Rb will be (1 X)

X (84.91) + (1-X) 86.92 = 85.47

X = 0.72(1-X) = 0.28

The % abundance of 85 Rb = 72%

The % abundance of 87 Rb = 28%

4) What is the atomic mass of element Y if Y consists of 57.25% of atoms with a mass of 120.90amu and42.75% of atoms with a mass of 122.90 amu?

Answer 4

(Abundance) (Mass) + (Abundance) Mass) = atomic weight

0.57 (120.90) + 0.42 (122.90) = 120.52

The atomic mass of element Y = 120.52

5) If an element X consists of 92.0% of atoms with a mass of 28.0amu, 5.0% of atoms with a mass of 29.0 amu

and 3.0% of atoms with a mass of 30.0 amu. What is the atomic mass of X?

0.92(28.0) + 0.05(29.0) + 0.03(30.0) = 27.71 amu

The atomic mass of element X = 27.71 amu


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j. S [Ne] 3s23p4

k. F[He] 2s22p5

l. Cl [Ne] 3s23p5

m. Br[Ar] 4s2 4p53d10

n. C[He] 2s22p2

9) Explain the following:

Aufbau Principle: Electrons go to the lowest energy orbital first.

Pauli Exclusion Principle: Every orbital can hold two electrons if they have opposite spin.

Hunds Rule: Electrons would rather be alone if another equal energy orbital is available.

10) Diagram and explain the major 4 different Atomic models? (Thompson, Rutherford, Bohr, Schrodinger)

Thomsons plum pudding model has the electrons floating around in a sphere of positive charge

Rutherfords model puts the massive and dense, positively charged nucleus in the center surrounded bynegative electrons


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Bohr Proposed that electrons travel in orbits of fixed energy around the nucleus, explaining atomic spectra

Schrodinger Quantum mechanical descriptions of the probability of locating an electron in a cloud or orbital.

11) Explain the following

a) The law of definite proportions.Each compound has a specific ratio of elements

b) The Law of Conservation of masses.

There is no detectable change in mass during the course of a chemical reaction.

c) The Law of Multiple proportions.

Dalton discovered that if two elements form more than one compound, the ratio of the second element thatcombines with 1 gram of the first element in each is a simple whole number

12) Summarize Daltons Atomic Theory? What were his 5 points?

Dalton outlined five fundamental postulates about atoms:

1. All matter consists of tiny, indivisible particles, which Dalton called atoms that cannot be broken down or

destroyed.2. All atoms of a particular element are exactly alike, but atoms of different elements are different.

3. All atoms of different elements are unchangeable and have different physical and chemical properties.

4. Atoms of elements combine to form "compound atoms" (i.e., molecules) of compounds in simple whole-number ratios.

5. In chemical reactions, atoms are neither created nor destroyed, but are only rearranged.

13) Define the term Isotopes?

When elements have same number of protons but different number of neutrons they form isotopes for that

element.

Example.C12 and C14 are isotopes. They have same number of protons but different number of neutrons

14) Why were different colors emitted when the salts were burned in the flame? (in the lab)?

When the salts were held in the flame the electrons from the salts absorbed the energy from the burner and they

jumped to a higher energy level (Excited state). When they go from this excited state to the ground state the

absorbed energy is given out in the form of light. The wavelength of light given out depends on the energylevel that the electrons are when they are in the excited state and to the energy level that they fall to when they

reach the ground state.


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15) Draw and label the different parts of a wave.

16) Explain the following statement in your own words.

The frequency and wavelength of a wave are inversely relateFrom the equation

Speed of light ( c ) = Frequency (v) X Wave length (lambda)

As wave length increases the frequency decreases in the same wayAs wave length decreases the frequency increases. Hence wave length and frequency are inversely related to

each other.

17) Define the term Frequency of a Wave.

The number of waves that pass a given point per second. Units are cycles/sec or hertz (hz)

Abbreviated in the Greek letter nu ()

18) What is the speed of light (c)? Write the value and the units of the speed of light.


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C = 2.9979 x 10 8 m/s is the speed of light.

19) Draw a wave that has low frequency and long wavelength.

Draw a wave that has high frequency and short wavelength.

20) Write an equation to show the relationship between Speed, wavelength and frequency of electromagnetic

waves.

Speed of light ( c ) = Frequency (v) X Wave length (lambda)

21) Explain the terms: Ground state and Excited state.

Exited state If you add energy to an electron it can jump from one energy level to another producing an excited

state.


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Ground state: When electrons of an atom are close to the nucleus, they are in the condition of lowest possible

energy, called Ground state.

22) How can an electron go to the exited state from the ground state?

The electron can go to the excited state when it absorbs energy.

23) What happens when an electron goes from an excited state back to the Ground state?

When an electron goes back to the ground state it emits the energy in the form of light.

24) Write the following particles in nuclear notation: particle, particle, positron, neutron, proton, gamma

ray.

particle 42He

particle 0-1e-1positron 0+1e

+1

neutron 10n

proton 1+1H+1

gamma ray 00

25) Classify each of the following as stable or unstable. Then identify whether or not they would be morelikely to emit a positron or an electron to achieve stability. 208Pb, 8B, 150Ho, 30Al, 120Sn, 94Kr

26) Write reactions for the following transmutations.

a. Nitrogen 14 is bombarded with an alpha particle to produce Fluorine 18.14

7N +4

2He 18

9Fb. Beryllium 9 emits an alpha particle to become Helium 5.

94Be

42He +

52He

c. Phosphorus 30 transmutes into sulfur 30.30

15P 30

16S +0

-1e-1

d. Tritium undergoes decay to Helium.3

1H 3

2He +0

-1e-1

e. Copper 64 emits a beta particle.

6429Cu

6430Zn +

0-1e

-1


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27) List and describe the 4 quantum numbers being sure to include the chemistry notation that describes them.

28) Compound XY is known to contain 45.0% X by mass. What is the % composition for element Y? How

much mass would Y have in a 20.00 g sample of compound XY?

29) A 50.0-g sample of sodium chloride is found to contain 19.50 g of Sodium. What is the percent

composition of Sodium in this compound? How much Chlorine would you find in a 70.00 g sample ofsodium chloride?

30) Compound AB is found to contain 30.00 grams of element A and 25.00 grams of element B. Calculate themass ratio of B to A.

31) What is the mass ratio of Chlorine to Lithium in Lithium Chloride if the percent composition of Lithium is

16.5%?

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