Atomic Structure Exam 1 and Problem Solutions

Atomic Structure Exam 1 and  Problem Solutions

1. Which one of the following statements are always true for neutral atoms;

I. Atomic number is equal to number of electrons

II. Mass number is equal to sum of electrons number and neutron number

III. Atomic number is equal to neutron number

Solution:

I. In neutral atoms, number of protons is equal to number of electrons. Since atomic number is equal to number of protons we can say that atomic number is also equal to number of electrons in neutral atoms. I is true.

II. Mass number is equal to sum of number of protons and number of neutrons. Since number of protons is equal to number of electrons in neutral atoms, we can say that mass number is also equal to sum of number of electrons and neutrons. II is true.

III. In neutral atoms, number of neutrons is not always equal to number of protons. Thus, III can be true or false.

2. If  number of electrons of ₃₇X^- and ₂₀Y⁺² are equal; find number of neutrons of ₃₇X^- .

Solution:

Number of electrons of ₂₀Y⁺² ;

20-2=18

So, X^- also has 18 electrons. Number of protons of ₃₇X^- is;

18-1=17

Number of neutrons of ₃₇X^- is;

37-17=20

3. Which ones of the following statements are true for ₂₉⁶⁴Cu atom;

I. Nuclear charge of Cu is 29

II. Mass number of Cu is 64

III. Number of neutrons of Cu is larger than number of protons

Solution:

Nuclear charge of atoms is equal to number of protons, so I is true.

Number written at left corner of atom shows mass number. II is true

We can find number of neutrons;

Mass number= number of protons + number of neutrons

64 = 29 + number of neutrons

number of neutrons = 35

III is also true number of neutrons is larger than number of protons.

4. Which ones of the following statements are true for ₁₂²⁴X and ₁₁²⁴Y atoms;

I. Relation between nuclear charges : X > Y

II. Relation between number of neutrons: Y = X + 1

III. Number of nucleons are equal.

Solution:

₁₂²⁴X has;

Mass Number=24 =number of nucleons

Atomic Number=12=number of protons =nuclear charge

Mass number = p + n

24=12 + n

n=12 number of neutrons of X

₁₁²⁴Y has;

Mass Number=24 =number of nucleons

Atomic Number=11=number of protons =nuclear charge

Mass number = p + n

24=11 + n

n=13 number of neutrons of Y

Thus, I, II and III are all true.

5. If S^-2ion has 18 electrons, find atomic number of S.

Solution:

S^-2 ion has 18 electrons, this means that, S takes 2 electrons from outside and has 18 electrons. Thus, in neutral S atom there are;

18-2=16 electrons. For neutral atoms;

number of protons=number of electrons=atomic number

So; atomic number of S is 16.

6. Chlorine has two isotopes; ₁₇³⁵Cl in 75 % and ₁₇³⁷Cl in 25%. Find average atomic mass of Cl atom.

Solution:

We use following formula to find average atomic mass of isotopes;

Average atomic mass=35.75/100 + 37.25/100

Average atomic mass=35,5 g