# 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 _{37}X^{-} and _{20}Y^{+2} are equal; find number of neutrons of _{37}X^{-} .

**Solution:**

Number of electrons of _{20}Y^{+2} ;

20-2=18

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

18-1=17

Number of neutrons of _{37}X^{-} is;

37-17=20

**3.** Which ones of the following statements are true for _{29}^{64}Cu 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 _{12}^{24}X and _{11}^{24}Y atoms;

**I.** Relation between nuclear charges : X > Y

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

**III.** Number of nucleons are equal.

**Solution:**

_{12}^{24}X 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

_{11}^{24}Y 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^{-2}ion 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; _{17}^{35}Cl in 75 % and _{17}^{37}Cl 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