## Atoms, Molecules, and Ions

In this set of practice problems, we will go over the atomic structure, subatomic particles, determining the number of protons, neutrons, and electrons, as well as writing correct chemical formulas.

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#### Practice

1.

All the atoms of a given element have the same ________.

a) number of neutrons
b) number of protons
c) atomic mass
d) number of electrons
e) volume

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2.

Which atom has the smallest number of protons?

a) Li
b) Mn
c) Be
d) Ag
e) As

a) Li

3.

Which atom has the largest number of protons?

a) 11B
b) 14C
c) 14N
d) 19F
e) 18O

d) 19F

4.

Which combination of protons, neutrons, and electrons is correct for the 59Ni isotope?

A) 29 p+, 30 n°, 29 e-
B) 28 p+, 30 n°, 28 e-
C) 28 p+, 31 n°, 28 e-
D) 28 p+, 32 n°, 31 e-
E) 27 p+, 32 n°, 27 e-

C) 28 p+, 31 n°, 28 e-

5.

How many electrons are in the O2- ion?

a) 9
b) 10
c) 8
d) 11
e) 6

b) 10

6.

What is the average atomic mass of the element X if it contains 92.23 % of isotope X-27.976, 4.683 % of  X-28.976, and 3.087 % X-29.973?

28.08 amu

Solution

In order to calculate the average atomic mass of the element, we need to multiply the abundance (decimal percentage) of each isotope by its atomic mass and sum these resulting masses.

amu (X) = 0.9223 x 27.976 + 0.04683 x 28.976 + 0.03087 x 29.973 = 25.80 + 1.357 + 0.9253 = 28.08

7.

Complete the following table using the periodic table of elements. Add the specific isotope with the charge where needed.  Solution

b1) This is the atomic number of Fe. Looking up in the periodic table, it is 26.

c1) The number of neutrons varies for isotopes. Therefore, you need to subtract the number of protons from the atomic mass to determine the number of neutrons for this specific isotope: 56 – 26 = 30

d1) For electrons, pay attention to the charge. 2+ means the Fe lost two electrons, and therefore, it has two electrons less than protons: e = 26 – 2 = 24

e1) The charge is given in this example.

a2) First, you need to identify the element based on the atomic number (protons). It is 13, so we have Al. It shows a 3+ charge, so it is the aluminum ion. and not the atom.

c2) The number of neutrons varies for isotopes. Therefore, you need to subtract the number of protons from the atomic mass to determine the number of neutrons for this specific isotope: 27 – 13 = 14.

d2) For electrons, pay attention to the charge. 3+ means the Al lost three electrons, and therefore, it has three electrons less than protons: e = 13 – 3 = 10.

a3) Normally, you’d look up the atomic number (protons) in the periodic table. However, it is not given here, so we need to determine the number of protons based on the number of electrons. Since it has no charge, these two must be equal. Therefore, there are 15 protons (based on d3) and we have phosphorous.

b3) From the periodic table, P has 15 protons. Again, we can also conclude this from the number of electrons since it is a neutral atom.

c3) The number of neutrons varies for isotopes. Therefore, you need to subtract the number of protons from the atomic mass to determine the number of neutrons for this specific isotope: 31 – 15 = 16.

b4) This is the atomic number of sulfur. Looking up in the periodic table, it is 16.

d4) For electrons, pay attention to the charge. 2- means the S gained two electrons, and therefore, it has two electrons more than protons: e = 16 + 2 = 18.

e4) The charge is given in this example – 2- (e4).

f4) Mass number is the sum of the number of protons and neutrons: 16 + 16 = 32.

a5) Like in (a3), we need to determine the number of protons based on the number of electrons. Since it has a 1+ charge, there must be one more proton than the electrons. So, it is 10 + 1 = 11, and we have Na+.

e5) The charge is shown on the symbol: Na+, so it is 1+.

f5) Mass number is the sum of the number of protons and neutrons: 11 + 12 = 23.

b6) This is the atomic number of Cl. Looking up in the periodic table, it is 17.

d6) There is no charge, and therefore, the number of electrons is equal to the number of protons – 17.