End of chapter exercises
Write down only the word/term for each of the following descriptions.

The sum of the number of protons and neutrons in an atom

The defined space around an atom's nucleus, where an electron is most likely to be found
a) atomic mass number
b) electron orbital
For each of the following, say whether the statement is true or false. If it is false, rewrite the statement correctly.

${}_{10}^{20}\text{Ne}$ and ${}_{10}^{22}\text{Ne}$ each have 10 protons, 12 electrons and 12 neutrons.

The atomic mass of any atom of a particular element is always the same.

It is safer to use helium gas rather than hydrogen gas in balloons.

Group 1 elements readily form negative ions.
False. They both have 10 protons and 10 electrons, but ${}_{10}^{20}\text{Ne}$ has 10 neutrons and ${}_{10}^{22}\text{Ne}$ has 12 neutrons.
 True.
 True.
False. Group 1 elements readily form positive ions.
The three basic components of an atom are:

protons, neutrons, and ions

protons, neutrons, and electrons

protons, neutrinos, and ions

protium, deuterium, and tritium
protons, neutrons, and electrons
The charge of an atom is:

positive

neutral

negative

none of the above
If Rutherford had used neutrons instead of alpha particles in his scattering experiment, the neutrons would:

not deflect because they have no charge

have deflected more often

have been attracted to the nucleus easily

have given the same results
not deflect because they have no charge
Consider the isotope ${}_{92}^{234}\text{U}$. Which of the following statements is true?

The element is an isotope of ${}_{94}^{234}\text{Pu}$

The element contains 234 neutrons

The element has the same electron configuration as ${}_{92}^{238}\text{U}$

The element has an atomic mass number of 92
 The element has the same electron configuration as ${}_{92}^{238}\text{U}$
The electron configuration of an atom of chlorine can be represented using the following notation:

$1{\text{s}}^{2}2{\text{s}}^{8}3{\text{s}}^{7}$

$1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{6}3{\text{s}}^{2}3{\text{p}}^{5}$

$1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{6}3{\text{s}}^{2}3{\text{p}}^{6}$

$1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{5}$
Give the standard notation for the following elements:

beryllium

carbon–12

titanium–48

fluorine
Beryllium has 4 protons and 5 neutrons. So the number of nucleons is 9. The standard notation is: ${}_{4}^{9}\text{Be}$
Carbon has 6 protons and 6 neutrons. So the number of nucleons is 12. The standard notation is: ${}_{6}^{12}\text{C}$
Titanium has 22 protons and 26 neutrons. So the number of nucleons is 48. The standard notation is: ${}_{22}^{48}\text{Ti}$
Fluorine has 9 protons and 10 neutrons. So the number of nucleons is 19. The standard notation is: ${}_{9}^{19}\text{F}$
Give the electron configurations and Aufbau diagrams for the following elements:

aluminium

phosphorus

carbon

oxygen ion

calcium ion
a) Aluminium has 13 electrons. So the electron configuration is: $$
b) Phosphorus has 15 electrons. So the electron configuration is: $$
c) Carbon has 6 electrons. So the electron configuration is: $$
d) Oxygen has 8 electrons. The oxygen ion has gained two electrons and so the total number of electrons is 10. The electron configurationis: $$
e) Calcium has 20 electrons. The calcium ion has lost two electrons and so the total number of electrons is 18. The electron configuration is: $$
For each of the following elements give the number of protons, neutrons and electrons in the element:

${}_{78}^{195}\text{Pt}$

${}_{18}^{40}\text{Ar}$

${}_{27}^{59}\text{Co}$

${}_{3}^{7}\text{Li}$

${}_{5}^{11}\text{B}$
a) Z = 78 and A = 195. So the number of protons is 78 and the number of neutrons is $N=AZ=19578=117$ . Since the element is neutral the number of electrons is also 78.
b) Z = 18 and A = 40. So the number of protons is 18 and the number of neutrons is $N=AZ=4018=22$ . Since the element is neutral the number of electrons is also 18.
c) Z = 27 and A = 59. So the number of protons is 27 and the number of neutrons is $N=AZ=5927=32$ . Since the element is neutral the number of electrons is also 27.
d) Z = 3 and A = 7. So the number of protons is 3 and the number of neutrons is $N=AZ=73=4$ . Since the element is neutral the number of electrons is also 3.
e) Z = 5 and A = 11. So the number of protons is 5 and the number of neutrons is $N=AZ=115=6$ . Since the element is neutral the number of electrons is also 5.
For each of the following elements give the element or number represented by x:

${}_{45}^{103}\text{X}$

${}_{\mathrm{x}}^{35}\text{Cl}$

${}_{4}^{\mathrm{x}}\text{Be}$
a) This element has 45 protons and atomic mass number of 103. Looking on the periodic table we find that the element with 45 protons and atomic mass number of 103 is Rhodium (Rh).
b) We are given that chlorine has 35 protons and neutrons. We need to find out how many protons it has. From the periodic table we find that x = 17
c) We are given that beryllium has 4 protons. We look at the atomic mass given on the periodic table and see that it is 9. So x = 5.
Which of the following are isotopes of ${}_{12}^{24}\text{Mg}$:

${}_{25}^{12}\text{Mg}$

${}_{12}^{26}\text{Mg}$

${}_{13}^{24}\text{Al}$
b) is the only correct answer. An isotope has the same number of protons, but a different number of neutrons. c) is a different element and a) has 25 protons, but 13 neutrons, which is not chemically possible.
If a sample contains 69% of copper–63 and 31% of copper–65, calculate the relative atomic mass of an atom in that sample.
The contribution from copper63 is: $\frac{69}{100}\times 63=43,47$
The contribution from copper45 is: $\frac{31}{100}\times 65=20,15$
Now we add the two values to get the relative atomic mass:
$43,47+20,15=63,62u$
This value is slightly higher than the one on the periodic table which is mainly due to rounding errors.
Complete the following table:

We first write down the number of electrons for each element:
B: 5 electrons, Ca: 20 electrons, Si: 14 electrons, Li: 2 electrons, Ne: 10 electrons
For the electron configuration we use the filling rules to write the electron configuration.
The core electrons are the electrons in the innermost shell. The valence electrons are the electrons in the outermost shell.
Element or ion  Electron configuration  Core electrons  Valence electrons 

Boron (B)  $1{\text{s}}^{2}2{\text{s}}^{1}$  2  3 
Calcium (Ca)  $1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{2}3{\text{s}}^{2}3{\text{p}}^{6}4{\text{s}}^{2}$  18  2 
Silicon (Si)  $1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{2}3{\text{s}}^{2}3{\text{p}}^{2}$  10  4 
Lithium ion (${\text{Li}}^{+}$)  $1{\text{s}}^{2}$  2  2 
Neon (Ne)  $1{\text{s}}^{2}2{\text{s}}^{2}2{\text{p}}^{6}$  10  8 