Chemistry - A Molecular Science

Chapter 1 The Early Experiments

Example 1.10

Identify each of the atoms or ions below based on the number of protons and electrons they contain.

Protons Electrons

a) 11 10 b) 16 18 c) 26 26

a) This element contains 11 protons

, so it must be Na (sodium). T

he fact that it

contains only

10 electrons means that it carries a char

ge of 11 - 10 = +1. Thus, the species is Na

1+, a

cation or positively-charged ion.

b) An atomic number of 16 signifies sulfur. It

has gained two electrons to carry a charge of

16 - 18 = -2. It is the S

2- anion or negatively-charged ion.

c) Fe (iron) has an atomic number of 26 and,

because the number of

electrons equals the

number of protons, this species is the Fe atom.

Example 1.11

How many protons, neutrons, and electrons are in each of the following?

a)

37 Cl

1-^

Chlorine’s atomic number is 17, so it has

17 protons. The number of neutrons (N) in an

element is the difference between mass number (A

) and the atomic number (Z): N = A - Z

= 37 - 17 = 20 neutrons.

37 Cl

1- has a charge of -1. Charge e

quals (the number of protons)

• (the number of electrons), so -1 = 17 – x for this ion. Solving for x, we find that there are x = 18 electrons in the ion.
  b)

40 Ca

2+^

Z = 20 for calcium, so it must have 20 protons. A= 40, so N = 40 - 20 = 20 neutrons for this isotope. The +2 charge means that th

e number of electrons is two less than the

number of protons. Thus, there are 18 electrons in this ion.

c)

127

I

There is no charge, so the num

ber of electrons equals the number of protons. Z = 53, so

the atom contains 53 protons

and 53 electrons. In addition, A = 127, so this isotope of

iodine contains 127 - 53 = 74 neutrons.

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