# 2.2.1: Practice Problems- The Structure of the Atom and How We Represent It (Optional)

PROBLEM $$\PageIndex{1}$$

In what way are isotopes of a given element always different? In what way(s) are they always the same?

They always have different masses due to different numbers of neutrons.

They always have the same number of protons (which determines the identity).

PROBLEM $$\PageIndex{2}$$

Write the symbol for each of the following ions:

(a) the ion with a 1+ charge, atomic number 55, and mass number 133

(b) the ion with 54 electrons, 53 protons, and 74 neutrons

(c) the ion with atomic number 15, mass number 31, and a 3− charge

(d) the ion with 24 electrons, 30 neutrons, and a 3+ charge

$$\ce{^{133}_55Cs}$$+

$$\ce{^{127}_53I}$$-

$$\ce{^{31}_15P}$$3-

$$\ce{^{57}_27Co}$$3+

PROBLEM $$\PageIndex{3}$$

Write the symbol for each of the following ions:

(a) the ion with a 3+ charge, 28 electrons, and a mass number of 71

(b) the ion with 36 electrons, 35 protons, and 45 neutrons

(c) the ion with 86 electrons, 142 neutrons, and a 4+ charge

(d) the ion with a 2+ charge, atomic number 38, and mass number 87

$$\ce{^{71}_31Ga}$$3+

$$\ce{^{80}_35Br}$$-

$$\ce{^{232}_90Th}$$4+
$$\ce{^{87}_38Sr}$$2+