Chemistry
Date:
§03.03a Atomic Number,
Most Important Idea(s):
Name:
Mass Number, and Isotopes
Background
ATOMS: All atoms of the same element have the same number of protons: the number of
protons determines the identity of the atom. For example, a carbon atom always has six
protons. If it has seven protons, it’s nitrogen, not carbon. The number of protons is called the
atomic number (Z).
ISOTOPES: Although all atoms of an element have the same number of protons, they don’t all
have the same number of neutrons. Isotopes are atoms of the same element that have different
masses (resulting from the different numbers of neutrons). An analogy is the flavors of ice
cream (even ones to which no specific flavor is added). Identifying an isotope requires knowing
both the atomic number of the element (Z) and the mass number (A) of the isotope. The mass
number is the total number of protons and neutrons that make up the nucleus of an isotope. For
example, carbon-14, commonly used to date biological objects (up to approximately 50,000
years old), has six protons (Z=6) and eight neutrons. To determine the number of neutrons in
an isotope:
Mass Number = Atomic Number + Number of Neutrons
For Carbon-14:
14
=
6
+
8
IONS: Just as isotopes have different numbers of neutrons, ions have different numbers of
electrons. Because protons are positively charged, and electrons are negatively charged, atoms
are considered neutral because the numbers of each are equal. However, ions have different
numbers of protons and electrons. For example, a magnesium ion with 12 protons (because it is
magnesium) but only ten electrons, the magnesium ion has a +2 charge (Mg2+). Similarly, then
chlorine ion, Cl–, has 17 protons and 18 electrons. When the coefficient has no number and only
the charge, we assume number is 1. (Similarly, we write H2O as such because it has two
hydrogen atoms and one oxygen atom.)
F:\2014-2015\330_ModChem\330_sections\330_03_Atoms_Hx\330.03.03a_Atomic'Number_Mass'Number_Isotopes_HW.docx (9/23/2014)
Chemistry
Isotopes
p. 2
Questions
1. Atomic number versus mass number:
A. Define mass number:
B. Define atomic number:
C. Which of these numbers (atomic number or mass number) can vary without changing
the identity of the element? Explain.
2. Define isotope.
3. Define ion.
4. Is an isotope always an atom? Explain.
5. Is an atom always an isotope? Explain.
31
31
32
6. Given the following isotopes 16 X , 15 X , 16 X ?
A. Which two of the following are isotopes of the same element? (Write the entire
isotopic symbol)
B. Explain.
C. What is the name of the element for these two isotopes?
7. A. What makes isotopes different for the same element?
B. What makes ions different for the same element?
C. What makes atoms different?
8. Fill in the table on the following page.
Chemistry
Element
Atomic Mass, Mass Number, and Isotopes
Atomic
Symbol
Isotope
Symbol
16
8
Atomic
Number
Mass
Number
3
7
Protons
p. 3
Number of
Neutrons
Electrons
Net
Charge
O
2
Chromium
28
24
52
6+
21
59Co3+
34
192
Antimony
46
76
36
2+
121
0
53
74
1–