Answers To Composition Of Atoms: The Sub-Atomic Particles Page 2
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4Answers to Isotopes and Average Atomic Mass
Question from Unit Outline:
Give three differences between mass number and average atomic mass:
Mass Number
Average Atomic Mass
(aka Relative Atomic Mass)
•
•
is the number of protons + neutrons in the
is the weighted average mass of all of the
nucleus of an atom
isotopes of an element (the mass is of the whole
atom, not just the nucleus so it includes the mass
of electrons)
•
•
is a counted value
is a measured value
•
•
it has no units
has the units amu or u
•
•
it is not reported on the Periodic Table
it is reported on the Periodic Table
Questions from page 29 of text:
2. the two particles which are responsible for most of the mass of an atom are the protons and neutrons
4. if an atom has 14 protons and 13 neutrons, its mass number is 27
5. if an atom has 15 protons and a mass number of 31:
a) its atomic number is 15
b) it has 16 neutrons (31 – 15 = 16)
c) the element is phosphorus (atomic number 15)
6. an atom of Cl – 37 has 20 neutrons in its nucleus. The “37” is the mass number of the atom and
chlorine has atomic number 17, so 37 – 17 = 20.
Questions from Handout:
2. a) What are isotopes?
Isotopes are atoms of the same element that have different numbers of neutrons. That is, isotopes
are atoms that have the same atomic number (same number of protons) but different mass numbers
(different numbers of neutrons).
b) Gold has four isotopes. Their mass numbers are 195, 196, 198 and 199. Find the number of
protons, electrons and neutrons in neutral atoms of these isotopes.
Isotope
Number of Protons
Number of electrons
Number of Neutrons
Au – 195
79
79 (neutral)
195 – 79 = 116 neutrons
Au – 196
79
79 (neutral)
196 – 79 = 117 neutrons
Au – 198
79
79 (neutral)
198 – 79 = 119 neutrons
Au – 199
79
79 (neutral)
199 – 79 = 120 neutrons
35
3. a) What information is given by each part of the expression
?
Cl
17
•
the Cl is the symbol for the atom chlorine
•
the “17” is the atomic number of chlorine which tells us it has 17 protons in the nucleus
•
the “35” is the mass number which tells us the number of protons + neutrons in the nucleus
b) We identify isotopes with symbols such as "U-238". What does the number "238" represent?
“238” is the mass number of that particular isotope of uranium.
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