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5 Mass of Protons, Neutrons, and Electrons
We have to look up this number. Note that the sum of the individual
masses of 8 protons and 10 neutrons alone should be larger than
18 u, but an actual O-18 atom has a mass slightly less than 18 u.
Most elements have two or more naturally-occurring isotopes. The masses of specific
isotopes are typically not listed in simple periodic tables; what is typically listed is the
average mass of atoms in a naturally-occurring sample. The numerical value of the average mass (in u) is called the atomic weight. For example, the atomic weight of Mg is often
listed as 24.305. This means that, in a naturally-occurring sample of Mg, the average mass
of the atoms is 24.305 u; there is not a single Mg atom that actually has a mass of 24.305 u.
If we want information about specific isotopes of an element, we need to look it up
from more extensive data sources. For example, we can look up the following information
about naturally-occurring isotopes of Mg from webelements.com:
• Mg-24, mass = 23.99 u, natural abundance = 78.99%.
• Mg-25, mass = 24.99 u, natural abundance = 10.00%.
• Mg-26, mass = 25.98 u, natural abundance = 11.01%.
A natural abundance of 78.99% for Mg-24 means that about 79 out of every 100 atoms
have a mass of 23.99 u. The average mass of can be calculated from the information above
using the formula:
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Average Mass = Sum of (relative abundance × mass)
In other words,
Average Mass of Mg = (78.99%)(23.99 u) + (10.00%)(24.99 u) + (11.01%)(25.98 u)
= 24.31 u
Electrons and protons are said to be electrically-charged, while neutrons are electricallyneutral. The unit for electric charge is the Coulomb (official abbreviation: C). Electrons
carry the smallest magnitude of charge known, which is −1.602 × 10−19 C. This amount
of charge is usually assigned a value of “−1” (atomic units). Protons have a charge of
+1.602 × 10−19 C, or “+1” (atomic units). Neutrons are not charged (charge = 0). All atoms
are electrically neutral. This means that they have equal numbers of protons and electrons.
An atom has 10 protons. How many electrons does it have?
Answer: 10 electrons
Chemistry: The Core Concepts