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5 Mass of Protons, Neutrons, and Electrons

5 Mass of Protons, Neutrons, and Electrons

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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


Electrical Charge

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

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