Chemistry Examples

$A r$

Step 1

Find the element $Ar$ on the periodic table.

18	Atomic Number
Ar	Element Symbol
Argon	Element Name
39.95	Average Atomic Mass

Step 2

The atomic weight is a measurement of the total number of particles in an atom's nucleus including both protons and neutrons. So the first step is to locate the atomic mass on the periodic table, and round the value to the nearest whole number.

$39.95$ round to $40$

Step 3

The mass number is the number of particles in an atom's nucleus. The nucleus is made up of protons and neutrons.

$Atomic Mass = Number of Protons + Number of Neutrons$

Step 4

Find the element $Ar$ on the periodic table.

18	Atomic Number
Ar	Element Symbol
Argon	Element Name
39.95	Average Atomic Mass

Step 5

To find the number of protons in $argon$ , first locate the element on the periodic table. Next, find the atomic number which is located above the element's symbol. Since $argon$ 's atomic number is $18$ , $Ar$ has $18$ protons.

$18$ protons

Step 6

Fill in the known values where $N$ represents the number of neutrons.

$40 = 18 + N$

Step 7

Rewrite the equation as $18 + N = 40$ .

$18 + N = 40$

Step 8

Move all terms not containing $N$ to the right side of the equation.

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Step 8.1

Subtract $18$ from both sides of the equation.

$N = 40 - 18$

Step 8.2

Subtract $18$ from $40$ .

$N = 22$