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Learning how to calculate the total mass of an atom using its subatomic particles.
If an atom were the size of a football stadium, the nucleus would be like a marble in the center, yet that tiny marble holds 99.9% of the entire atom's weight. How do scientists 'weigh' something so incredibly small?
In the world of atoms, not all particles are created equal. While protons and neutrons are the 'heavyweights' of the atom, electrons are so light they are practically weightless—like a speck of dust on a bowling ball. Because of this, we only look at the nucleus to find the atom's mass. The Mass Number () is the total count of these heavy particles. Since both protons and neutrons have a mass of approximately atomic mass unit (amu), adding them together gives us the atom's total 'weight.' Remember: electrons are ignored in this calculation because their mass is nearly zero.
Quick Check
If an atom has 8 protons and 9 neutrons, what is its mass number?
Answer
17
A Sodium (Na) atom has an Atomic Number of and a Mass Number of . How many neutrons does it have? 1. Identify (Mass Number): 2. Identify (Atomic Number): 3. Use the formula: 4. Result: Sodium has neutrons.
Quick Check
An atom has a mass number of 40 and an atomic number of 20. How many neutrons are in its nucleus?
Answer
20
It is easy to confuse the Atomic Number with the Mass Number, but they serve different purposes. The Atomic Number is the 'Social Security Number' of the element—it never changes. If you change the number of protons, you change the element itself! The Mass Number, however, is more like the 'Current Weight.' While the protons stay the same for an element, the number of neutrons can sometimes change (creating isotopes). This changes the mass but keeps the element's identity the same. Always check the Mass Number to see which 'version' of an atom you are looking at.
An unknown neutral atom has electrons and a Mass Number of . 1. In a neutral atom, . So, the Atomic Number () is . 2. Look at the periodic table: Atomic Number is Argon (Ar). 3. Calculate neutrons: . 4. If we added one more neutron, the Mass Number would become , but it would still be Argon!
Which subatomic particles are added together to find the mass number?
If an atom of Phosphorus has an atomic number of and a mass number of , how many neutrons does it have?
Changing the number of neutrons in an atom changes the element's identity.
Review Tomorrow
In 24 hours, try to recall the formula for finding neutrons and explain why we don't count electrons when calculating an atom's mass.
Practice Activity
Pick three elements from the periodic table. Find their atomic numbers and look up a common mass number for each, then calculate the number of neutrons for all three.