Chemistry Lesson #6 – Subatomic Particles – The Difference between Mass and Weight

A Single Atom

A Single Atom

To recap, we know that everything you can see and touch, literally every physical object around you can be made from only three particles, called subatomic particles: protons, neutrons and electrons.

These subatomic particles combine in various numbers and combinations to form every atom in the universe, and these atoms combine in various numbers and combinations to form every molecule in the universe, and ultimately every physical thing in the universe. In the last post we discussed how incredibly small these particles are and gave a few examples to help think about their size. Now we need to discuss the second of the three properties which are most important to us: Mass.

When I talk about mass, usually the first thing people ask is: this is like weight, right? The answer is – sort of. When we speak of mass we are referring to the amount of actual physical stuff (which is referred to as “matter”), within an object. Weight does not refer to the amount of matter within an object, rather weight refers to the amount of force that an object exerts downward due to gravity.

Here’s an example of what I mean.

Let’s assume you are standing on a standard bathroom scale. Since, you are a physical object and the earth’s gravity is pulling you down onto the scale, then you are exerting a force unto that scale. The number you see in front of you, your weight, is a measure of the amount of force which you are exerting on the scale due to the influence of gravity on your body.

So what happens when you gently bounce up and down? The scale starts to jump all over the place. If you bounce up for a moment, the scale will show a lower weight, and when you come back down, you exert a greater force on the scale from your downward motion pushing down onto the scale, which causes the scale read a larger weight. But has the actual amount of matter of which you are composed actually changed? No not at all!

This brings us to mass. The mass is a measure of the total quantity of matter – the total quantity of physical stuff within the thing you are trying to measure. So what gives matter its mass? The answer is protons and neutrons! Remember, every physical and tangible object (this is matter) in our universe is composed of only three subatomic particles – protons, neutrons and electrons. And at some level, these three particles are responsible for every property of matter.

But recognize that I said the mass is the due to the protons and neutrons, and I did not include the electrons. That is because the mass of an electron – much like its size (which we learned about in the last lesson) – is incredibly small compared to that of protons and neutrons. So small in fact, that we tend to disregard it completely. In the next lesson, we will talk more about the actual mass of each of these three subatomic particles in greater detail.

So to recap:

  • The mass of an object is the quantity or amount of actual matter (or physical stuff) within that object.
  • The only way to change the mass of an object is add or take away matter (or physical stuff) from that object.
  • The weight of an object is the force that object exert downward on the earth due to both its mass and the influence of gravity on that object.
  • The mass of an electron is so much smaller than the mass of a proton or neutron that the mass of the electrons is usually disregard in chemistry.

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