Mini-summary moment:

• Part of mass of atom/molecule is lost as energy, called mass defect
• Energy used to keep the substance stable
• Substance is kept stable through strong nuclear force, powered by energy
• We can calculate mass defect and energy it gives
  • Mass = Predicted mass - Actual mass
  • $E = mc^2$ (Einstein's theory)
• This calculation gives us binding energy, amount of energy that causes strong nuclear force to keep the entire atom together, as well as break it apart

Lesson moment:

We have special types of mass measurements for subatomic particles/constituents and such, named atomic mass unit or amu (wow so creative).

This measurement surprisingly is normally not displayed with units in the periodic table as it is standard to just display the number, though it does have a unit which is, as said before amu.

Though generally in chemistry (for now) we generalise a proton and neutron to have 1 amu while an electron has a negligible mass, we can better understand this lesson by calculating the amu's of the subatomic particles to the eighth decimal place.

A proton is 1.00727647 amu, a neutron has 1.00866490 amu and an electron is 0.000548597 amu (to ninth decimal place for the sake of the lesson).

Btw, the measurement is based on 1/12 of the mass of an atom of carbon-12 (the carbon you see on periodic table), so obviously carbon-12 is 12 amu.

Let's take a lithium-6 atom as an example; take a look at the diagram below (excalidraw my beloved):

(Btw this is Bohr’s atomic model which is a) wrong [the actual model has proved to be inaccurate] and b) the electron shells are supposed to be full circles and not broken lines)

As we can see, the nucleus consists of protons and neutrons, which are often referred to as nucleons, but is quite unnecessary as you can tell. Electrons orbit around the nucleus in electron cloud.

The strange thing is, according to laws of electrostatic forces (or magnetism for simpletons), since all the protons are positively charged, wouldn't they be repelling each other? How is it possible that they are able to stay together? Astute observation, we will get into that around the end of the lesson.

For now, let's get back to amu. Calculate the amount of amu's (btw the numbers after the element refer to the isotope of it. Lithium-6 is the lithium you see on the periodic table)