Physical Science · Unit 2: Inside the Atom · Elise's Elected Education

Meet the atomic family — every atom is basically a tiny household with three roommates.

Proton lives in the middle (the nucleus) and is the loudest one. Proton has a positive charge (+1) and is heavy. Proton is also the family name: if you have 1 proton, your last name is Hydrogen. If you have 6, your last name is Carbon. If you have 79, your last name is Gold. Change the proton count and the whole family becomes a different element. You cannot change the proton count without becoming a different person.

Neutron also lives in the nucleus, right next to Proton. Neutron has no charge (neutral — hence the name). Neutron's main job is to keep Proton stable, because if you crammed a bunch of positive Protons together with nothing in between, they'd shove each other apart. Neutron is the chill mediator.

Electron is the wild one. Electron has a negative charge (−1) and is tiny — about 1/2000th the mass of Proton. Electrons don't live in the nucleus at all; they zoom around it in fuzzy regions called shells. They're so light and so fast that you can never pin one down to an exact spot — only roughly where it probably is.

Here's the wild fact: the nucleus is INCREDIBLY small compared to the whole atom. If an atom were the size of a football stadium, the nucleus would be the size of a marble at center field. Everything else is just electrons whizzing around in mostly empty space. That table you're touching? Mostly empty space pretending to be solid.

Atoms can swap roommates. Pick up an extra electron → you become an ion (charged atom). Swap out a neutron → you become an isotope (same element, slightly different weight). But change protons? Different element entirely. That's the one rule that can't bend.

An atom looks roughly like a sun with planets — the nucleus sits in the middle, and electrons orbit around it in shells.

• • •
• •
• ( + + ) •
• ( n n ) •
• •
• • •

center = nucleus (protons + neutrons) · dots = electrons in their shells

Each particle is built differently:

particle	charge	where	mass
proton	+1	nucleus	1
neutron	0	nucleus	1
electron	−1	shells around	~1/2000

Electrons fill shells from the inside out, and each shell has a maximum:

shell 1: holds 2 electrons
shell 2: holds 8 electrons
shell 3: holds 8 electrons (for the basics)

memorize this pattern: 2, 8, 8

Example: Oxygen has 8 electrons. Shell 1 takes 2. Shell 2 takes the remaining 6. Oxygen's outer shell needs 2 more to fill up — that's why it grabs electrons (and why it loves to react with stuff).

Example: Sodium has 11 electrons. Shell 1 takes 2. Shell 2 takes 8. Shell 3 has just 1 lonely electron — which Sodium happily gives away.

How to read the periodic table to find proton, neutron, and electron counts — every time, same steps:

Step 1. Find your element on the periodic table.

Step 2. Look at the atomic number (the small number, usually on top). That number = number of protons. Always. No exceptions.

Step 3. A neutral atom has the same number of electrons as protons. So if the atomic number is 6 (Carbon), it has 6 electrons too.

Step 4. The mass number (the bigger decimal number) is roughly protons + neutrons. To find neutrons: round the mass number, then subtract the atomic number.

Worked example — Carbon:

atomic number = 6 → 6 protons
mass number ≈ 12
12 − 6 = 6 neutrons
neutral atom → 6 electrons

Step 5. To find the electron shell arrangement, fill 2, then 8, then 8. Carbon has 6 electrons → 2 in shell 1, 4 in shell 2.

That's the whole method. Atomic number = protons = electrons (if neutral). Mass − atomic number = neutrons. Shells fill 2, 8, 8.

Inside the Atom: Protons, Neutrons, Electrons

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