GED Science
Atoms, Elements & Matter
Everything is made of atoms — a heavy little nucleus of protons and neutrons, orbited by nearly weightless electrons. The proton count alone decides which element you have. Matter changes state (solid, liquid, gas) when heat makes its particles move faster or slower, without ever changing what the substance is.
The words first
- proton
- positive particle in the nucleus — its count IS the element's identity
- neutron
- neutral particle in the nucleus — adds mass only
- electron
- negative particle orbiting outside — traded in reactions
- atomic number
- the proton count, the element's ID number
- element vs compound
- one kind of atom (O) vs kinds bonded together (H₂O)
The method
THE ATOM AT A GLANCE
nucleus: protons (+) and neutrons (0) — almost all the mass
shells: electrons (−) — almost all the size
neutral atom: electrons = protons
READING A PERIODIC TABLE SQUARE
6 ← atomic number = protons (the identity)
C
12.01 ← atomic mass ≈ protons + neutrons
neutrons ≈ mass − atomic number → 12 − 6 = 6
STATES OF MATTER — same stuff, different particle speed
solid particles locked, vibrating fixed shape
liquid particles sliding past each other takes container's shape
gas particles flying free fills the container
add heat → melt → evaporate; remove heat → condense → freeze
Ice, water, and steam are ALL still H₂O.
Worked all the way through
The problem
An atom has atomic number 11 and a mass number of 23. How many protons, neutrons, and electrons does it have (assume it is neutral)?
- Step 1 — Protons come straight from the atomic number.Atomic number 11 → 11 protons. (That also makes it sodium, whatever the other counts are.)
- Step 2 — Neutrons are the leftover mass.Mass number − atomic number = 23 − 11 = 12 neutrons.
- Step 3 — Neutral means charges cancel.Electrons must equal protons: 11 electrons.
On the board
atomic number 11 → 11 protons
identity
23 − 11 = 12 neutrons
mass minus protons
neutral → 11 electrons
matches the protons
Answer
11 protons, 12 neutrons, 11 electrons
A second one, in a different form
A puddle disappears on a hot afternoon. What happened to the water, at the particle level?
heat = particles speeding up
fastest surface particles escape as gas
evaporation
still H₂O — nothing new was made
physical change only
Answer
It evaporated — the heated particles escaped as water vapor, chemically unchanged
The usual mistake
Calling dissolving a chemical change. The sugar is still sugar.
Check yourself
Protons answer "what element?", mass-minus-protons answers "how many neutrons?", and "neutral" answers "how many electrons?". Three questions, three separate lookups — never let one number answer another number's question.
Practice it
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