# Atomic Structure Cornell Notes

```Cornell Notes 2.1 - Atomic Structure
October 2, 2014 - Pages 41 & 43
•Atoms are the smallest particles of matter possible.
•Each different kind of atom is called an element
•Element also means a substance made up of only one kind of atom
•Atoms are super small – about 10 billionths of a centimeter wide!
•To measure an atom’s mass, we use a unit called the atomic mass unit (AMU)
•One gram is equal to 6.02*1023 (60200000000000000000000) AMU
•An atom has two areas – the nucleus and the electron clouds
•The nucleus is the center of the atom (just like the nucleus of a cell…)
•The nucleus holds most of the atom’s mass
•The electron clouds are filled with particles orbiting the atom.
•These are fairly distant from the atom. If the nucleus was the size of a pea, the energy
clouds would be about two miles away
•The atom is made up of three kinds of subatomic particles
–Protons have a positive (+) charge
–Electrons have a negative (-) charge
–Neutrons have no charge
•Protons and Neutrons are found in the nucleus of the atom
•Electrons are found in the electron clouds orbiting the nucleus
•Protons and neutrons are about 1 AMU in mass (neutrons are a tiny bit bigger)
•Electrons are about 1/2000 of an AMU – in other words REALLY REALLY SMALL
•Protons give atoms their IDENTITY. In other words, if you have an atom with one
proton, it HAS to be a Hydrogen atom. Two protons means it HAS to be a Helium atom
and so on.
•This means all atoms of the same element have the same number of protons
•Electrons give atoms their PROPERTIES (except for mass)
•Since electrons have a negative charge and protons have a positive charge, electrons are
attracted to protons and that’s why they orbit the nucleus.
•You might notice that since protons are all pluses, they should repel, but there’s an even
stronger force called strong nuclear force that only acts when particles are close
•Since neutrons don’t have a charge, but they do have strong force, they act like glue,
holding the atom together.
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