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Cornell Notes 2.1 - Atomic Structure
October 14 - Page 41
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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
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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
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An atom has two areas – the nucleus and the energy levels
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 energy levels (also called 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
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The nucleus of an atom makes up almost all of its mass (about 99.99%), but only a very small
part of its volume (About 0.01%)
The electrons in the energy levels don’t take up much mass or volume at all, but the electrons
move so fast, that they seem like a cloud or shell
Most of the volume of an atom is the empty space between the nucleus and the energy levels is
empty space!
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The atom is made up of three kinds of subatomic particles
o –Protons have a positive (+) charge
o –Electrons have a negative (-) charge
o –Neutrons have no charge
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Protons and Neutrons are found in the nucleus of the atom
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Electrons are found in the electron clouds orbiting the nucleus
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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
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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)
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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.