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An atom has three parts:
"   Proton = positive
"   Neutron = no charge
"   Electron = negative
"   The proton & neutron are found in the center of the atom, a place called the nucleus.
"   The electrons orbit the nucleus in the electron cloud.

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Atoms are often referred to as the building blocks of matter.
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Each element on the periodic table is composed of one type of atom and cannot be broken down into a simpler substance.

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Atoms are composed of smaller subatomic particles such as the proton , neutron , and electron .
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Atoms contain a nucleus surrounded by an electron cloud that consists of one or more energy levels.

Nucleus - Small, dense, positively charged center of the atom which contains most of the atom’s mass

The nucleus contains the following subatomic particles :
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Protons - positively (+) charged particles
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Neutrons - particles that have no charge (neutral), but contribute to the atom’s mass

•   Electron cloud - an area around the nucleus where electrons are likely to be found orbiting the nucleus in several energy levels

•   The electron cloud contains several energy levels
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Electrons - negatively (-) charged particles located in specific energy levels surrounding the nucleus

•   Multiple energy levels in the electron cloud completely surround the nucleus.
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Electrons follow a specific order to fill the energy levels. After 18 it jumps to 32, 32, 18 and 2.
Maximum of 2 electrons
Nucleus
Maximum of 8 electrons Maximum of 18 electrons*
*Applies to the first
18 elements only

•   The electrons in the outermost energy level are called valence electrons

Atomic
Number
Atomic
Mass
6
C
12.0
Atom’s
Symbol
Atom’s
Name

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Step 1 - The Number of Protons is...
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The   atomic number   is the number of protons in an atom of an element. In our example, krypton's atomic number is 36.
This tells us that an atom of krypton has
36 protons in its   nucleus .

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Step 2 - The Number of Electrons is...
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By definition, atoms have no overall electrical charge.
That means that there must be a balance between the positively charged protons and the negatively charged electrons. Atoms must have   equal numbers of protons and electrons. In our example, an atom of krypton must contain 36 electrons since it contains 36 protons.

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Step 3 - The Number of Neutrons is...
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The   atomic weight   is basically a measurement of the   total number of particles in an atom's nucleus. All you really need to find is something called the   mass number. Round the atomic weight to the nearest whole number. In our example, krypton's mass number is 84 since its atomic weight, 83.80, rounds up to 84.

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For any element:
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Number of Protons = Atomic Number
Number of Electrons = Number of Protons = Atomic
Number
Number of Neutrons = Mass Number - Atomic Number
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For krypton example:
Number of Protons = Atomic Number = 36
Number of Electrons = Number of Protons = Atomic Number = 36
Number of Neutrons = Mass Number - Atomic Number = 84 - 36 = 48

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The number of protons in the nucleus is the atomic number of that atom. Protons are used to identify elements.
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The atomic number represents the number of protons (+) and is equal to the number of electrons (-).

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The atomic mass is the mass of the protons plus the mass of the neutrons .
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Atomic mass is recorded in the SI units: atomic mass units ( amu ).
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Protons and neutrons each are given an amu of 1.
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Electrons have a mass of nearly zero.

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Be prepared for a quiz next week over these notes.