Notes on Atomic Structure
Atoms – have 3 parts called subatomic particles
1. Protons – positive particles found inside the nucleus of an atom (1800 times larger than an electron)
2. Electrons – negative particles found outside the nucleus ( 1/1800 the size of a proton)
3. Neutrons – neutral particles found inside the nucleus (same size and mass as a proton)
Atoms have the same number of protons and electrons. The number of neutrons can vary.
REMEMBER! P E N An atom is made up of protons, electrons, and neutrons
Note: Protons are positive and neutrons act as the glue to hold them inside the nucleus….otherwise they
would repel (move away from) each other. (Remember! Opposites attract and like charges repel)
Note: This is the current model. With newer technologies we have learned that there are more than the 3 subatomic particles – protons, electrons and neutrons. Using a machine called a hadron collider, scientists have
been able to move protons at the speed of light and collide them into one another expecting to fuse together and
create a new particle. Instead the collision caused them to break apart into even smaller pieces they named
quarks. Out study will not take into account the structure of quarks.
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CHEMICAL SYMBOL – the international abbreviation of the element name –always 1 or 2 letters
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PROTONS MAKE AN ATOM WHAT IT IS
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ATOMIC NUMBER TELLS HOW MANY PROTONS IN AN ELEMENT
* Atomic Number = the number of protons in an atom
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ATOMIC MASS TELLS THE WEIGHT OF AN ELEMENT –(How many protons AND neutrons
are in the nucleus)
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The number of neutrons do NOT make a difference in the identity of an atom
Atoms of the same element have the same number of protons, but can have different numbers of
neutrons
ISOTOPES – are atoms of the same element with a different number of neutrons and different atomic
masses….example… Gold has anatomic number of 79…that means there are 79 protons (and 79
electrons as an atom has no charge and the protons and electrons cancel each other out)…it has an
atomic mass of 196.97 and since the all mass is in the nucleus of the atom that means there are 118
neutrons in the nucleus ( 197 – 79 = 118)…there are gold atoms, however, that have 117 neutrons with
still 79 protons…that is why the given atomic mass is 196.97…this means that on the average gold has
118 neutrons and 79 protons.
To name an isotope you write the name of the element , a dash, and the atomic weight…ex. Cl-37
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Atomic Structure continued
Inside the Atom – protons (positive charge) and neutrons (no charge/neutral)
Outside the Atom - the electrons (negative charge)
Hydrogen – the simplest of all atoms
 1 proton
1 electron
0 neutrons
 Atomic number is 1(1 proton)
Atomic mass is 1 (only 1 particle in the nucleus)
Helium
 2 protons
2 electrons
 Atomic number is 2
2 neutrons
Atomic mass is 4 (2protons + 2 neutrons in the nucleus)
Uranium
 92 protons
92 electrons
 Atomic number is 92
146 neutrons
Atomic Mass is 238 (92 protons + 146 neutrons)
NOTE* An atom’s number of protons, electrons and atomic number will always be the same
The same element will always have the same number of protons, but can have different
numbers of neutrons (Isotope)
ELECTRONS:
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Travel around the nucleus in orbitals or shells
The first shell – K shell – holds 2 electrons…then it is full
The second shell – L shell – holds 8 electrons until full
The third shell – M shell – holds 8 electrons until full
The fourth shell – N shell – holds 18 electrons until full
The fifth shell – O shell – holds 18 electrons until full
The sixth shell – P shell - holds 32 electrons until full
The seventh shell – Q shell 0 holds 32 electrons until full
Example: Calcium atom has 20 protons, 20 electrons, and 20 neutrons
That means there are 2 e on the first level, 8 e on the second level, 8 e on the third level, and 2 on the
fourth and outer level – (2 + 8 + 8 + 2 = 20)
Tungston atom has 74 protons, 74 electrons and 109 neutrons (atomic mass = 183)
That means there are 2 e on K shell, 8 e on L shell, 8 e on M shell, 18 e on N shell, 18 e on O shell,
20 e on P shell - ( 2 + 8 + 8 + 18 + 18 + 20 = 74)