Structure of the
Atom
Structure of the Atom
• Subatomic particles are smaller than
the atom
• The three main subatomic particles are
the proton, neutron, and the electron
Nucleus
• Nucleus also called the core contains 99.9% of the
mass of the atom
Contains protons and neutrons
Has a positive charge
Protons
• Positively charged
particle
• They are identical
in every element
• Mass is 1 atomic
mass unit (amu)
Neutrons
•
•
•
•
Have no charge
All neutrons are identical in every element
Mass 1 amu
Number of neutrons may vary from
atom to atom
Quarks
• Quarks are the basic building block of
protons & neutrons
Atomic number
• The number of protons in the nucleus
determines the atomic number of the
element
Isotopes
• Atoms of the same element will have the same
number of protons, but can have different
numbers of neutrons
Hydrogen Isotopes
Mass Number and
Atomic Mass
• The mass number is the
sum (+) of the protons and
neutrons
• Used to distinguish one
isotope from another
• Atomic mass is the
average mass of all the
isotopes of an element
The Electrons
•
•
•
•
Electrons whirl around the nucleus
The mass is smaller than a proton or neutron
Have a negative charge
Spin around in energy levels billions of times a
second!
• They move so fast that the space they spin in
sometimes is referred to as an electron cloud
Each level/shell can hold a certain number of electrons
1st 2 electrons
2nd 8 electrons
3rd 18 electrons
4th 32 electrons
The further away from the
nucleus an electron is
the more energy it has
Valence Electrons: electrons on
the last level/shell that
determines bonding
The number of electrons an atom has
determines its chemical properties
Vocabulary Review Time
Counting subatomic particles
To find the number of subatomic particles follow
the rules below:
• Protons, Electrons and the Atomic Number
are all the same number
• Neutrons = Atomic Mass- Atomic Number
• Atomic Mass Number = Atomic Number +
neutrons
Practice Time
Drawing Atoms
1. Find the number of protons, neutrons and
electrons.
2. Draw a circle and label a P= and N= next to the
equal signs write in the # of protons and neutrons
3. Draw your “shells” and place an e- for each
electron, remember 2,8,18, 32
e-
e-
Magnesium (Mg)
eeee-
P= 12
e-
P= 12
N= 12
e-
N= 12
e-
e- e-
e-
e= 12
Counting Atoms
• The symbol of an element represents one atom of
that element.
• Subscript: # written at the lower right corner behind
the symbol of an element. The subscript is used to
indicate the number of atoms.
• Parentheses: A subscript outside a bracket multiples
all the elements inside the brackets.
• Coefficient: # written in front of a chemical symbol.
Indicates the # of atoms of that element or number of
molecules
• Coefficient multiples the # of molecules of each
chemical formula
4 Forces within the atom
1. The Strong Force: responsible for binding of
nuclei.
2. The Electromagnetic Force: The force
which exists between all particles which
have an electric charge.
3. The Weak Force: responsible for nuclear
decay
4. The Gravitational Force: On the scale of
atomic world it is of no importance at all.