
The smallest part of an element that retains the properties of that element

How Small is an atom?
20,000,000,000,000,000,000,000 atoms of copper and zinc in a penny

What is an atom made of?

Protons:



Neutron:


Particles of the nucleus that have no electrical charge
Electrons:




Positively charged particles in the nucleus
The atomic number is = to the number of protons
Negatively charged particles outside the nucleus
Orbiting around the nucleus in energy levels, shells, or clouds
Electrons have smallest amount of mass
Atomic Mass Unit:

Used to express the masses of particles (protons + neutrons) in atoms


Charges of protons and electrons are opposite
but equal, so their charges cancel out.
Atoms have no overall charge


Unless the number of electrons and protons are
unequal
Ion:

Charged particle formed when electrons and protons
are not equal



More electrons / negatively charged ion
Less electrons/ positively charged ion
Remember: Electrons and Ions go together!




All atoms contain protons and
electrons
Most atoms contain neutrons,
but not all ex. hydrogen
An atom does not have to have
an = number of protons and
neutrons
Atomic Number:

The # of protons in the
nucleus

Isotope:




Have the same number of protons but have
different numbers of neutrons
Atoms that are isotopes of each other are always
the same element because isotopes always have
the same number of protons
Have different # of neutrons which gives them
different masses
Remember: Neutrons and Isotopes go together!!




Some isotopes are unstable
An unstable atom is an atom with a nucleus that will change
over time – radioactive
Radioactive atoms spontaneously fall apart over time
Telling Isotopes Apart:

You can tell isotopes of an element apart by its mass number




protons + neutrons =mass number (figure 5 page 92)
+ H ion
H atom
- H ion
Electrons are not included in the mass number because they are so
small that they have little effect on the elements atomic mass.
Atomic Mass:


the mass of an atom expressed in atomic mass units
The weighted average of the masses of all the naturally occurring
isotopes of that element.

Each atom has forces that act between the
particles





Gravitational force
Electromagnetic force
Strong force
Weak force
(page 94 figure 7)