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Atom Characteristics
• All matter (stuff with mass and volume) is
made of atoms.
• Space (without any particles) has no atoms.
• Atoms are made of smaller particles :
protons (+), neutrons ( ), and electrons (-).
• Protons, neutrons, and electrons, of course,
are made of even smaller particles…
Atomic Size
• Atoms are unbelievably tiny. How tiny?
Incredibly smaller than plant and animal
cells. Billions of atoms make up the writing
tip of your pen or pencil.
A key discovery leading to the current model of the
atom was that atoms contain charged particles.
The charge on a particle can be either positive or
negative. Particles with the same type of charge
repel each other—they are pushed apart. Particles
with different charges attract each other—they are
drawn toward each other.
The neutron has approximately the same mass
as a proton.
Review Time!!!!!!
The protons and neutrons of an atom are
grouped together in the atom’s center. This
combination of protons and neutrons is
called the nucleus of the atom. Because it
contains protons, the nucleus has a positive
charge. Electrons are negatively charged
particles that move around outside the
nucleus.
Atomic Numbers
If all atoms are composed of the same particles,
how can there be more than 100 different
elements? The identity of an atom is
determined by the number of protons in its
nucleus, called the atomic number.
Every hydrogen atom—atomic number 1—has
exactly one proton in its nucleus. Every gold
atom has 79 protons, which means the atomic
number of gold is 79.
Atomic Number and
Mass Number.
• The number of protons present in the
nucleus of an atom of an element is known
as its atomic number.
• The total number of protons and neutrons in
the nucleus of an atom of an element is
known as its mass number.
Atomic Mass Number
The total number of protons and neutrons in
an atom’s nucleus is called its atomic mass
number. While the atoms of a certain
element always have the same number of
protons, they may not always have the same
number of neutrons, so not all atoms of an
element have the same atomic mass
number.
Isotopes
• Atoms of the same element can be
different (they can have different #
of neutrons)
–Thus, different mass numbers.
• These are called isotopes.
Isotopes are atoms of the same element
having different masses, due to varying
numbers of neutrons.
Isotope
Hydrogen–1
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Protons Electrons
Neutrons
1
1
0
1
1
1
1
1
2
Nucleus