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4-3 Atomic Structure
Elements Contain a Single Kind of
Atom
• Robert Brown– Discovered direct evidence of the existence of
atoms (1827).
– His discovery is named after him.
– Brownian Motion – perpetual jiggling of
particles resulting from the collisions between
visible particles (ie: dust) and invisible atoms.
Brownian Motion in action
More Brownian Motion In Action
Elements Contain a Single Kind of
Atom
• Protons, Neutrons, and Electrons
(subatomic particles)
– Nucleus – The center of the atom, contains
almost all of the mass of the atom. Contains
both neutrons and protons.
– Electron Cloud – Region outside the nucleus
where the electrons travel. Sometimes
referred to as electron shells.
Typical model of a Helium atom
In this model, we see the very small nucleus surrounded by a
“cloud” of electrons whose shading relates to the likelihood
of finding an electron at a given point at a given time.
Atoms are Mostly Empty Space
• Atoms contain mostly empty space due to
their volumetrically large electron clouds.
• Matter doesn’t “overlap” because rather
than coming into complete contact, the +/repulsions prevent this, leaving an
infinitely small gap between two objects
moving toward one another.
The Nucleus is Made of Protons
and Neutrons
• Proton (p+) – Found in the atomic nucleus
– Carries a positive charge (1+)
– About 2000 times more massive than an electron.
• Neutron (no) – Found in the nucleus
– Carries a neutral charge (0)
– About the same mass as a proton.
• Nucleons- a term that represents the protons
and neutrons in the nucleus
The Nucleus is Made of Protons
and Neutrons
• The number of protons in any atom of the
same element will be the same, however,
the number of neutrons can vary from
atom to atom
– This means that the number of protons in an
atom determine the identity of that atom.
• The number of protons (positive charge)
equals the number of electrons (negative
charge) in an atom. (All atoms are neutral
in charge)
The Nucleus is Made of Protons
and Neutrons
• Table of subatomic particles:
Particle
Charge
Mass (in
relationship to
an electron
Amu (Atomic
Mass Unit)
Proton (p+)
1+
1,836 times the
mass of an
electron
1
Electron (e-)
1-
1
0
Neutron (no)
0
1841
1
Protons and Neutrons Determine the
Mass Number and Atomic Mass
• Atomic Mass - The average mass of all
the different isotopes of an element.
(Usually not a whole number)
• Mass Number – The sum of the number
of protons and neutrons. It is usually
equal to the rounded atomic mass and is
always expressed as a whole number.
Protons and Neutrons Determine the
Mass Number and Atomic Mass
• Things to remember:
– Mass number = # or protons + # of neutrons
– Atomic number = # of protons
– # of protons = # of electrons (for neutral
atoms)
Protons and Neutrons Determine the
Mass Number and Atomic Mass
• Isotope notation – a way to express the
number of protons and mass number of a
specific atom of an element.
• Example: Lithium
Mass #  7
Li
Atomic #  3
Protons and Neutrons Determine the
Mass Number and Atomic Mass
• Chlorine
Mass # 35
Cl
Atomic # 17
Now You Try
• Osmium
190
76
Os