Atomic Structure
Objectives
• Explain Dalton’s atomic theory
• Be able to state the charge, mass, &
location of each part of an atom
• Be able to compare & contrast Bohr’s model
with the modern model of the atom
• Explain orbitals, their structure, & valence
electrons
History of Atoms
• The idea of the atom first came from a
Greek philosopher Democritus nearly 2500
years ago
• It started with a set of simple questions,
“If you take a piece of chalk and break it in
half, are both halves still chalk? If you
continue to divide each piece, is there a
limit to the division before it becomes
something else?”
• Democritus thought there was a limit which
he called atomos, meaning “unable to be
cut”
Models of the Atom
• Democritus
• John Dalton
• Niels Bohr’s
Democritus' Theory of Atoms
1. All matter consists of invisible particles
called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position,
and arrangement.
• He believed that the movements of atoms
caused the changes in matter he observed
• However, he had no way of proving the
existence of atoms
John Dalton’s Atomic Theory 1808
1. All matter consists of atoms
2. Atoms are indestructible & unchangeable
3. Elements are characterized by the weight
of their atoms
4. In chemical reactions, atoms combine in
small, whole number ratios
5. When elements react, their atoms may
combine in more than one whole-number
ratio
Characteristics of Atoms
• Are the building blocks of molecules
• Smallest part of an element that still has
the element’s properties
• Unreacted atoms have no overall charge
• Atoms have 5 basic parts
• Atoms are made of protons, neutrons, &
electrons (subatomic particles)
• The protons & neutrons are housed in the
center of the atom in the nucleus
• Electrons are moving around outside of the
nucleus within the electron cloud
Atom Model
Subatomic Particles
• Protons have a __________ charge
• Neutrons have ___________ charge
• Electrons have a __________ charge
• The number of protons and electrons for an
atom is unique for each element.
Bohr’s Model of the Atom
• In 1913, Niels Bohr suggested that electrons
in an atom move in set paths around the
nucleus
• “Like planets in orbit”
• The path defines the electron’s energy level
1. Electrons can only be in certain energy
levels
2. Electrons must gain energy to move to a
higher energy level
3. Electrons must lose energy to move to a
lower level
Bohr’s Model
Modern Atomic Model
• By 1925, Bohr’s model didn’t explain
electron behavior
• The new model proposed that electrons
behave like waves on a vibrating string
• This is known as the “wave-particle duality
of nature”
• This model was developed by Louis de
Broglie, Albert Einstein & Max Planck
Modern Atomic Model
• An electron’s exact location can not be
determined nor can its speed or direction
• Electrons are constantly moving
• To illustrate the location of electron we now
use the electron cloud
• Electrons still exist in energy levels
• The number of filled energy levels depends on
the number of electrons
Energy Levels
• 1st energy level = 2 electrons
• 2nd energy level = 8 electrons
• 3rd energy level = 18 electrons
• 4th energy level = 32 electrons
• For example: Na has 11 electrons
• 2 in level 1
• 8 in level 2
• 1 in level 3
Orbital Theory
• Electrons are found in orbitals within
energy levels
• Within each level, electrons occupy orbitals
that have the lowest energy
• 4 different kinds of orbitals
•s
•p
•d
•f
s Orbitals
• Are the simplest
• Are spherical in shape
• Have the lowest
energy
• Hold only 2 electrons
p Orbitals
• Are dumbbell shaped
• They can be oriented three different ways
in space
• Has more energy than a s orbital
• Each p orbital can hold 2 electrons for a
total of 6 electrons
p Orbitals
d & f Orbitals
• Are much more complex
• There are 5 possible d orbitals
• There are 7 possible f orbitals
• f orbitals have the greatest energy
• Each orbital holds a maximum of 2
electrons
d Orbitals
f Orbitals
Relationship between Levels
• 1st energy level has 1 sublevel = s
So it contains 1 orbital and holds 2 electrons
• 2nd energy level has 1 s orbital and 3 p
orbitals
So it can hold 8 electrons
• 3rd energy level has s, p, & d orbitals
So it can hold 18 electrons
• 4th energy level has s, p, d, & f orbitals
So it can hold 32 electrons
Valence Electrons
• Every atom has between 1 and 8 valence electrons
• Valence electrons are electrons in the outermost
energy level of an atom
• They determine an atom’s chemical properties and
its ability for form bonds
• For example: Neon
Has 10 electrons
2 electrons in the lowest level
8 electrons in the 2nd level
Thus, it has 8 valence electrons!
Quantum Model