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The Bohr Model
Today’s Objectives
 To
Be Able To (TBAT) find the number of protons
and electrons for any given element
 TBAT
calculate the number of neutrons for any
element
 TBAT
use the 4 Step Method for Calculations
 TBAT
use the proper number of electrons in
orbitals for each element
 TBAT
identify the correct number of Valence
Electrons for each element
Electrons in Atoms
 We
know from our previous day that atoms are a
theoretical idea.
 Because
they are theoretical, we cannot show
them exactly as they are
 Instead we use models
 Models- a way of showing/picturing how
something works. Based on a Theory
Problems with the Rutherford
Model
 Rutherford’s
Model did not account for
the fact that opposite charges attract
 His
model did not explain why electrons
did not crash into the protons in an atom
 Because
refined.
of this, the model needed to be
Double Slit Experiment
 Discovered
that electrons act as both
waves and particles
 Due
to the characteristics of waves, they
had to exist at certain energy levels
(distances) from the nucleus.
Bohr Model:
From Denmark
 Niels
Bohr was a Danish
Physicist
 In 1913 made a
planetary model of the
atom:

Atoms have a nucleus in
the middle


Nucleus contains the ALL
the mass
Electrons exist around
the nucleus at certain
energy levels
Bohr’s Planetary Model

Nucleus in the middle

Electrons ‘orbit’ (go around) the
nucleus in orbitals


Orbital – area that electrons are
allowed to exist at.
Electrons are at exact distances
from the nucleus

Looks like planets orbiting the Sun
Rules of Bohr Diagrams
 1st
orbital – maximum of 2 electrons
 2nd
and 3rd orbital – maximum of 8 electrons
 Each
energy level must have a full amount
of electrons before they go in the next
orbital
Protons
 The
nucleus contains protons
and neutrons
 Protons
= positively charge
particles (p+)

Give an atom its identity
 Each
element has a different
number of protons
 The
Atomic Number is the
number of protons
10
Neutrons

Neutrons are found in the nucleus

The provide stability to the nucleus

Elements always have the same number
of protons but can have different numbers
of neutrons in their nucleus

The same element, with different numbers
of neutrons are called Isotopes
Atomic Mass
 The
atomic mass is the mass of an atom
 Since
the mass comes from the nucleus, we only
include the protons and neutrons
 Each
proton and neutron have about the same
mass so we call that mass an ATOMIC MASS
UNIT


1 proton has a mass of 1 amu
1 neutron has a mass of 1 amu
Calculating using the Atomic
Mass
Atomic Mass = protons + neutrons
A.M. = p+ + n0
Eg. How many neutrons are in an atom of carbon-13
(carbon atom with a mass of 13amu?)
Bohr Model Examples
 Aluminum
 Chlorine