2.1.d Bohr Model

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UNIT 2 - ATOMS & THE
PERIODIC TABLE
2.1.d Bohr’s Model of
Electron Arrangement
Section 2.1 Models of the Atom
BOHR’S MODEL OF ELECTRON ARRANGEMENT
You have seen that electrons occupy the space outside the nucleus of an atom. When atoms approach
one another, it is their electrons that interact. The first widely accepted explanation for the
arrangement of these electrons was put forward by a Danish physicist, Niels Bohr (1885-1962)
(shells)
In 1913, Bohr proposed a model of the
atom that helped explain the movement
of the electrons around the nucleus of an
atom. He suggested that the electrons
exist around the nucleus of an atom in
specific energy levels. Each energy level
is a certain distance from the nucleus and
can contain a certain maximum number of
electrons. Energy levels are often referred
to as shells.
Bohr said that electrons can exist only in these energy levels or shells, but they could not exist in
between the shells!
Because electrons are negative, they are attracted to the positive nucleus. Therefore electrons will fill
the shells closest to the nucleus before entering shells that are further away. The maximum number of
electrons in each shell are shown in Table 1.
Shell
1
2
3
4
Maxiumum
number of
electrons
2
8
8
(18*)
*The Bohr model kind of breaks down at the 4th shell and so you’ll not be asked to place more than 1 or 2 electrons in it.
For example, the lithium atom , which has the atomic number 3, has 3 electrons. The first two
electrons will be in the first shell, at which point the first shell will be full. The third electron will be in
the second shell.
atomic number = # protons
= # of electrons in
a neutral atom
UNIT 2 - ATOMS & THE
PERIODIC TABLE
Section 2.1 Models of the Atom
2.1.d Bohr’s Model of
Electron Arrangement
The element nitrogen, which has the atomic number of 7, has 7
electrons. Two of the electrons are in the first orbital and the
other 5 electrons are in the second orbital.
atomic number
= # protons
= # of electrons in
a neutral atom
Notice the position of the electrons in the second shell: at the top, bottom, left and right of the
diagram.
•
•
When placing electrons, it can be helpful to think of the atom as a hotel, and the shells as the
floors of the hotel.
Each floor has a certain number of rooms.
First “floor” has only ______ room:
Second “floor” has ______ rooms:
Third “floor” has _______ rooms:
•
This hotel has no elevator, so electrons will fill the lower floors first
•
Each room can accommodate 2 electrons, maximum.
•
Nobody wants to share a room if they don’t have to, so electrons will slot in singly (one at a
time) at first and only pair when there are no more empty rooms on the lowest available
floor
•
I like to start at the top and go around the shell like a clock when filling in the e−
UNIT 2 - ATOMS & THE
PERIODIC TABLE
NOTE:
2.1.d Bohr’s Model of
Electron Arrangement
Section 2.1 Models of the Atom
We often write the number of protons and neutrons in the nucleus, but for this worksheet
you don’t have to.
Ex. Oxygen is atomic number 8, so an oxygen atom has 8 electrons.
The numbers below show the order in which they fill in:
3 7
1 2
6
4
8
5
If we show all the numbers, the order of filling looks like this:
19
11 15
3 7
1 2
14
18
66
4
12
10
8
16
55 9
13
5 917
NOTE:
Bohr models don’t really work after element #20, so we stop there.
20
UNIT 2 - ATOMS & THE
PERIODIC TABLE
2.1.d Bohr’s Model of
Electron Arrangement
Section 2.1 Models of the Atom
Exercises:
1.
Give 2 names for the “circles” on which elements “live” in the Bohr Model.
2.
How many electrons are there maximum in the (a) 1st shell __________
(b) 2nd shell __________
(c) 3rd shell __________
3.
If you imagine an atom to be like a hotel, how many “rooms” are there to hold electrons
on the
(a) 1st shell ________
(b) 2nd shell ________
(c) 3rd shell ________
4.
How many electrons can each “room” hold? ____________
5.
Electrons go into the rooms o_______ at a time, and only p___________ up when they have
to.
6.
Electrons always go into the
shell when that one is full.
7.
How can you find the number of electrons in a neutral atom?
8.
Write numbers in the boxes on the Bohr
model shown here, in the order of filling.
Start by writing the number 1, then
the number 2 and continue up to #20.
highest / lowest
circle the correct answer
Order of filling:
Start in the lowest shell.
Electrons go into orbitals (boxes)
singly and don’t pair up until all
orbitals in an energy level (shell)
contain 1 electron.
Only move to the next shell when all
the orbitals in a lower shell are filled.
Start at the top and go around like a clock.
shell first, and only go into the next
UNIT 2 - ATOMS & THE
PERIODIC TABLE
9.
Section 2.1 Models of the Atom
2.1.d Bohr’s Model of
Electron Arrangement
Use the periodic table to find the # of electrons for each atom and write that on the
line. (The number of electrons is equal to the atomic number for that element.)
Draw in spots or small circles to represent the correct number and placement of
electrons in each shell (energy level).
UNIT 2 - ATOMS & THE
PERIODIC TABLE
Section 2.1 Models of the Atom
2.1.d Bohr’s Model of
Electron Arrangement
UNIT 2 - ATOMS & THE
PERIODIC TABLE
2.1.d Bohr’s Model of
Electron Arrangement
Section 2.1 Models of the Atom
2.1.d KEY
ANSWERS:
1.
Give 2 names for the “circles” on which elements “live” in the Bohr Model.
① shells
② energy levels
2.
How many electrons are there maximum in the (a) 1st shell __________
2
nd
(b) 2 shell __________
8
rd
8
(c) 3 shell __________
3.
If you imagine an atom to be like a hotel, how many “rooms” are there to hold electrons
on the
1
(a) 1st shell ________
4
(b) 2nd shell ________
4.
2
How many electrons can each “room” hold? ____________
5.
Electrons go into the rooms one
6.
Electrons always go into the
shell when that one is full.
7.
How can you find the number of electrons in a neutral atom?
at a time, and only pair up when they have to.
highest / lowest
shell first, and only go into the next
circle the correct answer
The # of e– in a neutral atom is equal to the
8.
4
(c) 3rd shell ________
# of p+ = atomic #
Write numbers in the boxes on the Bohr
model shown here, in the order of filling.
Start by writing the number 1, then
the number 2 and continue up to #20.
(Remember, the Bohr model
breaks down after
element #20, Ca.)
19
11 15
3 7
1 2
14
18
66
10
55 9
13
5 917
4
12
8
16
20
UNIT 2 - ATOMS & THE
PERIODIC TABLE
Section 2.1 Models of the Atom
2.1.d Bohr’s Model of
Electron Arrangement
2.1.d KEY
9.
Use the periodic table to find the # of electrons for each atom and write that on the
line. (The number of electrons is equal to the atomic number for that element.)
Draw in spots or small circles to represent the correct number and placement of
electrons in each shell (energy level).
UNIT 2 - ATOMS & THE
PERIODIC TABLE
Section 2.1 Models of the Atom
2.1.d Bohr’s Model of
Electron Arrangement
2.1.d KEY
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