Transcript The Mighty Electron

The Mighty Electron
Charges
• Remember,
– protons always have a positive charge
– electrons always have a negative charge
Neutral Atoms vs. Ions
• When an atom is neutral it means that it has an
equal number of protons and electrons. .
• A neutral atom of boron would have 5 protons,
so it would have 5 electrons. (Neutrons don’t
effect the charge.)
• Ions have an uneven number of protons
and electrons. This gives them a charge.
• If it has more protons, it’s a positive
charge, more electrons is a negative
charge.
This atom of oxygen has 8
protons and 10 electrons,
giving it a -2 charge!
• To find the number of electrons and
protons:
– Look at the symbol. If there is a little plus or
minus sign, its an ion! If there is no sign, its
neutral.
• P = neutral atom of phosphorous
• O-2 = ion of oxygen w/a -2 charge
• Li+ = ion of lithium w/a +1 charge
• Find the atomic number. This is ALWAYS
the number of protons
– All that can change is the number of
electrons.
– So F- has 1 extra electron.
• Fluorine’s atomic number is 9, so this ion has 9
protons and 10 electrons.
9
proton
s
– Ca+2 is missing two electrons (more protons
gives it a positive charge)
• Calcium’s atomic number is 20, so it has 20
protons and 18 electrons.
– Ar is neutral (no + or -), so it has an equal
number of protons and electrons
• Argon’s atomic number is 18, so it has 18 protons
and 18 electrons.
To Find Neutrons
• The atomic mass is the average number of
protons plus neutrons, so we need to
round it.
– So atomic mass (rounded) minus atomic
number is the average number of neutrons.
• Ex. Chlorine
– Mass = 35.453
» Round it to 35
– Atomic number = 17
35 – 17 = 18
Chlorine has 18 neutrons on average.
Element
Atomic #
Atomic mass
# protons
# electrons
# neutrons
F
F-
Na
Na+
S-2
As-3
Kr
K+
• Find protons electrons, and neutrons for each
of these:
Element
S-2
Na+
As -3
Kr
Fe+2
Protons
Electrons
Neutrons
Bohr Models
• Named after Neils Bohr - discovered
much of the structure of the atom.
• First, find out the number of protons and
electrons.
• Then fill in the shells of the model from the
center outward.
– The first shell can hold 2, the next 8, and the
third 8 as well.
Holds 8
Write number
of protons and
neutrons in
the nucleus
Holds 2
P+ =
N0=
Holds 8
• Ex.
– C has an atomic number of 6, so it has 6
protons and 6 electrons (if its neutral!) as well
as an average of 6 neutrons.
1. First, put
the number
of protons
(6) and
neutrons
(12-6=6) in
the nucleus
2. Then, add
electrons,
starting with 2 in
the inner shell.
P+ = 6
N0 =6
3. Finally, add
electrons to the
next shell
(maximum of 8)
until you have
added them all!
e- = (16+2) = 18
…2 in the inner
shell,
S-2
Protons = 16
P+ =16
N0=16
Neutrons =
32-16=16
8 in next shell (it
can only hold 8!)
…and finally, 8
in the last shell!
Valence Electrons
• The electrons in the outermost shell of an
atom are called valence electrons.
– These are VERY important! They determine
how (and if) the atom will bond to other
atoms.
Valence
Electrons
P+ =
N0=