Download Atomic Theory - Somerset Academy

Atomic Theory
• Atoms are incredibly small!
• What we know about them is based on
indirect evidence.
History of atomic theory
• 500 BC Democritus proposed idea of atombut he had no experimental data to support
it.
• 1800 Dalton proposed an atomic theory
supported by experimental data.
Dalton’s Atomic theory
• Support for his theory came when he began
looking at the masses of elements that
combined with eachother.
• He found that elements combine in simple
ratios by mass.
Dalton’s atomic theory
• Dalton’s atomic theory
– all matter is composed of tiny, indivisible
particles called atoms.
– All atoms of the same element are identical.
– Atoms can neither be created nor destroyed.
– Atoms of two or more different elements can
combine in small whole number ratios to form
compounds.
Dalton’s atomic theory
• 2 problems with his theory:
– Atoms of the same element have different masses
– atoms have subatomic particles (electrons, protons
and neutrons)
So Dalton’s theory is not completely correct.
The Subatomic particles
MASS
Charge
proton
1 amu
+1
neutron
1 amu
0
electron
“0amu”
-1
The subatomic particles
• Electron discovered ~1900 by JJ Thomson
using his cathode ray tube.
• Electron charge found by American Robert
Millikan ~1910. Oil drop experiment
• Proton found using adaptation of cathode
ray tube ~1900.
• Neutron discovered in 1932 by Chadwick.
Rutherford’s Gold Foil
Experiment
• A consequence of the plum pudding model
was that the atom would have a weak
positive charge.
• Rutherford tested this idea by firing
positively charged particles at gold foil.
• He expected particles to pass through.
• His results surprised him.
Results of Gold Foil Experiment
• Atom is mostly empty space.
• Protons are found in a dense nucleus in the
center of the atom.
Problems with Rutherford’s model
• Could not explain why electrons would not
spiral into the nucleus.
Atoms to Elements
• What makes elements different from one
another?
– The number of protons, electrons and neutrons
in them.
– C has 6 protons, 6 neutrons and 6 electrons,
while H has 1 proton, 0 neutrons and 1electron.
Atomic number
• Atomic number: the number of protons in
an atom.
– Because atoms are neutral we know that
protons must equal electrons and so atomic
number is also equal to the number of electrons.
Atomic number also equals the number of
electrons in the neutral atom
How many protons in these
atoms? How many electrons?
•
•
•
•
•
•
•
H
He
Li
Be
B
F
Ne
Mass number
• Mass number- the total number of neutrons
and protons in an atom.
• If we know the mass number and the atomic
number we can find the number of neutrons
in an atom.
Mass number - atomic number = number of neutrons
How many neutrons? How many electrons
Atomic number
Mass number
Beryllium
4
9
Neon
10
20
Sodium
11
23
Notation
Mass Number
65
29
Atomic Number
Cu
Another notation
Cu-65
Element
Mass number
Isotopes
• Isotopes- elements with same number of
protons and electrons, but different numbers
of neutrons.
– For example:
65
29
63
Cu
Cu
29
How many protons, neutrons and electrons does each Isotope have?
Mass #
Isotopes
65
63
Cu
29
Atomic #
Cu
29
protons = 29
protons = 29
electrons = 29
electrons = 29
neutrons = 65-29 = 36
neutrons = 63-29 = 34
FAQ: Average Atomic Masses
• “If protons and neutrons weigh 1amu, then
how come the periodic table has atomic
masses with decimals? How does this
happen?”
– for example: the 2 isotopes of copper weigh 65
and 63amu’s. In periodic table it says
63.55amu.
Average atomic masses
• The average atomic mass is an average of
the masses of the isotopes of an element.
– The average atomic mass of Cu is an average of
the two masses of the isotopes: 65amu and
63amu.
– It’s a weighted average that takes into account
the relative abundance of each isotope.
Atoms and ions
• Atoms are electrically neutral because they
have equal numbers of protons and
electrons.
– EXAMPLE: Hydrogen has 1proton and
1 electron. They cancel eachother.
• Atoms can gain or lose electrons when they
form compounds.
• When atoms lose or gain electrons they become
charged.
• Atoms having a positive or negative charge are
called ions.
Element
Symbol
Sodium
Iron
Atomic
Number
Mass
Protons
Number
11
11
Fe
Sn
26
56
106
33
Electrons
11
28
Pd
Nickel
Neutrons
Atoms and ions
• Sodium, Na, loses an electron when it forms
NaCl.
23
11
How many electrons in Na?
Na
Atoms and ions
23
11
Na
Sodium has atomic number 11,
meaning it has 11 protons and if
it is neutral it should have 11
electrons.
11 protons + 11electrons = 0 charge
So how many electrons would
Sodium have if it lost an electron?
Atoms and ions
23
11
Na
If Sodium lost one electron it
would still have 11 protons, but
now it would have 10 electrons.
11 protons + 10 electrons = +1 charge
We write the positively charged ion,
or cation, as Na+
Atoms and ions
35
17
Cl
Chlorine gains an electron when it
forms compounds.
How many electrons does chlorine
have?
Atoms and ions
35
17
Cl
Chlorine has atomic number 17,
meaning it has 17 protons and if
it is neutral it should have 17
electrons.
17 protons + 17electrons = 0 charge
So how many electrons would
Chlorine have if it gained an
electron?
Atoms and ions
35
17
Cl
If chlorine gained one electron it
would still have 17 protons, but
now it would have 18 electrons.
17 protons + 18 electrons = -1charge
We write the negatively charged ion,
or anion, as Cl-
Write the symbol and name of
the ion formed:
A. Strontium atom loses two electrons
B. Iodine atom gains one electron
C. Magnesium atom loses two electrons
D. Oxygen atom gains two electrons
Complete the table
Element Change Formula Name
symbol
in
of ion of ion
electrons
Al
3 lost
S
____
_____
____
____
____
2-
____
2+
____
S
Sr