Transcript Isotopes and Ions

Isotopes and Ions
Variations on the Atom
Dr. M. Hazlett
Mandeville High School
Isotopes
• All atoms of an element have the SAME
number of protons (p+)
• The p+ number is the atomic number (Z)
– This is a constant
– For example: All Sodium (Na) atoms have 11 p+
– If an atom loses a proton, it becomes a different
element
• If Na loses 1 p+, then it has become Neon (Ne)
Z = atomic number = p+
• The number of protons identifies the atom
and which element it is
• In a stable atom:
– # p+ = # n0 = # e– Thus, Na in its stable form has 11 p+; 11 n0; and 11
e– If it has an unequal number of p+ and n0, then it is
called an ISOTOPE
• Theoretically – an element can have as many
isotopes of itself as it has neutrons, or it can
add an unlimited number of n0
• For example: H has 3; C has 16; Al has 25
– These can be looked up in the CRC (the
Chemistry/Physics Data Bible) or on the internet
– Remember – a change in the number of n0 does
not change the element’s atom – only a change in
the number of protons can do that!
The Carbon Isotope
Ions
• Ions are when an atom has an unequal
number of p+ and e• Remember – a stable atom has a neutral
overall charge due its equal number of p+ and
e• When an atom loses or gains an e-, its charge
changes accordingly
– Loss of e- means a + charge; gaining an e- means
a – charge for the atom
Losing or Gaining e- . . . . .
• If an atom loses an e-, then it has more p+ than
e- and it will have an overall positive charge
• Different elements’ atoms can lose 1, 2, 3, or
even 4 electrons depending on various factors
• If an atom has LOST e-, then it is called a
CATION or a positive ion
– A Cation would be written as Al+ (the one being
understood) or Al+3
• Atoms can also gain electrons
• If an atom gains electrons (from 1 up to 4), then it
will have more e- than p+ and will end up having
an overall negative charge
• A negatively charged ion is called an ANION
– The element is shown this like: Na- (the 1 is
understood) or Na-2
• The losing or gaining of electrons determines
what type of bonds the atoms will form, and
which atoms will bond to others
Ions in Water Solution
Using the Periodic Table
• Elements in the Main Groups (A), form fairly
consistent ions – LEARN TO USE THE CHART
• Group IA will form +1 ions; Group 2A form up to +2;
Group 3A form up to +3 ions
• Group 4A will form either up to -4 or +4 ions
• Group 5A will form up to -3 ions; Group 6A up to -2;
Group 7A form -1; and Group 8A will not form ions at
all
• Those elements in the B groups vary and we’ll learn
those later
Ions and Isotopes in Review
• Stable atom: #p+ = #n0 = #e• Atomic Mass - #n0 = # p+
• Atomic Mass - #p+ = #n0
• If charge is 0, then #p+ = #e• If charge is positive, then #p+ > #e- Cation
• If charge is negative, then #p+ < #e- Anion
Examples:
• Li-1 has gained an electron, meaning there is
one more negative charge than positive ones
– It has 3 p+ and 4 e-
• Li+1 has lost an electron, meaning there is one
more positive charge than negative ones
– It has 3 p+ and 2 e• REMEMBER: The # of p+ DO NOT CHANGE
• Only the number of n0 (isotope) and e- (ion) change
• Cf-3 has an atomic number of 98
– This means it has 98 p+
– Its atomic mass is 216
– It has 118 n0, (216 – 98), making it an ion and an
isotope!
– Since it has a -3 charge, the number of e- will be
101; (98 + 3)
– Zn+1 has 30 p+ and n0; but due to the +1 charge, it
has only 29 e-
Mass Number and Atomic Mass
• An atom’s mass number = # p+ + # n0
• The atomic mass unit (amu or u) is a little
more complex
– It is an average of all of an atom’s isotopes and
what percent abundance that isotope is in nature
• Abundances will add up close to 100%
• The closer to a whole number the amu is, the fewer the
isotopes that exist
Determining the average atomic mass:
• Average Atomic Mass =
(Mass of Isotope 1)(% Abundance of Isotope 1) +
(Mass of Isotope 2)(% Abundance of Isotope 2) +
(Mass of Isotope 3)(% Abundance of Isotope 3) +
(Mass of Isotope ∞)(% Abundance of Isotope ∞)
AMU is a little different. . . . . . .
AMU (sometimes just an ‘u’)
• Average Mass Unit
– It uses C-12 as a reference point
• C-12 has 6 protons and 6 neutrons
• 1 amu is the equivalent of 1/12 of a Carbon’s mass
Mass
n0 1.675 x 10-24 g
p+ 1.673 x 10-24 g
e- 9.1 x 10-28 g
amu
1.008665
1.007276
0.000549
• Average Atomic Weight example:
For an unknown element we know that:
• the mass of Isotope 1 is 6.015 amu and its abundance
is 7.5%
• The mass of Isotope 2 is 7.016 amu with a 92.5%
abundance
• Therefore –
– (6.015)(.075) + (7.016)(.925) = 6.941 amu
– Looking on the Periodic Chart we can see the
element is Lithium (Li)
Another example:
• N 14 and N 15 have a total amu of 14.007.
What are the percentages of abundance?
Make the abundances equal to x and (x-1).
Thus:
14(x) + 15(1 - x) = 14.007
14x + (15 – 15x) = 14.007
- x = 14.007 - 15
so, x = 99.3 % for N14
and, 1 – x = 0.7% for N15
On the Periodic Table:
The top number is Z, the Atomic Number or
number of p+
The Element’s Symbol
The element average atomic weight set by isotopes
and abundances
• If the Atomic Weight is in (parentheses), then
it is a synthetically made element and it has
no known isotopes
• The closer to a whole number the atomic
weight is, the fewer isotopes the element has
• To discover known isotopes and abundances –
use the CRC Handbook
Conservation of Mass
• Conservation of Mass means that the mass of
the reactants will equal the mass of the
products after the reaction
– This is true no matter how many reactants or
products exist in the reaction
– Example: Fe with a mass of 15.72 g; placed in a
solution of 21.2 g Cu(II)Sulfate. Cu separates.
How much Fe (II) Sulfate created?
– The final masses of the reaction (rxn) are Fe = 8.33
g; and Cu = 8.41 g
– Thus – 15.72 g – 8.33 g = 7.39 g
– mreactant 1 + mreactant 2 = mproduct 1 + mproduct 2
• mFe + mCu = mCuS + mFeS
• mFeS = mFe + mCuS - mCu
• mFeS = 7.39 g + 21.12 g – 8.41 g = 20.10 g
Law of Definite Proportions
• In a compound, the same elements will be in
the same proportion by mass
• Example:
– 100 g H2O contains 11.19 g of H2 and 88.81 g O
– % Composition = mass element x 100
mass compound
Well, what does it equal???????
OK – try another one . . . .
• 25 g of a compound with 6.77 g tin and 18.23
g bromine. What percent is tin by mass?
–
mass tin x 100 = 6.77 x 100 =
mass compound
25
Did you get the answer?
The End
Now, onto the
Periodic Table!