Isotopes and Ions Variations on the Atom Dr. M. Hazlett Mandeville High School.

Isotopes and Ions

Variations on the Atom

Dr. M. HazlettMandeville 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 amun0 1.675 x 10-24 g 1.008665p+ 1.673 x 10-24 g 1.007276e- 9.1 x 10-28 g 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 EndNow, onto the Periodic Table!