How atoms differ

Mass Number

• The sum of the protons and neutrons

• BUT…Why is it not an even number??

Isotopes:Dalton was wrong in saying that every atom

of an element is the same.

Neutral atoms of an element always have the same number of protons and

electrons.

But, the number of neutrons of an element differs in different atoms

Isotopes:Atoms with the same number of protons in the

nucleus but different number of neutrons

Isotopes• Isotopes have a different mass

• Isotopes containing more neutrons have a larger mass

• But, isotopes react the same – WHY?

Examples:

• EX: Potassium has 3 isotopes – one with 20 neutrons, one with 21 neutrons, and another with 22 neutrons

• Potassium has 19 protons • Potassium 39 – 19 protons 20 neutrons

• Potassium 40 – 19 protons 21 neutrons

• Potassium 41 – 19 protons 22 neutrons

3919K

4019K

4119K

Isotopes• In nature, elements are found as a

mixture of isotopes

• There is a set percent present of each isotope

• Example – Potassium in a banana 93.25% is 20 neutrons; 6.7302% is 22 neutrons; and 0.017% is 21 neutrons

Mass of an element:

• For elements - Instead of listing each isotope of every atom – scientists combine the isotopes to make the atomic mass of the element

Atomic Mass:

• Atomic mass of an element is the weighted average of the mass of the isotopes of that element

• To find the Atomic Mass • (Isotope 1 mass X % abundance) +

(Isotope 2 mass X % abundance), etc…

Atomic Mass:

• EXAMPLE• Chlorine has two isotopes

–Chlorine – 35 with 75.770% abundance–Chlorine – 37 with 24.230% abundance

Isotope Number of protons

Number of neutrons

Mass Relative abundance

N14 7 7 14 99.64%

N15 7 8 15 0.36%

• Carbon has two isotopes, C-12 and C-13. C-12 has a percent abundance of 98.89%. C-13 has a percent abundance of 1.11%. Calculate the average atomic mass of Carbon.