Question

q1 chemistry: u2l3 isotopes
prework part 2: average atomic mass
directions: read the passage below. as you read, annotate the important information and vocabulary words. then answer the questions.
**part 1: read & annotate:
what is average atomic mass?
every element on the periodic table has an atomic mass listed beneath its symbol. this number is not usually a whole number because it represents the average atomic mass, not the mass of a single atom.
the average atomic mass is calculated by considering the different isotopes of an element and their relative abundance—how common each isotope is in nature. since isotopes of the same element have different numbers of neutrons, they have different masses. the average atomic mass is a weighted average, not a simple average, and it reflects the contribution of each isotope based on how much of it is found naturally.
for example, chlorine has two major isotopes:

• chlorine - 35 (mass = 35 amu, abundance = 75%)
• chlorine - 37 (mass = 37 amu, abundance = 25%)

the average atomic mass of chlorine is: note: the percentages are converted to decimal before plugging them into the formula: (ex: 75% = 0.75)
(35×0.75)+(37×0.25)=26.25 + 9.25 = 35.5 amu
the average atomic mass will be closest to the isotope with the greatest percent abundance. the average atomic mass for chlorine is 35.5 amu which is closest to 35 because chlorine - 35 has 75% abundance in nature.
**part 2: calculation
directions: use the information below to calculate the average atomic mass of the given elements.
element isotope (mass) & percent abundance average atomic mass (amu)
magnesium 24 (78.99%), 25 (10.00%), 26 (11.01%)
part 3: reflection questions

1. why is the average atomic mass rarely a whole number?
2. what information do you need to calculate the average atomic mass of an element?

Explanation:

Step1: Convert percentages to decimals

$78.99\% = 0.7899$, $10.00\%=0.1000$, $11.01\% = 0.1101$

Step2: Calculate the contribution of each isotope

For magnesium - 24: $24\times0.7899 = 18.9576$
For magnesium - 25: $25\times0.1000 = 2.5$
For magnesium - 26: $26\times0.1101=2.8626$

Step3: Find the average atomic mass

$18.9576 + 2.5+2.8626=24.3202$ amu

1. The average atomic mass is rarely a whole number because it is a weighted - average of the masses of an element's isotopes, which have different abundances in nature. Since isotopes of the same element have different numbers of neutrons and thus different masses, and their contributions are based on their natural abundances, the result is usually a non - whole number.
2. To calculate the average atomic mass of an element, you need to know the mass of each isotope of the element and the relative abundance (percentage abundance) of each isotope. The relative abundances are converted to decimals and then used in the weighted - average calculation.

Answer:

24.3202 amu