Chapter 1 Activity 9Part A

What Determines and Limits an Atom’s Mass?

What Do You See?

What Do You Think?

• What do you think makes up the nucleus of the atom?

electron

neutron

proton

Investigate

Part A: What’s in the Nucleus? 1. How many protons are there:

a) in a hydrogen atom?

b) in a sodium atom?

c) in a carbon atom?

d) in a uranium atom?

Atomic Mass Unit• Atomic mass is the average mass of the atoms of each

element. A proton has a mass of 1.67 x 10-24 g, this is approximately 1 amu.

2. Answer the following questions

a) What is the mass of a hydrogen atom? (round to the nearest whole number)

b) How many protons are there in a helium atom?

c) Considering that the electrons mass is negligible, what would you expect the mass of a helium atom to be?

d) What is the mass of a helium atom to the nearest whole number?

Mystery particle?• The helium atom has a mass four times the mass of a

hydrogen atom however, the electric charge on the helium nucleus is only twice that of the hydrogen atom. This suggests the presence of another particle with about the same mass as the proton but with no electric charge.

• 3. What particle is this?

a) How many protons would you expect to find in the nucleus of a helium atom?

b) How many neutrons would you expect of find in helium?

c) How many protons and neutrons would you expect to find in the nucleus of an atom of: Li, B, N, F, Be, C, O, Ne.

Mystery particle?

• Boron has an atomic mass of 10.811 amu. When you subtract the number of protons from this, it would indicate that there are 5.811 neutrons in the nucleus of Boron. There cannot be pieces of neutrons.

• 4. How can you explain this? Since the number of protons will always be 5, what must change?

a) What are the atomic masses of Mg, Cl, Na and F?

b) What are the probable number of protons and neutrons in each of these four elements?

Mystery particle? • Example: Carbon (atomic mass 12.011, atomic number 6)

carbon-12 atoms with 6 neutrons and carbon-13 atoms with 7 neutrons. Because the atomic mass is so close to 12, there must be more carbon-12 atoms with 6 neutrons.

• 5. Answer the following questions.a) What isotopes do you expect to account for the known

atomic masses of the following elements? H, B, Mg, Be, Na.

b) How many neutrons and protons are present in the following isotopes?

– i) 32 He and 4

2 He– ii) 6

3 Li and 73 Li

– iii) 126 C and 13

6C– iv) 14

7 N and 157N

What is an isotope?

• Only the number of protons, the atomic number, is the same in all atoms of a given element.

Atoms of the same element with different number of neutrons in the nucleus are known as isotopes.

Example of an Average Atomic MassEasy Method

Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine.

35 x 0.755 = 26.4

37 x 0.245 = +9.07

35.47

= 35.5

Step 1 Read the problem.

• Four isotopes of lead are shown below, each with its percent mass abundance and composition of its nucleus. Using the data, calculate the average atomic mass

82 p

122 n

82 p124 n

82 p125 n

82 p126 n

1.37% 22.26% 51.55%20.82%

Step 2 Change % to decimals.• Divide the percentages by 100

1.37÷100 = 0.0137

26.26 ÷100 = 0 .2626

20.82 ÷100 = 0.2082

51.55 ÷100 = 0.5155

Step 3 Multiple the decimals by the mass.204x 0.0137 = 2.7948

206 x 0.2626 = 54.0956

207 x 0.2082 = 43.0974

208 x 0.5155 = 107.22

Step 4 Add all the products.

2.7948 + 54.0956 + 43.0974 + 107.224 = 207.2118 amu

Problems

• 1. Element X has two natural isotopes. The isotope with a mass of 10.012 amu (10X) has a relative abundance of 19.91%. The isotope with a mass of 11.000 amu (11X) has an abundance of 80.09%. Calculate the average atomic mass and identify the element.

• 2. What is the atomic mass of silicon if 92.21% of its atoms have a mass of 27.977 amu, 4.7% have a mass of 28.976 amu, and 3.09% have mass of 29.974 amu?

Homework Assignment

• Average Atomic Mass Worksheet on Blackboard.

• Due by Wednesday at the beginning of class.