1. Balancing Nuclear Equations & Calculating Half-Life
West Valley High School
General Chemistry
Mr. Mata

2. Balancing Nuclear Equations
Step 1: Add atomic masses & atomic numbers on each side of equation.
Step 2: Balance atomic masses & atomic numbers to find the atomic mass & atomic number of the unknown particle.
Step 3: Determine the unknown symbol by using the atomic number.

3. Example 1
1 H H -> ?
1 H H -> 4 He

4. Example 2 9 Be + 1 H -> ? + 4 He 4 1 2 9 Be + 1 H -> 6 Li + 4 He
9 Be H -> 6 Li He

5. Example 3
42 K -> 0 e + ?
42 K -> 0 e Ca

6. Calculating Half-Life
A radioisotope has a half-life of 4 days. How much of a 20 gram sample would remain at the end of each time period?
Known: t ½ = 4 days 20 gram sample
Unknown: amount left after each half-life

7. Problem 1 Solution 4 days 4 days 4 days 4 days
20 g g g g g

8. Calculating Half-Life
2. The mass of cobalt-60 is found to have decreased from grams to grams in a period of 10.5 years. Calculate the half-life of cobalt-60.
Known: g -> g decrease in 10.5 yrs
Unknown: t ½ cobalt-60

9. Problem 2 Solution 0.800 g 0.400 g 0.200 g 10.5 years
5.25 yr yr
0.800 g g g
10.5 years
t ½ cobalt-60 = 5.25 years

10. Calculating Half-Life
3. Carbon-14 emits beta radiation and decays with half-life of 5730 years. Assume you start with a mass of 2 grams of carbon-14. How long is three half-lives? How many grams of the isotope remain at the end of three half-lives?
Known: t ½ = 5730 years 2 grams
Unknown: How long is three half-lives?
How much after three half-lives?

11. Problem 3 Solution 3 t ½ = 3 (5730 years) = 17,190 years
5730 yrs yrs yrs
2 g g g g