Question

Fluorine-18 has a half-life of 110 minutes.

Indicate which statements are true, if any, about fluorine-18 by checking the box in front of each true statement.

		A fluorine-18 atom has an equal number of protons and neutrons in its nucleus.
		A fluorine-18 atom has an odd number of protons and an odd number of neutrons.
		A fluorine-18 atom undergoes alpha-decay.
		A fluorine-18 atom undergoes beta-decay.
		A fluorine-18 atom undergoes positron-decay (positron emission).
		The half-life for 1.00 g of fluorine-18 atoms is greater than the half-life for 0.50 g of fluorine-18 atoms.
		It takes 6.1 hours for 90.0% of a given quantity of fluorine-18 to decay.

Answer 1

1.Atomic number of electrons= 9, mass number= 18

No of electrons= no of protons= 9, No of neutrons= mass number- no of electrons=18-9=9

Hence no of protons= no of neutrons

2. since 9 is odd number, no of protons= number of neutrons, fluorine will have odd no of protons and neutrons.

3. Alpha decay is associated with heavy nuclei , Fluorine is not heavy nuclei and does not undergo alpha decay

4. Beta decay refer to electron emission and F18 has a tendency to accept electrons.

Fluorine has got isotopes ranging from 14F to 31F. Out of this, F20 undegoes beta decay to form more stable 20Ne which is more stable. So F18 does not undergo beta decay.

5. Fluorine undergoes positroon decay, 18F---->O18+e,

6, Decay process is first order reaction and the rate constant is calculated as rate constant= 0.693/ half life

So half life is independent of initial mass.

7. Half life of F-18 = 1.1*100min=110 min, Rate constant =0.693/110 =0.0063/min

Since decay is 1st order, NA= NAO*e(-Kt), NA/NAO= ratiof atoms of F-18 at time t/ ratio of atoms of F-18 at t=0

t= 6.1 hrs= 6.1*60min= 366 min

NA/NAO= e(-0.0063*366)= 0.099

(NA/NAO)*100 =10

Hence NA remaining = 10% of NAO

% disintegrated in 6.1 hours= 100-10= 90%

so statements 1, 2,5, 7 are correct and 3.4.6 are incorrect.