Add up the mass of everything going into a nuclear change, then add up the mass of everything it produces. Chemistry taught you to expect a perfect match. A nuclear change that releases energy breaks the match: the products' total mass comes out smaller, and measurably so.
A chemical change hides its mass change far below any balance's reach. A nuclear change does not: its energy release is so large that the missing mass shows up in careful measurements. This lesson names the missing amount.
A nuclear change is also called a nuclear reaction.
In an energy-releasing nuclear reaction, the products together have slightly less mass than the reactants.
The missing mass is the amount converted into the released energy.
A helium-4 nucleus has about 0.7% less mass than its two protons and two neutrons have when separate.
Nothing vanished from the count: the same two protons and two neutrons are all still there.
The mass defect is missing mass, never a missing proton or neutron.
To find a mass defect, subtract the products' total mass from the reactants' total mass.
What this missing mass buys, and why every energy-releasing nuclear reaction shows it, is where the next topic begins.
Two protons and two neutrons have a total mass of 6.696 × 10⁻²⁷ kg when separate. Bound together as a helium-4 nucleus, their mass is 6.646 × 10⁻²⁷ kg. Find the mass defect.
Write down the values:
reactants' total 6.696 × 10⁻²⁷ kg, products' total 6.646 × 10⁻²⁷ kg
Subtract the products' total from the reactants' total:
6.696 minus 6.646 is 0.050, so the difference is 0.050 × 10⁻²⁷ kg, written 5.0 × 10⁻²⁹ kg.
The mass defect is 5.0 × 10⁻²⁹ kg, about 0.7% of the total.
A nuclear reaction's reactants total 3.346 × 10⁻²⁶ kg, and its products total 3.340 × 10⁻²⁶ kg. Find the mass defect.
Write down the values:
reactants' total 3.346 × 10⁻²⁶ kg, products' total 3.340 × 10⁻²⁶ kg
Subtract the products' total from the reactants' total:
3.346 minus 3.340 is 0.006, so the difference is 0.006 × 10⁻²⁶ kg, written 6.0 × 10⁻²⁹ kg.
The mass defect is 6.0 × 10⁻²⁹ kg.
In a nuclear reaction that releases energy, how does the products' total mass compare with the reactants' total mass?
Mireille says: 'In an energy-releasing nuclear reaction, the missing mass is the amount converted into the released energy.' Is Mireille correct?
In an energy-releasing nuclear reaction, the reactants' total mass is 4.520 × 10⁻²⁶ kg and the products' total mass is 4.512 × 10⁻²⁶ kg. Calculate the reaction's mass defect, in kilograms, to two significant figures.
A nuclear reaction releases energy, and careful measurement shows the products together have 0.3% less mass than the reactants together. Where has the missing mass gone?
A nuclear reaction's reactants total 7.150 × 10⁻²⁷ kg, and its products total 7.130 × 10⁻²⁷ kg. Calculate the reaction's mass defect, in kilograms, to two significant figures.
In an energy-releasing nuclear reaction, the products together have slightly less mass than the reactants. What is the missing amount called?