# What do we mean by half life? With what kinds of materials do we use this term?

Jun 23, 2017

A half-life is how long it takes half of any amount of a radioactive element to break down.

#### Explanation:

What we mean when we say half life is how long it takes for half of any amount of something to break down.

This doesn't apply to things like compost and paper, which break down into their components and become soil, but radioactive things. Radioactive elements break down into simpler ones due to their instability.
Some radioactive elements, with the highest atomic numbers, have half-lives of seconds. More stable (but still very dangerous) ones, have far longer ones. Uranium's most common isotope has a half life of 4,500,000,000 billion years. Bismuth, the most stable radioactive element, has a half-life of 19,000,000,000,000,000,000 years.

Half-lives can vary widely, but all radioactive elements have them.

An important concept to understand is that half-lives apply to any amount of a radioactive material.

I'm using Uranium as an example.

If I had a cubic millimeter of Uranium, in 4.5 billion years one of my great-great-great... grandchildren would have half a cubic millimeter of Uranium.

If I had a cubic kilometer of Uranium, in 4.5 billion years one of my great-great-great... grandchildren would have half a cubic kilometer of Uranium, as well as radiation poisoning.

The half-life concept applies to any possible amount of a radio active element: If I have $y$ amount of element $x$, after the span of element $x$'s half-life, I will have $\frac{y}{2}$ amount of element $x$.

That means that no matter how many half-lives of element $x$ have passed, some tiny amount of element $x$ will remain.

The decay of all radioactive elements can be shown with this graph:
graph{y = 0.5^x [-0.151, 2.974, -0.379, 1.184]}

The x-axis is the number of half-lives passed, and the y-axis is how much of the original amount of the radioactive element is left.