# A uranium-235 sample starts with 200 atoms, and 700 million years later, there are 100 atoms. What is the half-life of uranium-235?

##### 1 Answer

#### Explanation:

The is a great example of a problem that wants to test your understanding of *what* exactly we mean when we say **half-life**.

The **half-life** of a radioactive nuclide, **time needed** for a sample of said nuclide to decay to **half** of its initial size.

In other words, if you start with number of atoms and wait until **half** of them undergo radioactive decay, the period of time needed for this to happen is nuclide's **half-life**.

In your case, a sample of **atoms** of uranium-235 needs **million years** to decay to **atoms**.

Since **half** of the initial sample

#"200 atoms" * 1/2 = "100 atoms"#

it follows that the time needed for your sample to decay from **half-life** of uranium-235.

Therefore, you can say that

#color(green)(|bar(ul(color(white)(a/a)color(black)(t_"1/2" = "700 million years")color(white)(a/a)|)))#