How many atoms are in a 3.5 mole sample of the element lithium?

2.1 x 10^24

Explanation:

The number of atoms in a mole will always be the same. The fact that it's lithium is completely irrelevant.

One mole of anything is 6.022 x 10^23 atoms

So if we need to find the number of atoms in 3.5 mol we just need to multiply Avogadro's number by 3.5

3.5 x 6.022 x 10^23

= 21.077 x 10^23

To make it proper scientific notation the first number can't be greater than 10 so let's move the decimal place and increase the exponent

2.1077 x 10^24

Remove the numbers that aren't significant figures

2.1 x 10^24

Boom there's your answer