# Question #93c80

Dec 9, 2015

Here's what I got.

#### Explanation:

Your strategy here will be to

• figure out what chemical element you're dealing with
• determine what $57$ and $139$ actually represent

So, pull up a periodic table and look for the element with the chemical symbol $\text{La}$. AZ quick search will reveal that you're dealing with lanthanum.

Now, notice that one of the two numbers given to you, $57$, is listed next to lanthanum's chemical symbol.

This number represents the element's atomic number, which tells you how many protons must be located in an atom's nucleus in order for that atom to be an atom of lanthanum.

So, any atom that has $57$ protons in its nucleus will be a lanthanum atom.

Now for the second number, $139$. When isotopes are given to you, they are written using isotope notation.

In your case, the lanthanum isotope given to you is written like this

$\text{_(color(white)(1)57)^139"La}$

This tells you that its atomic number is equal to $57$, like we've already figured out, and that its mass number is equal to $139$.

An isotope's mass number tells you how many protons and neutrons it contains in its nucleus.

$\textcolor{b l u e}{\text{mass number" = "no. of protons" + "no. of neutrons}}$

Since you know that you have $57$ protons and that the isotope's mass number is equal to $139$, it follows that its nucleus will also contain

$\text{no. of neutrons} = 139 - 57 = 82$

Finally, the number of electrons found in a neutral atom will always be equal to the number of protons found in that atom's nucleus.

That happens because each electron carries a $1 -$ charge and each proton carries a $1 +$ charge, so equal number of electrons and neutrons will result in a neutral atom.

In your case, the number of electrons found in a neutral lanthanum-139 atom is equal to $57$.