Identifying Element

Question

Hi,

I am looking for the element that has a proton of 12 and a mass of 21. I thought it was magnesium, but a neutral magnesium atom would have a mass of 24. I feel like its a magnesium isotope, but would it be magnesium 21? If so, how would that have 9 neutrons? and 12 electrons?

Any help would be grateful!

Answer 1

The number of protons identifies the element. The protons (atomic #) never changes for an element. If the proton number for the element is 12 it can only be identified as Magnesium. Therefore, you are correct. It is an isotope of Mg. An isotopes are just different atom versions of an element with different numbers of neutrons.

atomic mass = protons + neutrons.

21 (mass) = 12p + ?n

21(mass) - 12p = 9n

It is an isotope of Mg with 9 neutrons.

Magnesium-21 (mass)

Answer 2

You are correct! The identity of an element is determined by its atomic number, in this case 12. Therefore, this is a magnesium atom with a mass of 21, so it is an isotope: magnesium-21. It is still neutral because it still has 12 electrons to balance the charge of the 12 protons.

Answer 3

Hi "Asked",

Bear also in mind that you'd better look at it quickly, since it has a half-life of about 118 ms.

So it's not as bad as H-7 (half-life 23 * 10^-24 sec), but it's not exactly Te-128 (half-life 69*10^30 sec, or 2.2*10^24 years, or 1.6 * 10^11 times the estimated age of the observable universe).

-- Cheers, -- Mr. d.