Main content
Chemistry library
Unit 1: Lesson 1
Introduction to the atom- Introduction to chemistry
- Preparing to study chemistry
- Elements and atoms
- Average atomic mass
- Worked example: Atomic weight calculation
- The mole and Avogadro's number
- Atomic number, mass number, and isotopes
- Worked example: Identifying isotopes and ions
- Isotope composition: Counting protons, electrons, and neutrons
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice
Worked example: Identifying isotopes and ions
Identifying isotopes and ions from the number of electrons, protons and neutrons, and vice versa.
Want to join the conversation?
what is the difference between the element hydrogen and the isotope of hydrogen?(17 votes)
Hydrogen is the element ! , in that element there are various types of isotopes as protium, deuterium and tritium all are hydrogen elements. Think like this Human is the Element and Male and Female are isotopes.(44 votes)
Great video! I do have a question though. Where do elements actually pick up extra neutrons? I am assuming the non-synthetics exist in nature as what they are on the periodic table. Am I correct in assuming as such?(19 votes)
We are all made of stardust.
Almost every element on Earth was formed at the heart of a star.
It started after the Big Bang, when hydrogen and helium gathered together to form stars.
At the stars’ cores, hydrogen and helium nuclei fused to beryllium and carbon.
As these heavier nuclei were produced, they too combined inside stars to form all sorts of nuclei with different numbers of neutrons.
During supernovae, the different elements disperse across the universe, and these now make up the planets including Earth.(19 votes)
- what is ion? what is the relationship between isotopes and ions?(8 votes)
An ion is an atom with a non neutral electric charge; an atom missing or having too many electrons. All atoms are isotopes, regardless of whether or not they are ions. Which isotope the atom is depends on the atomic number (number of protons) and the number of neutrons.(16 votes)
What is the difference between an isotope and an ion?(6 votes)
Isotopes are those atoms having same atomic number (number of protons are same) but different mass number (number of neutrons differ).
Ions are atoms which contain an overall charge (where number of protons ≠ number of electrons )(10 votes)
- so does that mean that you can figure out the number of protons by looking at the top of the element?(3 votes)
He means that if you look at the periodic table, then each element is in a box and the uppermost number in the box is usually the atomic number, which is the number of protons. (In the table in the video, the top number in the hydrogen box is 1, for helium it is 2, lithium 3, etc.)
There are lots of different ways of presenting the periodic table, so you will find exceptions to this. However, the atomic number is always shown somewhere and it is always an integer that increases by 1 as you move from element to element across the table, from left to right.(7 votes)
i know this is a stupid question but i m confuse .. how can we so sure that an element has same no. of protons as mentioned in periodic table? as we know that atoms are very small and protons are even smaller then how no. of proton is counted?? Actually i want to ask how do we count no. of protons ??(5 votes)
Well, we have defined the elements in such a way that any atom with 1 proton is a hydrogen atom, any atom with 2 protons is a helium atom, etc.
You can't count them as like you said, atoms are far too small, but over 100 years ago a scientist found a way to find the atomic number of elements: http://www.rsc.org/education/teachers/resources/periodictable/pre16/order/atomicnumber.htm(3 votes)
What's the difference between an Isotope and an Ion?(1 vote)- Isotopes are atoms that have the same numbers of protons but different numbers of neutrons.
An ion is an atom that has gained or lost electrons, so it now has more or fewer electrons than it does protons. So an ion has a negative or positive charge.
All atoms are isotopes and if an isotope gains or loses electrons it becomes an ion.(7 votes)
And here is where I got confused. My chemistry teacher said the atomic # of an element is equal to the # of proton likewise the electron. Example Carbon's atomic #is 6 and atomic mass of 12 so, the no. Of proton=6 electron= 6. But here, it's just different. And that's why also I can't answer your practices correctly. Why?how? Somebody(2 votes)
It isn't different. For protons, the number always equals the atomic number of the element.
Ions are atoms don't have the same number of electrons as protons. If you are told an atom has a +1 charge, that means there is one less electron than protons. If it has a -2 charge, there must be two more electrons than protons. Carbon with a -2 charge must have 8 electrons (6 protons/electrons in neutral atom plus 2 more electrons to give it a -2 charge = 8)
Isotopes are simply specifying the number of neutrons and protons (together called nucleons) in the atom. So, Carbon-12, which has an atomic mass number of 12, has 6 neutrons (12 nucleons - 6 protons = 6 neutrons). Carbon-13, which has an atomic mass number of 13, has 7 neutrons (13 nucleons - 6 protons = 7 neutrons).(6 votes)
Can an atom have less neutrons than its Protons?(except hydrogen)(3 votes)- Sure. Many elements have isotopes with fewer neutrons than protons. However, most of those are unstable.(3 votes)
Why does an element tend to have isotopes? I mean why and how would a neutron get added and how will it affect the atom if the element will remain the same?(1 vote)- Atoms can vary in the number of particles they have. They can have different numbers of protons, neutrons and electrons.
When we refer to an element, we are specifying a specific number of protons.
When we refer to an ion, we are specifying a certain number of electrons not equal to the protons.
When we refer to an isotope, we are referring to the total number of nucleons for an element, which an element has a specific number of protons so the neutrons can vary.
We can take it another direction and refer to isobars, which have the same number of nucleons, yet can vary in protons and neutrons.
We can also refer to isotones which have the same number of neutrons yet different protons counts.
Back to neutrons and isotopes, the neutrons can vary by the production of the atom, by fusing different combinations to give varying neutron counts. Also neutron capture, where a free neutron directly interacts and fuses to a nucleus can change the neutron count. Finally, radioactive decay from various other atoms can produce various different isotopes of an element.(5 votes)
Video transcript
- [Narrator] An isotope
contains 16 protons, 18 electrons, and 16 neutrons. What is the identity of the isotope? And I encourage you to
pause the video and see if you can figure it out
and I'll give you a hint, you might want to use
this periodic table here. All right, so I'm assuming
you've had a go at it. So, an element is defined by the number of protons it has. So if someone tells you
the number of protons, you should be able to look at
a periodic table and figure out what element they are talking about. So, because it is 16
protons, well we can go right over here to the atomic
number, what has 16 protons, well anything that has 16
protons by definition is going to be sulfur right over here. So I could write a big S. Now, the next thing we
might want to think about is the mass number of
this particular isotope. Remember, an isotope, all
sulfur atoms are going to have 16 protons, but
they might have different numbers of neutrons. So, the sulfurs that have
different number of neutrons, those would be different isotopes. So, this case we have
16 protons and we have 16 neutrons, so if you add
the protons plus the neutrons together, you're going
to get your mass number. So 16 plus 16 is 32. Now let's figure out if there's
going to be any charge here. Well, the protons have a positive charge. The electrons have a negative charge. If you have an equal amount
of protons and electrons, then you would have no charge. But in this case, we have
a surplus of electrons. We have two more electrons
than protons and since we have a surplus of the
negative charged particles we, and we have two
more, we're going to have a negative two charge and
we write that as two minus. So this is actually an
ion, it has a charge. So this is the isotope
of sulfur that has a mass number of 32, the protons
plus the neutrons are 32, and it has two more electrons
than protons which gives it this negative charge. Let's do another example
where we go the other way. Where we are told, we are
given some information about what isotope and
really what ion we're dealing with because this
has a negative charge and we need to figure out
the protons, electrons, and neutrons. Well, the first thing that
I would say is, well look, they tell us that this is fluorine. As soon as you know what
element we're dealing with, you know what it's atomic
number is when you look at the periodic table and
you can figure out the number of protons. Remember, your atomic number
is the number of protons and that's what defines the element. That's what makes this one fluorine. So let's go up to the, our periodic table and we see fluorine right
over here has an atomic number of nine. That means any fluorine has nine protons. So, let's scroll back down. So, must because it is
fluorine, we know we have nine protons. Now what else can we figure out? Well, we know we have a
negative charge right here and this is, you can use
as a negative one charge and so we have one more
electron than we have protons. And so since we have nine
protons, we're going to have 10 electrons. And then finally how many neutrons? Well, remember, the
neutrons plus the protons add up to give us this mass number. So, if you have nine protons,
well how many neutrons do you have to add to
that to get to 18, well you're going to have
to have nine neutrons. Nine plus nine is 18.