# GCSE Chemistry help

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Thread starter 7 years ago
#1
Does the periodic table show the mass number for each element or the relative atomic mass?
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7 years ago
#2
relative atomic mass

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7 years ago
#3
The bigger number is the mass number (the total number of the protons and neutrons in an atom) The atmoic number is the number of electrons and protons.

To work out the realative atomic mass:

(Relative abundance of isotope 1 x mass number) + (Relative abundance of isotope 2
x mass number) Divided by 100
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Thread starter 7 years ago
#4
(Original post by DiamondsForever)
The bigger number is the mass number (the total number of the protons and neutrons in an atom) The atmoic number is the number of electrons and protons.

To work out the realative atomic mass:

(Relative abundance of isotope 1 x mass number) + (Relative abundance of isotope 2
x mass number) Divided by 100
But then how can chlorine have mass number of 35.5? this is not a whole number?
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7 years ago
#5
It doesn't matter. What's the question, what's the other isotope?
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7 years ago
#6
(Original post by cherrypiez)
But then how can chlorine have mass number of 35.5? this is not a whole number?
Chlorine has a mass number of 35.5 because it is an average of the different isotopes of chlorine.
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Thread starter 7 years ago
#7
(Original post by DiamondsForever)
It doesn't matter. What's the question, what's the other isotope?
no I'm just confused about why chlorine doesn't have a whole number for the mass number and how you can have half a proton or neutron?
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Thread starter 7 years ago
#8
(Original post by hadeelxxo)
Chlorine has a mass number of 35.5 because it is an average of the different isotopes of chlorine.
so this is the relative atomic mass?
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7 years ago
#9
Oh okay well its just because chlorine has 2 isotopes, its just decimals
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Thread starter 7 years ago
#10
(Original post by DiamondsForever)
Oh okay well its just because chlorine has 2 isotopes, its just decimals
So how do you calculate the mass number of chlorine if it has more than one isotopes? What's the formula for it? I know that to find it you need to add the number of protons and neutrons, but how do you take into account the other isotopes?
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7 years ago
#11
(Original post by cherrypiez)
So how do you calculate the mass number of chlorine if it has more than one isotopes? What's the formula for it? I know that to find it you need to add the number of protons and neutrons, but how do you take into account the other isotopes?

Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons because of the two different isotopes
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7 years ago
#12
Chlorine has 2 isotopes Cl-35 and Cl-37.

The mass number of an element is the average mass of all the existing isotopes. Roughly the ratio of abundance of Cl-35 to the abundance of Cl-37 is 3:1.

Accordingly, calculated is calculated as:

(3 x 35) + (37)
-------------------- = 35.5.
3+1

That's it.
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Thread starter 7 years ago
#13
(Original post by hadeelxxo)
Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons because of the two different isotopes
so how do scientists know which is the original one? shouldn't it be the one with 18 neutrons as there is 75% of it?
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Thread starter 7 years ago
#14
(Original post by DiamondsForever)
Chlorine has 2 isotopes Cl-35 and Cl-37.

The mass number of an element is the average mass of all the existing isotopes. Roughly the ratio of abundance of Cl-35 to the abundance of Cl-37 is 3:1.

Accordingly, calculated is calculated as:

(3 x 35) + (37)
-------------------- = 35.5.
3+1

That's it.
isn't that the formula for relative atomic mass?????
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7 years ago
#15
Here's the formula:

(Relative abundance of isotope 1 x mass number) + (Relative abundance of isotope 2 x mass number)
--------------------------------------------------------------------------------------------------------------------------------
100
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7 years ago
#16
(Original post by cherrypiez)
so how do scientists know which is the original one? shouldn't it be the one with 18 neutrons as there is 75% of it?

They just work out an average they're both original. Btw are you doing Core science or additional?
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Thread starter 7 years ago
#17
(Original post by DiamondsForever)
They just work out an average they're both original. Btw are you doing Core science or additional?
I'm doing additional right now. I'm so confused why is the formula the same for working out the mass number and relative atomic mass?
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7 years ago
#18
Me too! No there is no formula for mass number, mass number is just the largest number (the number at the top of the element)

All you need to know is the formula for relative atomic mass and then the question will say what the relative abundance is, you just have to work out the relative atomic mass using the formula that I mentioned in the above posts.
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7 years ago
#19
(Original post by cherrypiez)
so how do scientists know which is the original one? shouldn't it be the one with 18 neutrons as there is 75% of it?
You don't need to know about the different isotopes in too much detail. Just as long as you know that 35.5 is an average of the two isotopes of chlorine you'll be fine 0
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Thread starter 7 years ago
#20
(Original post by DiamondsForever)
Me too! No there is no formula for mass number, mass number is just the largest number (the number at the top of the element)

All you need to know is the formula for relative atomic mass and then the question will say what the relative abundance is, you just have to work out the relative atomic mass using the formula that I mentioned in the above posts.
ok thanks!
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