[rishch]

I was reading my science textbook and they have introduced the concepts of atomic mass,relative atomic mass and mass number.Here are some doubts I have-

1)Is atomic mass a ratio ?
2)1/12th of a C-12 atom is half a proton,half a neutron and half an electron.Since mass of an electron is negligible and since 1 proton=1 neutron,1/12th of a C-12 atom is basically the mass of one proton or one neutron.A hydrogen atom has 1 proton so why didn't they take 1u = mass of 1 hydrogen atom?
3)Also,if 1u = mass of a proton = mass of a neutron aren't mass number and atomic mass the exact same thing   ?
4)In my book the atomic mass of Chlorine is given as 35.5.How can an atom have half a neutron or proton ?
5)Can someone please explain what relative atomic mass is along with an example as in my book just the definition is given and I couldn't understand a thing. 

[siriusorion]

Hello.
I think that along with those you would have learnt isotopes? atoms of the same element with the same no. of protons but different no. of neutrons. so isotopes occur naturally, like for chlorine there are cl-35 and cl-37. so in your book what you have is most likely the relative atomic mass. 35.5 is a calculation using the relative abundance of the the types of chlorine. your teacher will probably teach you later then i think you will understand better

[rishch]

Yes my teacher did explain this but I'm getting confused because in my book they have referred to relative atomic mass as atomic mass.In my book it says the atomic mass of chlorine is 35.5.

[UG]

In my book it says the atomic mass of chlorine is 35.5.

This can be explained with a bit of maths. In your book and periodic tables, the atomic mass of an element is the average of all the isotopes which exist. There are two common isotopes of chlorine in nature, about 75% is Cl-35 and 25% is Cl-37. The atomic mass for chlorine is then calculated by 35 x 0.75 + 37 x 0.25 = 35.5

[Borek]

Is atomic mass a ratio ?

In a way all masses are expressed as "ratios". If you express your own mass in kg, you tell us what is the ratio of your mass to the mass of 1 kg object.

Atomic mass is expressed in amu - so it tells us what is a ratio of atom mass to the amu. If you express the mass of an atom in kg, it will be called "absolute mass", but even then it will be relative - just not to amu, but to kg.

[vmelkon]

2)1/12th of a C-12 atom is half a proton,half a neutron and half an electron.Since mass of an electron is negligible and since 1 proton=1 neutron,1/12th of a C-12 atom is basically the mass of one proton or one neutron.A hydrogen atom has 1 proton so why didn't they take 1u = mass of 1 hydrogen atom?

It used to be hydrogen the standard in the 19th century.
The standard changed to oxygen 16 and then carbon 12 in the 20th century.
The explanation is given here
http://www.iupac.org/publications/ci/2004/2601/1_holden.html

"In April 1957 at the bar in the Hotel Krasnapolski in Amsterdam, ."

however, I don't quite understand it.

[Arkcon]

2)1/12th of a C-12 atom is half a proton,half a neutron and half an electron.Since mass of an electron is negligible and since 1 proton=1 neutron,1/12th of a C-12 atom is basically the mass of one proton or one neutron.A hydrogen atom has 1 proton so why didn't they take 1u = mass of 1 hydrogen atom?

This is simply wrong.  Look up the subatomic constituents of the carbon nucleus and the hydrogen nucleus, since you correctly assume that the electron's mass is negligible.  See if you can reconcile the relative masses of the two elemental nucleii without fractions needed for the hydrogen nucleus.