Why does hydrogen's atomic mass differ from proton and neutron masses?

[mathzeroh]

i hope everyone's in fine health in this fine thursday evening.

ok i just had some general questions on atomic masses, and to the chemists in here, i appologize if these questions are a bit too basic for you. but ur help and time is greatly appreciated nonetheless!

so here are my questions (these are based on readings I've come across in the textbook):

Why is it that the number of neutrons determine the mass of an element? I ask this because to me, this idea doesn't fit completely altogether because an isotope is identified by its mass number, A, the total number of protons and neutrons in the nucleus.

mass number (A) = number of protons (Z) + number of neutrons

Even for Hydrogen, on periodic tables, it's stated that hydrogen has 1.0078 amu, but this isn't right according to the rule of "the number of neutrons determine the mass of an element," because one Neutron has the following mass:

neutron = 1.00867 amu

and that doesn't at all look anything like 1.0078 amu. so what i did was i added the weight of ONE proton which is 1.00728 amu (looks a lot closer to the actual mass of H ) and then divided by two (for reasons unknown, or not that clear to myself). that gave me this:

1.007975

what do i do??

(thanks in advance for the help and your time!)

 

[DrMark]

No neutrons in hydrogen, 1 neutron is deuterium, 2 neutrons is tritium. The amu of hydrogen ( in my textbooks) is 1.0079. The amu of a proton is 1.0073 the amu of an electron is .00055, so the hydorgen amu is 1.0079.

 

[Pengwuino]

And at add to that, whatever book said that neutrons determine the atomic mass is horribly inaccurate. Nothing by itself determines what the atomic mass is. Its all the electrons + the protons + the neutrons which will equal the atomic mass. Dont let it fool you, its still a basic mass. Its like adding dumbells together. Wanna find the weight of a 'system' of 3 dumb bells, you find hte masses of each dumb bell and add them all together.

 

[Gokul43201]

mass number (A) = number of protons (Z) + number of neutrons

This is perfectly correct.

but this isn't right according to the rule of "the number of neutrons determine the mass of an element,"

This "rule" is wrong ! I'm not sure where you read or heard that, but it is not correct. The mass is determined by both protons and neutrons (as well as, to a much smaller extent, electrons).

The mass of the H-atom is the sum of the proton's and electron's masses.

 

[kusal]

Mass of a atom is the mass of the protones and neutrone not just neutrones.and mass of the hidrogen differs from this a little bit because its the average of the mass of all the hidrogen isotpes available.the value so close to the mass of H1 because deuterium and tritium are really rare.

If this doesn't solve youre question contact me

 

[kusal]

Mass of a atom is the mass of the protones and neutrone not just neutrones.and mass of the hidrogen differs from this a little bit because its the average of the mass of all the hidrogen isotpes available.the value so close to the mass of H1 because deuterium and tritium are really rare.

If this doesn't solve youre question contact me.feel free to contact me at
kusal.w@gmail.com

 

[Pengwuino]

Oh and watch out guys, don't want to confuse this guy by forgetting electrons. Electrons do contribute to the mass but...

mass of proton: 1.6726 x 10 ^ -27 kg
mass of neutron: 1.6749 x 10 ^ -27 kg
mass of electron: 9.11 x 10 ^ -31 kg

So they don't really contribute when your dealing with that few digits... but they are still counted.

 

[mathzeroh]

OHHHHH wow thanks a lot everybody! that cleared everything up, thanks a bunch!

but i had another question, why are the protons all packed on top of the neutrons in the neuclus and why do the electrons spin around them? that's one concept I've never understood really. is there a site where i can read a bit on it, i mean one that'll put it in simple terms? :) thanks again!

 

[Gokul43201]

This is hardly a trivial question.

http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html#c1

Another important distinction is that neutrons and protons belong to a class of particles known as hadrons, while electrons are member of another class known as leptons. This makes the protons and neutrons similar - in a fashion - to each other, and different from the electrons.

http://hyperphysics.phy-astr.gsu.edu/hbase/particles/hadron.html#c5
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/lepton.html#c1