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Topic: How do we know?(periodic table)  (Read 1551 times)

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Offline 1210

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How do we know?(periodic table)
« on: April 22, 2016, 11:44:05 AM »
I've just studied periodic table..
My teacher said "we'll know how many protons the element has from atomic number.."

We can't see atom.. so,How do we know Carbon has 6protons? (+I want to know how Mendeleev knew that this element has this atomic number..)

Thank you in advance :)

Offline mikasaur

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Re: How do we know?(periodic table)
« Reply #1 on: April 22, 2016, 11:53:31 AM »
The scientific method is all about making hypotheses and testing them against reality. It's an iterative and evolutionary process for the most part. Through theories, experiments, and observations we have slowly come to know that an atom is composed of a dense nucleus of protons and neutrons surrounded by an electron cloud.

So there's no one answer to your question. The chemistry we do in everyday life corroborates our understanding of the atom. But through a quick Google search I found some resources. Take them with a grain of salt as they do come from the internet:

General overview of scientists and their experiments
Mosley's Law, using X-rays to measure Z
An excerpt from Rutherford's famous experiment
« Last Edit: April 22, 2016, 12:42:53 PM by mikasaur »
Or you could, you know, Google it.

Offline AdiDex

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Re: How do we know?(periodic table)
« Reply #2 on: April 22, 2016, 01:22:37 PM »
I want to know how Mendeleev knew that this element has this atomic number

Actually he didn't know how many protons the carbon atom have ..!! he just arranged the elements in the increasing order of atomic weight . He died in 1907 , He published his work in 1870's where as Plum Pudding model was proposed in 1904 and electron itself discovered in 1897 . So He didn't know about atomic structure when he published his work .

We can't see atom.. so,How do we know Carbon has 6protons?

Yes it is true we can't see nucleus . But there are indirect evidences . Like We have proof that electron exist as Beta rays are emitted when we apply high voltage through a gas at very low pressure . Also that whole atom is neutral  as it does not get deflected in the electric field .  So there must be both electron and proton in the atom (for electrical neutrality) .
Then we can calculate charge/mass ratio .
Check how we know electron exist and how we know the charge by mass ratio .
https://www.youtube.com/watch?v=p7YHjohpJrs


[url=https://www.youtube.com/watch?v=7WtdF4B5fmo

https://www.youtube.com/watch?v=37E40LwBrls


Then there are chemical reactions which proves that Carbon must have 6 electrons and there are so many ways to explain this .
« Last Edit: April 22, 2016, 01:41:39 PM by AdiDex »

Offline jameschadwick

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Re: How do we know?(periodic table)
« Reply #3 on: February 17, 2021, 06:37:01 AM »
1. How do we get the number of protons now?

For charge neutrality, the number of electrons has to equal the number of protons. So, the question really is how to know how many protons and how many electrons. This is actually a very interesting question dating back to the early days (and even before) of nuclear physics and chemistry.

Now, one can do it fairly easily with an ion accelerator and a magnet. With the right set up, one can get a beam of mono-energetic ions out of the accelerator, send them through a stripper to remove electrons, and then send them through a magnet to separate the components out by velocity (which depends on the ion mass through the energy). From the magnet deflections, one can get charge-to-mass ratios, and get back to the number of protons (charge on a fully stripped ion) and the number of neutrons (from the number of protons and the mass). This is the heart of mass spectrometry (although one can use time-of-flight techniques rather than, or in addition to, magnetic separation).

Back then chemists agreed that hydrogen is basically one positive and one negative ie one electron and one positive particle and then all the other elements were taken as ratios with respect to it.


2. How did Mendeleev know atomic numbers?
Mendeleev didn't consider atomic numbers. He worked with atomic mass and arranged the elements based on atomic mass. Then came Henry Moseley with Moseley's law wherein he established the atomic number of protons as a more fundamental property.

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