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Another important finding was the discovery of isotopes. During the early 1900s, scientists identified several substances that appeared to be new elements, isolating them from radioactive ores. For example, a “new element” produced by the radioactive decay of thorium was initially given the name mesothorium. However, a more detailed analysis showed that mesothorium was chemically identical to radium (another decay product), despite having a different atomic mass. This result, along with similar findings for other elements, led the English chemist Frederick Soddy to realize that an element could have types of atoms with different masses that were chemically indistinguishable. These different types are called isotopes    —atoms of the same element that differ in mass. Soddy was awarded the Nobel Prize in Chemistry in 1921 for this discovery.

One puzzle remained: The nucleus was known to contain almost all of the mass of an atom, with the number of protons only providing half, or less, of that mass. Different proposals were made to explain what constituted the remaining mass, including the existence of neutral particles in the nucleus. As you might expect, detecting uncharged particles is very challenging, and it was not until 1932 that James Chadwick found evidence of neutrons , uncharged, subatomic particles with a mass approximately the same as that of protons. The existence of the neutron also explained isotopes: They differ in mass because they have different numbers of neutrons, but they are chemically identical because they have the same number of protons. This will be explained in more detail later in this chapter.

Key concepts and summary

Although no one has actually seen the inside of an atom, experiments have demonstrated much about atomic structure. Thomson’s cathode ray tube showed that atoms contain small, negatively charged particles called electrons. Millikan discovered that there is a fundamental electric charge—the charge of an electron. Rutherford’s gold foil experiment showed that atoms have a small, dense, positively charged nucleus; the positively charged particles within the nucleus are called protons. Chadwick discovered that the nucleus also contains neutral particles called neutrons. Soddy demonstrated that atoms of the same element can differ in mass; these are called isotopes.

Chemistry end of chapter exercises

The existence of isotopes violates one of the original ideas of Dalton’s atomic theory. Which one?

Dalton originally thought that all atoms of a particular element had identical properties, including mass. Thus, the concept of isotopes, in which an element has different masses, was a violation of the original idea. To account for the existence of isotopes, the second postulate of his atomic theory was modified to state that atoms of the same element must have identical chemical properties.

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Questions & Answers

how to complete and balance ionic equation?
BARAKA Reply
what is molar mass?
Wendy Reply
Molar mass is defined as the mass of a sample of a compound divided by the amount of substance of that sample.
Love
right
xavi
or M=m\n mathematically.
xavi
atom is the smallest indivisible part of an element that takes place in chemical reaction
Obinna Reply
An atom is the smallest indivisible part of an element
Vibes
what is chemistry?
Mathias Reply
what is an atom
Mathias
what is an atom?
Mathias
is the study of matter and the changes it under goes.
xavi
under goes through three state they are solid liquid and gas
Samuel
in a sturctural way formula for C6H12
Samuel
sorry ,it's properties and structures
xavi
please what is hydroxyl group
Gold Reply
the hydroxyl group is a functional group consisting of a hydrogen atom covalently bonded to an oxygen atom.
Itz
Names are of homologous series
Josiah Reply
what's an hydroxyl group?
VICTORY Reply
A hydroxy or hydroxyl group is the entity with the formula OH. It contains oxygen bonded to hydrogen. In organic chemistry, alcohols and carboxylic acids contain hydroxy groups. The anion [OH⁻], called hydroxide, consists of a hydroxyl group. 
okwara
what are the steps to follow when writing the condensed formula of elements
Jessicafaith Reply
What is organic chemistry
John Reply
what's organic chemistry?
Mr Reply
why do electrons have no mass
Skynetic Reply
hello
Samuel
hello there
Oliver
hi and who say electrons has no mass ? it does have but most at time consider negleable
Eyong
hello
Lagazani
please what is maa
Lagazani
yeah i will calculate it
divine
Because it's mass it very very small compare with mass of atom
Yasmeen
it's simple
lola
consider the rxn of butadiene .Butadiene is prepared by the gas phase catalytic dehydrogenation of 1-butene , at 900K and 1 bar. c4H8 (g)_ C4H6 (g) + H2 (g) (a) In order to suppress side reactions , the butane is diluted with steam before it passes into the reactor.Estimate the conversion
Rodrick
Estimate the conversion of 1-butene for a feed consisting of 10 moles of steam per mole of 1-butene. (b) find the conversion if the inerts were absent and side reactions are ignored .
Rodrick
help me please I have tried
Rodrick
wat is the difference between electrochemical chemical corrosion
Uzor
please how can i solve this project work..modern aid science process and product
Lagazani
what is the definition of chemistry
Sophia Reply
Sophia pls check on fb u will see transcendence Internationale is a group from here just join over, and ask any question in science from there we here created it
Eyong
the icon is clear white then writing transcendence Internationale study group
Eyong
U will see it
Eyong
for now chemistry is the study of matter and it properties, why and how element combined to form other substances
Eyong
Chemistry is a group that identifies properties of matter and many more
Alona
Wow and chemistry also is the systematic study of nature
Eyong
chemistry is the study of the structure, properties of matter and the effect of addition or removal of heat from substances.
Bra
calculate the relative molecular Mass of the following 1 ZnCl2 2 NaNo3 3 HNO3 4 CaO
Reuben Reply
2) 23+14+(16×3)= 3)1+14+(16×3)= 4) 40+16= 1) Zn+71= that's how they do it but for Zn I don't know its mass number
Eyong
Reuben u are welcome to join the group transcendence Internationale on fb we created it for science students is a question and answering platform for all science subjects
Eyong
yeah it's fun
Christian
hi guys I'm new here, please I need definition of acid and base
Gold
please some one should tell me the meaning of hydro carbon
Glory
What is radioactivity
Aaron Reply
radioactivity is the spontaneous disintegration of the nucleus of an atom of an element to emmit radiation
Vivian
When dividing a certain and uncertain number, you still use sig figs? Ex: 12.86g divided by 12 M&Ms
Kristen Reply
Practice Key Terms 6

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Source:  OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9
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