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The atomic theory has changed much since John Dalton's time. In the
late seventeen hundreds John Dalton first discovered that elements combine
in specific proportions because they are made of individual atoms. For
example, hydrogen and chlorine always combine in a specific proportion to
create hydrochloric acid. Dalton thought that atoms were small particles,
like a small marble. Dalton's atomic theory consisted of three parts. They
were:
*All substances are made of atoms. Atoms are small particles that cannot
be created, divided, or destroyed.
*Atoms of the same substance are exactly alike, atoms of different atoms
are different.
*Atoms join with other atoms of other substances to make new substances.
Dalton's theory was proved wrong when new information was discovered
and Dalton's theory could not explain it. The atomic theory has evolved.
In 1897, a scientist named J.J. Thompson found and identified a
problem in Dalton's theory. He discovered that there were particles inside
the atom. So, Thompson said, atoms can be divided into even smaller parts.
The particles that were found inside the atom had a negative force. The
negatively charged particles were called electrons. Scientists had already
found out that atoms had no overall charge. Thompson didn't know the
location of the positively charged material so he made a theory that the
negatively charged electrons were embedded in the atom which was made
of overall positive matter. Thompson's model was called the plum-pudding
model. The electrons looked like plums that were in the pudding .
In 1909, a man named Ernest Rutheford, a former student of
Thompson wanted to investigate his former teacher's theory. He aimed a
small beam of radium particles at a sheet of thin gold foil. He thought if
atoms were only soft blobs embedded with electrons like Thompson had
suggested then the beam of particles should go straight through the gold
foil and keep going in a straight line. Some of the particles were deflected
though. The particles couldn't be soft blobs then. He said the electrons
floated around in empty space, around the nucleus. He proposed that the
nucleus was a small, extremely dense, positively charged region. He
guessed that the electrons floated around this nucleus like small planets
revolving around the sun. He also calculated from the experiment that the
nucleus was 100,000 times smaller than the entire atom. This would be like
walking to the middle of a basketball stadium and putting a penny in the
jump-ball circle.
Next, a Danish scientist named Niels Bohr proposed that electrons
didn't just float around the nucleus, but that they moved around in certain
paths. He said that the electrons could jump up and down levels. These
paths would be a certain way away from the nucleus. Bohrs model seemed
too simple to scientists though.
Scientists now think that the definite path of an atom cannot be
predicted. They travel around in a predicted region because they do not
move around in straight paths. These regions where they are likely to be
found are called electron clouds. These clouds are like the paths of Bohr's
model.
Scientists still aren't sure about the definite structure of atoms , but they think that they
are very close
Keywords:
atomic theory changed much since john dalton time late seventeen hundreds john dalton
first discovered that elements combine specific proportions because they made individual
atoms example hydrogen chlorine always combine specific proportion create
hydrochloric acid dalton thought that atoms were small particles like small marble atomic
theory consisted three parts they were substances made atoms small particles that cannot
created divided destroyed same substance exactly alike different different join with other
other substances make substances theory proved wrong when information discovered
could explain atomic evolved scientist named thompson found identified problem
discovered there were particles inside atom thompson said divided into even smaller parts
found inside atom negative force negatively charged called electrons scientists already
found overall charge thompson didn know location positively charged material made
negatively charged electrons embedded atom which overall positive matter model called
plum pudding model electrons looked like plums pudding named ernest rutheford former
student wanted investigate former teacher aimed beam radium sheet thin gold foil thought
only soft blobs embedded with like suggested then beam should straight through gold foil
keep going straight line some deflected though couldn soft blobs then said floated around
empty space around nucleus proposed nucleus extremely dense positively region guessed
floated around this nucleus planets revolving also calculated from experiment times
smaller than entire this would walking middle basketball stadium putting penny jump ball
circle next danish scientist named niels bohr proposed didn just float they moved certain
paths said could jump down levels these paths would certain away from bohrs model
seemed simple scientists though scientists think definite path cannot predicted travel
predicted region because move straight paths these regions where likely called electron
clouds these clouds bohr still aren sure about definite structure think very close
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