Download PODCAST 1 Atomic Structure

PODCAST 1 – 19/6/2015 Atomic Structure and its history.
Hello and welcome to this podcast from allerytutors.com. My name is Chris Harris and I will guiding
you through the topic of atomic structure and its history. In brief I will take you through the
structure of the atom including relative charge and mass of subatomic particles, isotopes and their
physical and chemical properties and then onto the rather interesting history of atomic models with
a nod to the modern quantum theory model we use today.
This podcast is designed to go through content required for the new A Level Chemistry specifications
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OK we are going to start with the structure of the atom. The atom is spherical in shape and is made
up of positive protons and neutral neutrons in the middle called the nucleus. There are negatively
charged electrons that orbit around the nucleus in orbitals. Most of the mass of the atom is actually
concentrated in the middle where the protons and neutrons are found however the electron shells
take up most of the space which makes the nucleus look tiny! To put this into perspective, imagine a
football ground and there is a garden pea sitting in the centre circle (this is the nucleus) and the
perimeter of the stadium is the size of the full atom. When sitting in the stand it would be very
difficult to see the pea yet the vast majority of the mass of an atom is concentrated in this tiny, tiny
u leus! Al ost diffi ult to o p ehe d is t it!?
Sub-atomic particles (e.g. Protons, Neutrons and Electrons) have a tiny mass so we normally give
them a relative mass to make life a little easier. So Protons and Neutrons have a relative mass of 1
each, with the electron having a relative mass of approximately 1/2000. This means the electron is
2000 times lighter than a proton or neutron! We also have relative charges too; a proton is +1, a
neutron is 0 and an electron is -1.
In the periodic table you will find 2 numbers for each element. The bigger number is the atomic
mass number and will tell you the number of protons and neutrons in an atom. The smaller number
is the atomic (proton) number and tells you the number of protons. Every element has a unique
number of protons (a little bit like its bar code). E.g. Carbon will always have 6 protons, if an atom
has 20 protons it will be Calcium. In an atom the number of protons = the number of electrons and
this makes the atom neutral in charge as there are an equal number of positive protons and negative
electrons. How do we o k out the u e of eut o s? Well tha kfull it s si ple, e just take the
mass number and minus the atomic number.
Seen as though we are on the topic of atoms we should also talk about ions! Ions are formed when
we add or lose electrons in an atom that leaves an overall charge depending on the ratio of negative
electrons to positive protons that is left behind.
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For example O2- has a double negative charge. This means that this ion has 2 more negative electrons
than positive protons hence the 2 negative charge. On the other hand if we look at Li+ it has a single
positive charge which suggests it has one more positive proton than negative electrons hence the
positive charge on the ion.
We can have elements where we have the same number of protons but a different number of
neutrons. We call these ISOTOPES! This is the same element because they have the same number of
protons but will weigh differently as they have a different number of neutrons. Let s look at an
exampleCarbon has 3 common isotopes – Carbon-12, Carbon-13, and Carbon-14. The numbers tell us the
mass number. The mass number is increasing as they have a different number of neutrons, but
crucially they are all CARBON atoms and hence all these isotopes have 6 protons. These isotopes
exist in different amounts (we call this abundance) e.g. Carbon 12 is the most abundant isotope,
followed by Carbon 13 and then Carbon 14 which is not stable and is radioactive.
All isotopes of the same element react in the same way as they have the same number of electrons
as each other. Chemical properties are governed by the number and configuration of electrons in
the shells. The Physical properties of isotopes are different e.g. density. This is because they have
different masses and this is one of the main factors that determine the Physical properties of
isotopes.
The history of the atom
No
e ha e all the st u tu e of the ato all so ted let s take a st oll i to the past he e e go
back to the 5th Century BC in ancient Greece. Philosopher Democritus came up with the idea that if
we keep cutting a material i half a d i half agai the e ill get to so ethi g that a t e ut
any more and he alled this at os ea i g u utta le.
This was all we knew about the atom for an incredible 2,400 years, when, at the start of the 19th
century the next development of the atom was introduced by an English Scientist called John Dalton.
Dalton came up with an idea that an atom was a solid sphere and also said that different elements
had a different type of sphere. This explained the different properties of the elements that had been
discovered at the time.
I
a othe E glish a , this ti e a ph si ist, alled JJ Tho so e pe i e ted ith Dalto s
idea a d o luded that the ato ould t e a solid sphe e as he fou d evidence of charges in
atoms and suggested that the atom was a sphere that was positively charged with negative charges
in it (we call these ele t o s o ). JJ Tho so s odel as k o as the plu
puddi g odel as
the atomic structure resembled a desert that was well known at the time.
Just 12 years later a New-Zealand born British Physicist called Ernest Rutherford decided to
experiment further and came up with evidence that proves the plumb pudding model was not right!
His experiment really revolutionised the way we look at the structure of an atom and was known as
the gold leaf experiment. Rutherford fired positively charged alpha particles at a very thin sheet of
gold. Rutherford expected most of the alpha particles to deflect as the plumb pudding model
suggested a positively charged sphere with negative charges in it. Rutherford (along with his
students – Hans Geiger and Ernest Marsden) were surprised to find out that most of the alpha
particles went straight through with no deflection and only a few were deflected back completely.
This disp o ed Tho so s odel a d ‘uthe fo d suggested a e
odel-
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1. Most of the atom was just empty space.
2. There was a very small positive charge in the middle where most of the mass of the atom is
concentrated (we call this the nucleus).
3. A cloud of electrons surrounded the nucleus.
Well after all this you would think that would be that. A good model that explains all that was
u e plai ed at the ti e. But s ie tists ould t esist a othe e pe i e t to fi d out if this model
was really correct! As it turns out Henry Moseley discovered that elemental charge increased by 1
between neighbouring elements in the periodic table. Rutherford, like a dog with a bone, went back
to his original model and did further experiments to finally come up with a unit of positive charge.
The proton was born! He said that elements had a different number of protons.
Ho e e the ato
as hea ie tha thought a d this ould t just e do to p oto s. I
English Physicist James Chadwick came up with another sub-atomic particle that had mass but no
charge, the neutron. It was so good that he was awarded a Nobel prize for Physics in 1935!
It does t stop there! Danish physicist Niels Bohr suggested that the cloud of electrons hypothesised
‘uthe fo d ould t e o e t as the ele t o s ould just ash i to the u leus due to the
attractive forces between negative electrons and positive nucleus. Bohr suggested a shell structure
where a fixed number of electrons could fit into the shell and each had a different amount of energy
(quantisation). At the time scientists knew about radiation being emitted and absorbed by atoms
a d Boh s odel suggests that ele t o s ould o e et ee shells if the e e gi e o lost
energy in the form of radiation. Due to the fixed nature of the shells the frequency of the wave
needed to promote an electron from a lower energy shell to a higher one was fixed too. His shell
theory also stated that there was a maximum number of electrons that could occupy each shell. This
explained why noble gases were unreactive as they had the maximum number of electrons in their
outer shell.
Fi all it i gs us to hat e k o a out the ato toda . Boh s odel is a odel e use to help
explain trends in the reactivity of elements and draw atomic structure. On a more complex note we
k o the ato has ele t o s su ou di g a u leus ho e e e do t k o
he e the ele t o s a e
or where they are moving to at any one time and the electrons can behave as a wave and a particle!
Ouch! We call this quantum mechanics. This theory explains the likelihood of where an electron will
be at any one time. Schrodinger had an equation that helps to explain what is going on however
tha kfull ou do t eed to k o this at A Le el. Just take a look at S h odi ge s Cat theo to get
a flavour of how Schrodinger thinks.
Well that s the e d of this pod ast o the ato . I hope this has p o ed useful a d e e e to
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That s all folks! Lo e Che ist !
© C Harris (AlleryTutors.com)