Atomic Structure: From Ancient Idea to Quantum Reality

Name: Introduction to the Atom
Uploaded: 2015-05-07T14:23:03+00:00
Duration: 21 min 5 s
Channel: Saylor University
Description: Summary and key takeaways on Introduction to the Atom: Summary & Key Takeaways, covering Historical and Philosophical Context The word “atom” comes from the

Saylor University

May 07, 2015

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21 min video

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YouTube video ID: muGua29QPl4

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The word “atom” comes from the Greek meaning “uncuttable” or “indivisible.” Ancient philosophers imagined the atom as the smallest piece of matter that could not be divided further. Modern chemistry treats the atom as a mental abstraction that helps describe observable phenomena, blurring the line between physical reality and information.

Atomic Structure and Mechanics

An atom consists of a dense nucleus made of protons and neutrons, surrounded by a cloud of electrons. Protons carry a positive charge, electrons a negative charge, and neutrons are neutral. Opposite charges attract, shaping how electrons arrange themselves around the nucleus.

Electrons do not travel in simple planetary orbits. Instead, they occupy orbitals, which are mathematical probability functions that indicate where an electron is likely to be found. Because position and momentum cannot be known simultaneously with precision, the orbital model replaces the classical “planetary” picture.

Defining Elements and Isotopes

The element’s identity rests on its atomic number, the count of protons in the nucleus. Changing the proton count creates a different element. Neutron numbers can vary without changing the element; such variants are called isotopes. The atomic weight of an element is a weighted average of the masses of its naturally occurring isotopes.

Mass, Scale, and Empty Space

A proton (or neutron) has a mass of roughly 1 atomic mass unit (amu), about 1,836 times the mass of an electron. Consequently, atomic mass calculations ignore electron mass and focus on the nucleus. The nucleus occupies an infinitesimally small fraction of the atom’s volume—approximately one‑hundred‑thousandth.

This extreme size disparity means that ordinary matter is overwhelmingly empty space. “If you look at your hand or if you look at the wall or if you look at your computer, 99.999 % of it is free space.” In other words, most things we perceive as solid are largely void.

Quantum Model vs. Classical Orbit

The planetary orbit model fails to explain observed quantum phenomena, prompting the shift to the orbital probability framework. This quantum model treats the atom as a mental abstraction that captures the statistical nature of electron locations, aligning with Coulomb’s Law, which governs the attraction between opposite charges.

Takeaways

The term “atom” originates from the Greek word meaning “uncuttable” and historically described an indivisible piece of matter.
Modern chemistry treats atoms as mental abstractions that consist of a dense nucleus of protons and neutrons surrounded by electrons occupying probability‑based orbitals.
An element is defined solely by its atomic number—the count of protons—while isotopes differ only in neutron number and affect the element’s average atomic weight.
Because a proton’s mass is about 1,836 times that of an electron, atomic mass is essentially the combined mass of protons and neutrons, with the nucleus occupying roughly one hundred‑thousandth of the atom’s volume.
Consequently, over 99.999 % of ordinary matter is empty space, meaning objects we perceive as solid are largely void.

Frequently Asked Questions

Why are electrons described by orbitals instead of planetary orbits?

Electrons are described by orbitals because the classical planetary orbit model cannot account for observed quantum behavior; orbitals are mathematical probability functions that indicate where an electron is likely to be found, reflecting the uncertainty principle that prevents simultaneous precise knowledge of position and momentum.

How does the atomic number determine an element’s identity?

The atomic number equals the number of protons in the nucleus, and this count uniquely identifies each chemical element; changing the proton count creates a different element, while varying the neutron count produces isotopes that share the same element but have different atomic masses.

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Large Wall Periodic Table Of Elements Recommended

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Molecular Model Kit For Chemistry Students

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Introductory Chemistry Textbook For College Students

Offers a comprehensive, structured deep dive into the atomic theory and quantum concepts introduced in the video.

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Quantum Physics Book For Beginners

Explains the probability-based nature of electrons and orbitals in a more accessible, conceptual format.

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Full Transcript YouTube

in most topics you have to get pretty
Advanced before you start addressing the
philosophically interesting things but
in chemistry it just starts right from
the get-go with what's arguably the most
philosophically interesting part of the
whole topic and that's the atom and the
idea of than Adam is philosophers long
ago and you could look it up on the the
different philosophers who first
philosophized about it they said hey you
know if I started off with I don't know
if I started off with an apple if I
started off with an apple and I just
kept cutting the Apple let me draw a
nice looking Apple just so it doesn't
look just like a hard or there you go
have a nice looking apple and you just
kept cutting it smaller and smaller
pieces So eventually you get a piece so
small so tiny that you can't cut it
anymore and I'm sure some of these
philosophers went out there with a knife
and tried to do it and they just felt
that oh if I could just get my knife a
little bit sharper I could cut it again
and again so it was completely
philosophical construct which frankly in
a lot of ways isn't too different to how
to how the atom is today it's really
just a a a mental abstraction that
allows us to describe a lot of
observations we see in the universe but
anyway these philosophers said well at
some point we think that there's going
to be some little Point part of a of an
apple that they won't be able to divide
anymore and they call that an Adam it
doesn't have to just be for an apple
they said this is true for any substance
or any element that you encounter in the
universe and so the word Adam is really
Greek for
uncuttable uncuttable or
indivisible
uncuttable now we know that it actually
is cutable and even though it is not a
trivial thing it's it's not the the
smallest form of matter we know we now
know that an atom is made up of other
more fundamental particles and let me
let me write them so we have
the we have the
neutron and I'll draw in a second of how
they all fit together in in kind of the
structure of an atom we have a neutron
do have a
proton and we have
electrons
electrons and you might already be
familiar with this if you look at uh old
videos about about Atomic projects
you'll see a drawing that looks
something like this let me see if I can
draw one so you'll have you know
something like that and you'll have
these things spinning
around that look like this you know they
have orbits that look like that and
maybe something that looks like
that and the general notion Behind these
kind of nuclear drawings and I'm sure
that they still show up at some
government defense Labs or something
like that is that you have a nucleus at
the center of an
atom you have a nucleus at the center of
an atom and we know that a nucleus has
neutrons and
protons neutrons and protons and we'll
talk a little bit more about which
elements have how many neutrons and how
many protons and then or orbiting
orbiting and I'm going to use the word
orbit right now although we learn in
about 2 minutes that the word orbit is
actually the incorrect or even the
mentally incorrect way of visualizing
What electron is doing but this the old
idea was that you have these electrons
that are orbiting around the nucleus
very similar the way the Earth orbits
around the Sun or the moon orbits around
the earth and it's been shown that
that's actually a very wrong way and it
there's a lot of um and and when we when
we cover quantum mechanics we'll learn
why this doesn't work what are kind of
the contradictions that emerge when you
try to model an electron like a planet
going around going around the Sun but
this was kind of the original idea and
frankly I think this is kind of the idea
that is is is the most mainstream way of
of viewing a a an atom now I said an
atom is philosophically interesting why
is it philosophically interesting
because the what we now view as the
accepted way of viewing an atom really
starts to blur the line between kind of
re you know our our physical reality and
and you know everything in the world is
just information there really isn't any
such thing as true matter or true
particles is the way we Define them in
our everyday life you know what for me a
particle it's like a grain of sand I can
pick it up touch it while you know a
wave that could be like a sound wave it
could be this you know change in energy
over time but we'll learn especially
when we do quantum mechanics that it all
gets jumbled up as we start we start
approaching the scales or the size of an
atom but anyway I said this was an
incorrect way of vieing it what's the
correct
way so it turns out let's see this is a
picture not a picture really this is
also a depiction so so it's an
interesting question what I just said
you know how can you have a picture of
an atom because it actually turns out
that the wave most wavelengths of light
that we especially the visible
wavelengths of light are much larger
than the size of an atom so it's not
like something you can even you know
everything else we quote unquote observe
in life it's by reflect light but all of
a sudden when you're dealing with an
atom reflective light is almost you
could almost view it as too big or too
blunt of an instrument with which to
observe an atom any this is a depiction
of a helium atom a helium atom a helium
atom has two protons and two neutrons or
at least this helium atom has two
protons and two neutrons and they
depicted here in the nucleus right there
maybe these are the two I'm assuming
they're using red for proton and purple
for Neutron
Neutron seems like purple seems like
more of a neutral color and they're
sitting at the center of of of this of
this of this atom and then this whole
haze around there those are the two
electrons that helium has or at least
this helium atom has maybe it could you
know gain or lose an electron but these
are the two electrons and you say hey s
that doesn't make you know how can two
electrons be this this blur that's kind
of schmeared around this this atom and
that's where it gets philosophically
interesting so you cannot describe an
electron's path around a around a a
nucleus with the kind of traditional
orbit idea that we've encountered when
we look at planets or if we just you
know imagine uh things at kind of a
larger scale it turns out that an
electron you cannot know exactly its
momentum and location at any given point
in time all you can know is a
probability distribution on where it is
likely to be and the way they depicted
this black is a higher probability so
you're much more likely to find the
electron here than you are here but the
electron really could be anywhere it
could even be here even though it's
completely white there but with some
very very very very very low probability
and so this function of where an
electron is this is called an orbital
orbital not to be confused with orbit
orbital orbital remember an orbit was
something like this it's like the you
know Venus going around the sun it's
just it's very physically easy for us to
imagine while an orbital is actually a
mathematical probability function that
tells us where we're likely to find an
electron and we'll deal a lot more of
that when we cover quantum mechanics but
that's not going to be in the scope of
this kind of introductory set of
chemistry lectures but it's interesting
right an electron is it it's Behavior so
bizarre at that scale that you can't I
mean to call it a particle is almost
misleading it is called a particle but
it's not a particle in the sense that
we're used to in our everyday life it's
this thing that is that you can't even
say exactly where it is it's it can be
anywhere in this Haze and we'll learn
later that there are different shapes of
the hazes as we add more and more
electrons to an atom but that to me
that's you know it starts to address
kind of philosophical issues of of what
matter even is or you know do the things
we look at how real are they or how real
are they at least as as we've defined
reality anyway I don't want to get too
philosophical on you but the the whole
notion of of of electrons protons
they're all kind of predicated on this
notion of charge and we've talked about
it before when we learned about koloms
law and you could you could review K
kul's law videos in the physics playlist
but the idea is that an electron has a
an electron has a negative charge a
proton sometimes written like that has a
positive charge and a neutron has no
charge and so that that's what was
tempting about the original
model of an electron is that they say
okay if this thing has positive charges
right because let's say this is two
neutrons and two protons let's say it's
the helium atom then we'll have some
positive charges here we have some
negative charges out here opposite
charges attract and so you know if these
things had some had some I guess you
could call some some velocity they would
or enough velocity they would orbit
around this is just the way uh uh uh
just the way a planet will orbit around
the Sun but now we learn even though
this is kind of partially true that you
know the further away the further away
an electron
is from the nucleus the further away an
electron is from the nucleus it does
have more I guess you could I mean it's
true potential energy in that it will
want to move towards the nucleus but
because of all these the the the
mechanics at the quantum level it won't
just do something simple like moving a
path like that like a comet would do
around the Sun it actually has a this
kind of wav like Behavior where it just
has this probability function that
describes it but the the kind of the
further away an orbital it does have
more potential and we're going to go a
lot more into that in in future videos
but anyway how do you recognize what an
element is I've talked a lot about the
philosophy and all of that but how do I
know this is helium is it by the number
of neutrons it has is it by the number
of protons it has is it by the number of
electrons well the answer is it's by the
number of protons so if you know the
number of protons in an element you know
what that element is and the number of
protons number of
protons this is defined as the atomic
number atomic
number now so how do you if let's say I
said something has four
protons four protons how do we know what
it is well we could if we haven't
memorized it we could look it up on the
Periodic Table of Elements which we'll
be dealing with a lot in this playlist
and you'd say oh four protons that is
brillium right there and the atomic M
the atomic number is the number that you
see up there and that's literally the
number of protons and that is the single
most different that is what
differentiates one atom from another if
you have 15 protons you're dealing with
phosphorus now all of a sudden if you
have if you have seven let's say you
have seven your protons you're dealing
with nitrogen if you have eight you're
dealing with oxygen that is what defines
the element now we'll talk in the future
about what happens with with charge and
all of that but or what happens when you
gain or lose electrons but that does not
change what element you're dealing with
and likewise when you have the when you
change the number of neutrons that also
does not change the element you're
dealing with
but that leads to an obvious question of
well how many neutrons and and electrons
do you have well if a if a atom is
atomically neutral is is charg neutral
that means it has the same number of
electrons and so let's say that I have a
carbon let's say this this let's say
carbon it has its atomic number is six
and let's say it's its mass
number is 12 now what does this mean so
let's and list me let me say further
that this is a this is a neutral
particle this is a neutral atom so the
atomic number for carbon is six that
tells us exactly how many protons it
have so if I were to draw a little model
here and this is in no way an accurate
model I'll draw six 2 3 four five six
protons in the center now and and the
weight of these protons each proton is
one atomic mass unit and we'll talk more
about that how that relates to kilogram
it's a very small fraction of kilogram
roughly one I think it's like 1.6 * 10
the - 27th of a kilogram so let's say
each of these let me write that each of
these are one atomic mass unit and
that's approximately equal to I think
1.67 * 10- 27 kg this is a very small
number it's actually it's almost
impossible to to visualize at least it
is for me this tells me the the the mass
of the entire carbon atom this
particular carbon atom and this can
actually change from carbon atom to
carbon atom and this is essentially the
mass of all of the protons plus all of
the neutrons and each proton has an
atomic mass of one in atomic mass units
and each neutron has an atomic mass of
one atomic mass unit so this is really
the number of protons number of
protons plus the number of of neutron
plus number of
neutrons so in this case we have six
protons so we must also have six
neutrons six
neutrons plus six protons now where are
the electrons well I said it's neutral
so the the proton has an equal positive
charge as the electron's negative charge
so this is a neutral atom and it has six
protons so it also has six electrons let
me draw that so we said it has six
neutrons in here 1 2 3 4 5 6 so that's
it's that's the nucleus right there and
then if we were to draw the
electrons well I I could draw it to a
shmear but if we want to kind of
visualize a little better we could say
okay there's going to be six electrons
orbiting 1 2 3 4 five six and they're
going to be moving around in this
unpredictable way that we would have to
describe with a probability function and
so and and the interesting thing about
it is most of the mass of a of a of a of
an atom is sitting right in here I mean
you might notice that when people care
about the mass of a when they care about
kind of the atomic mass number of a of
an atom they ignore the electrons and
that's because the mass of a proton one
proton Mass wise is equal to
1,836 electrons so for thinking about
the mass of a atom for all you know for
for all basic purposes you can ignore
the mass of an electron that it's really
the the mass of the the mass of the
nucleus that that counts as the of the
as the kind of mass of the atom now you
might see this periodic table here and
you say okay they gave us the atomic
number up there right the atomic number
of oxygen is eight it means it has eight
protons the atomic number of silicon is
14 it has 14 protons now what is this
right here let's see let in carbon
carbon they have this
12.17 that is the atomic weight of
carbon so let me write this right down
Atomic
atomic
weight of
carbon and the atomic weight of carbon
is
12.0
12017 now what does that mean does that
mean that you you know Carbon has six
protons six protons and then the
remainder the remaining
6.0 107 neutrons it has kind of this
fraction of a neutron no it means if you
were to average all of the carbon all of
the different carbon versions of carbon
you find on the planet and you were to
average the number of neutrons based on
on the different on on the on the kind
of the quantity of the different types
of carbon this is the average you would
get so so it turns out that carbon it
the the two major forms the main one
you'll find is carbon 12 so that's like
this so that has six protons and six
neutrons and then another isotope of
carbon now an isotope is the same
element with a different number of
neutrons another isotope of carbon is
carbon 14 which is much more scarce in
the in on on the planet we don't know
how much you know in the universe but on
the planet now if you were to average
these not just a straight up Aver then
you would get you know Carbon 13 and
then the atomic weight would be 13 but
you weight this one much higher because
this exists in much larger quantities on
Earth I mean this is pretty much all of
the carbon that you see but there's a
little bit of this so if you weight them
appropriately the average becomes this
so most of the carbon you'll find so if
you just took found carbon in the in
someplace on average its weight in
atomic mass units is going to be 12.0
12.01 07 but that idea of an isotope is
an remember when you change the neutrons
you're not changing the actual
fundamental element you're just getting
a different isotope a different version
of the element so these two versions of
carbon are both Isotopes now I want to
leave this video which I think is kind
of the neatest idea behind atoms and
it's kind of the most philosophically
interesting things about them is that
the the relative size so you know we
have this electrons which ma represent
very little of the mass of of a of a of
an atom I mean you know one 1 2000th of
of the mass of an atom are in the
electrons and even them those are it's
hard to even describe them as particles
because they you can't even tell me
exactly where and how fast one of these
particles is moving they just have a
probability function so most of the atom
is sitting that inside the nucleus and
this is the interesting thing if you
look at an an atom on average if you say
you know this is my atom and let's say I
had two atoms that are bonded to each
other and I would say what you know how
much of this is actual stuff and when I
say stuff that's a very abstract concept
because we're talking about the nucleus
right because the nucleus is where all
the mass is all the stuff it turns out
that it's actually an infinitism small
fraction of the volume of the atom where
if you know the volume of the atom it's
hard to to find because the electron can
pretty much be anywhere but if you view
the volume as where you're most likely
to find the electron or you know with
90% probability you're likely to find
the electron then the nucleus is on a
lot of cases and the way I think is
about 1,000th of the volume so if you
think about when you look at something
if you look at your hand or if you look
at the wall or if you look at your
computer
99.99 9% of it is free space it's
nothing it's vacuum it's if if you were
to just you know if you had Ultra small
I guess we could call them particles or
something most of them would pass
straight through whatever you look at so
it already starts to kind of question
our holds on reality what is there when
if you know and this is this is kind of
this is fact this isn't Theory right
here that if you take anything down to
the building blocks down to the atomic
level most of the space on of that kind
of quote unquote object is free vacuum
space you could go straight through it
if you could get down to that scale I
mean this is this image of a helium atom
they say right here this is one
femtometer right one
fomer one fto meter they this is the
scale of the nucleus of a helium atom
right one femtometer this is one
angstrom right and they say that equals
100,000 femtometers and just to get a
sense of scale one angstrom is 1 * 10
-10 M right so the atom is roughly on
the scale of an angstrom in the case of
helium the nucleus is even a smaller
fraction it's 100,000th so when you if
you had you know let's say you had
liquid helium which you'd have to get
very cold to get if you're looking at
that most of it is free space if you're
looking at an iron bar the great great
great great great great majority of it
is free space and we're not even talking
about what's you know maybe there's some
free space inside the nucleus that we
could talk about in the future but to me
that just blows my mind that most things
we look at are not really solid they're
really just empty space but they look
solid because of the way light reflects
on them or the forces that repel us but
there really isn't kind of something to
to touch there that most of this right
here is all free space I think I've said
the word free space now and I think I'll
leave leave further mind-blowing to the
next video

Help & FAQ

Atomic Structure and Mechanics

Defining Elements and Isotopes

Mass, Scale, and Empty Space

Quantum Model vs. Classical Orbit

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