# Introduction and models  (Page 3/3)

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## Models of the atom

Match the information in column A, with the key discoverer in column B.

 Column A Column B Discovery of electrons and the plum pudding model Niels Bohr Arrangement of electrons Marie Curie and her husband, Pierre Atoms as the smallest building block of matter Ancient Greeks Discovery of the nucleus JJ Thomson Discovery of radiation Rutherford

## Atomic mass and diameter

It is difficult sometimes to imagine the size of an atom, or its mass, because we cannot see an atom and also because we are not used to working with such small measurements.

## How heavy is an atom?

It is possible to determine the mass of a single atom in kilograms. But to do this, you would need very modern mass spectrometers and the values you would get would be very clumsy and difficult to use. The mass of a carbon atom, for example, is about $1,99×{10}^{-26}\phantom{\rule{2pt}{0ex}}\mathrm{kg}$ , while the mass of an atom of hydrogen is about $1,67×{10}^{-27}\phantom{\rule{2pt}{0ex}}\mathrm{kg}$ . Looking at these very small numbers makes it difficult to compare how much bigger the mass of one atom is when compared to another.

To make the situation simpler, scientists use a different unit of mass when they are describing the mass of an atom. This unit is called the atomic mass unit (amu). We can abbreviate (shorten) this unit to just 'u'. Scientists use the carbon standard to determine amu. The carbon standard assigns carbon an atomic mass of 12 u. Using the carbon standard the mass of an atom of hydrogen will be 1 u. You can check this by dividing the mass of a carbon atom in kilograms (see above) by the mass of a hydrogen atom in kilograms (you will need to use a calculator for this!). If you do this calculation, you will see that the mass of a carbon atom is twelve times greater than the mass of a hydrogen atom. When we use atomic mass units instead of kilograms, it becomes easier to see this. Atomic mass units are therefore not giving us the actual mass of an atom, but rather its mass relative to the mass of one (carefully chosen) atom in the Periodic Table. Although carbon is the usual element to compare other elements to, oxygen and hydrogen have also been used. The important thing to remember here is that the atomic mass unit is relative to one (carefully chosen) element. The atomic masses of some elements are shown in the table below.

 Element Atomic mass (u) Carbon ( $\mathrm{C}$ ) 12 Nitrogen ( $\mathrm{N}$ ) 14 Bromine ( $\mathrm{Br}$ ) 80 Magnesium ( $\mathrm{Mg}$ ) 24 Potassium ( $\mathrm{K}$ ) 39 Calcium ( $\mathrm{Ca}$ ) 40 Oxygen ( $\mathrm{O}$ ) 16

The actual value of 1 atomic mass unit is $1,67×{10}^{-24}\phantom{\rule{2pt}{0ex}}\mathrm{g}$ or $1,67×{10}^{-27}\phantom{\rule{2pt}{0ex}}\mathrm{kg}$ . This is a very tiny mass!

## How big is an atom?

pm stands for picometres . $1\phantom{\rule{2pt}{0ex}}\mathrm{pm}={10}^{-12}\phantom{\rule{2pt}{0ex}}\mathrm{m}$

Atomic radius also varies depending on the element. On average, the radius of an atom ranges from $32\phantom{\rule{2pt}{0ex}}\mathrm{pm}$ (Helium) to $225\phantom{\rule{2pt}{0ex}}\mathrm{pm}$ (Caesium). Using different units, $100\phantom{\rule{2pt}{0ex}}\mathrm{pm}=1\phantom{\rule{2pt}{0ex}}\mathrm{Angstrom}$ , and $1\phantom{\rule{2pt}{0ex}}\mathrm{Angstrom}={10}^{-10}\phantom{\rule{2pt}{0ex}}\mathrm{m}$ . That is the same as saying that $1\phantom{\rule{2pt}{0ex}}\mathrm{Angstrom}=0,0000000010\phantom{\rule{2pt}{0ex}}\mathrm{m}$ or that $100\phantom{\rule{2pt}{0ex}}\mathrm{pm}=0,0000000010\phantom{\rule{2pt}{0ex}}\mathrm{m}$ ! In other words, the diameter of an atom ranges from $0,0000000010\phantom{\rule{2pt}{0ex}}m$ to $0,0000000067\phantom{\rule{2pt}{0ex}}m$ . This is very small indeed.

The atomic radii given above are for the whole atom (nucleus and electrons). The nucleus itself is even smaller than this by a factor of about 23 000 in uranium and 145 000 in hydrogen. If the nucleus were the size of a golf ball, then the nearest electrons would be about one kilometer away! This should help you realise that the atom is mostly made up of empty space.

## Rutherfords alpha-particle scattering experiment

Radioactive elements emit different types of particles. Some of these are positively charged alpha ( $\alpha$ ) particles. Rutherford carried out a series of experiments where he bombarded sheets of gold foil with these particles, to try to get a better understanding of where the positive charge in the atom was. A simplified diagram of his experiment is shown in [link] .

Rutherford set up his experiment so that a beam of alpha particles was directed at the gold sheets. Behind the gold sheets was a screen made of zinc sulphide. This screen allowed Rutherford to see where the alpha particles were landing. Rutherford knew that the electrons in the gold atoms would not really affect the path of the alpha particles, because the mass of an electron is so much smaller than that of a proton. He reasoned that the positively charged protons would be the ones to repel the positively charged alpha particles and alter their path.

What he discovered was that most of the alpha particles passed through the foil undisturbed and could be detected on the screen directly behind the foil (A). Some of the particles ended up being slightly deflected onto other parts of the screen (B). But what was even more interesting was that some of the particles were deflected straight back in the direction from where they had come (C)! These were the particles that had been repelled by the positive protons in the gold atoms. If the Plum Pudding model of the atom were true then Rutherford would have expected much more repulsion, since the positive charge according to that model is distributed throughout the atom. But this was not the case. The fact that most particles passed straight through suggested that the positive charge was concentrated in one part of the atom only.

## Relative atomic mass

Relative atomic mass

Relative atomic mass is the average mass of one atom of all the naturally occurring isotopes of a particular chemical element, expressed in atomic mass units.

The relative atomic mass of an element is the number you will find on the periodic table.

what is the stm
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
what is Nano technology ?
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
why we need to study biomolecules, molecular biology in nanotechnology?
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
why?
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
research.net
kanaga
sciencedirect big data base
Ernesto
Introduction about quantum dots in nanotechnology
what does nano mean?
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
for teaching engĺish at school how nano technology help us
Anassong
How can I make nanorobot?
Lily
Do somebody tell me a best nano engineering book for beginners?
there is no specific books for beginners but there is book called principle of nanotechnology
NANO
how can I make nanorobot?
Lily
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
how did you get the value of 2000N.What calculations are needed to arrive at it
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Good
The fundamental frequency of a sonometer wire streached by a load of relative density 's'are n¹ and n² when the load is in air and completly immersed in water respectively then the lation n²/na is
Properties of longitudinal waves