Chemistry: Structure of the atom
today we define an atom as the
smallest particle of an element that
retains the chemical properties of that
element.
J.J Thompson 1807
J.J THOMPSON is the responsible for
the discovery of the electron. An
electron is surrounding the nucleus, it’s
a region occupied by negatively
charged particles.
It is connected to a vacuum pump because charges (Electrons)
will flow through gases only at very low pressures.
experiments were performed in which
electric current was passed through
various gases at low pressures. (Gases at atmospheric pressure don’t conduct electricity well.)
These experiments were carried out in glass tubes that had been hooked up to a vacuum
pump. Such tubes are known as cathode-ray tubes.
Investigators noticed that when current was passed through the tube, the surface of the tube
directly opposite the cathode glowed. They hypothesized that the glow was caused by a stream
of particles, which they called a cathode ray. The ray traveled from the cathode to the anode
when current was passed through the tube.
J.J THOMPSON measure how much heat the cathode rays generated he estimate the mass of
the ray produced which the mass was a thousand time lighter than hydrogen, the smallest bit
of matter knows at the time, he concluded that they weren’t “rays” but tactually very light
and small negatively charged particles.
In one investigation, he was able to measure the ratio of the charge of cathode-ray particles to
their mass. He found that this ratio was always the same, regardless of the metal used to make
the cathode or the nature of the gas inside the cathode-ray tube.
Thomson concluded that all cathode rays must be composed of identical negatively charged
particles, which named electrons.
Ernest Rutherford
ERNEST RUTHERFOR is responsible for the discovery of the proton and the nucleus. While
experimenting he used Alpha Particles, which are positively charged particles with about
four times the mass of a hydrogen atom. The nucleus is a very small region located at the
center of an atom. In every atom, the nucleus is made up of at least one positively charged
particle called a proton and usually one or more neutral particles called neutrons.
Ernest Rutherford design for the experiment used a thin gold sheet and a screen to detect
the deflected particles. He bombarded the sheet with Alpha particles (which are positively
charged and are produced by the decay of radium). He expected then alpha particle to go thru
the foil with no deflection (that mass and charge were uniformly distributed throughout the
atoms of the gold foil) and in many try they did just that, BUT in one occasion roughly 1 in
8000 of the alpha particles had actually been deflected back toward the source.
He concluded that the force must be caused by a very densely packed bundle of matter with
a positive electric charge. Rutherford called this positive bundle of matter the nucleus.
Therefore, if alpha particles a positive that means that the nucleus must contain another
subatomic particle that has a positive charge (equals repel).MOST OF THE ATOM IS
EMPTYSPACE AND MOST OF ITS MASS IS ON THE NUCLEUS (99%).
where were the electrons? This question was not answered until Rutherford’s student, Niels
Bohr, proposed a model in which electrons surrounded the positively charged nucleus as the
planets surround the sun.
James Chadwick
JAME CHADWICK is the responsible for the discovery of the neutron. the nucleus is made up
of at least one positively charged particle called a proton and usually one or more neutral
particles called neutrons.
Since opposites attract and equals repel people thought that there must be some other
subatomic particle that “holds the protons together”. This particle was the neutron and it does
exactly that.
James Chadwick shoots alpha particles to beryllium and he notices that rays get release from
the atoms of Beryllium, what he did was that when he shot the alpha particles thru the nuclei
of the beryllium he was knocking out the “unknown” particles out into a beam that Chadwick
would be able to study, he then used this beam and tested it with the cathode ray, to his
surprised the beam did not bend towards any charge when going thru the plates which meant
it was neutral particles (0 charge).
Nuclear Forces
The problem was that in the nucleus there are protons and if we follow the electromagnetic
forces, they should repel each other but they were not, in fact they had a strong bond,
therefore there must be a force holding them together.
How does it work? When protons are really close together instead of repelling each other
the created a strong attraction between them, there’s a similar attraction that neutrons
close to each other have or when protons and neutrons are very close together the same
strong bond forms.
If the subatomic particles didn’t have these short-range distance they would repel normally.
Properties of the subatomic particles
The radii of atoms are expressed in picometers.
The radius of an atom is the distance from the center of the nucleus to the outer portion of
this electron cloud. Because atomic radii are so small, they are expressed using a unit that is
more convenient for the sizes of atoms. This unit is the picometer. The abbreviation for the
picometer is pm (1 pm = 10-12 m = 10-10 cm). To get an idea of how small a picometer is,
consider that 1 cm is the same fractional part of 103 km (about 600 mi) as 100 pm is of 1 cm.
Atomic radii range from about 40 to 270 pm. By contrast, the nuclei of atoms have much
smaller radii, about 0.001 pm. Nuclei also have incredibly high densities, about 2 × 108
metric tons/cm3.