6-1
The present model for the structure of the atom is one in which a small, massive positively
charged nucleus is surrounded by electrons in orbitals. When two objects such as cloth and
amber are rubbed together the electrons in the orbitals of one of the objects (cloth) are
transferred to the orbitals of the other object (amber). This results in the development of
charge also known as static electricity. In the example given, electrons are transferred from
the cloth to the amber resulting in a net negative charge on the amber.
6-2
Du Fay's two-fluid model is consistent with the present model for the structure of the atom if
one thinks of the positively charged nucleus as one of the "fluids" and the negatively charged
electrons as the other "fluid." Objects that are not charged have equal amounts of both
fluids (equal numbers of protons and electrons). This model is only consistent for materials
that lose electron when rubbed leaving a positive charge due to excess protons.
Franklins one-fluid model is also consistent if the fluid is considered to be the electrons of
the atoms making up the object being studied. An object picks up electric charge when
some electrons are transferred from one body to another.
6-3
The evidence that lead to the belief that cathode rays consist of a beam of negatively
charged particles includes: the fact that magnetic fields are deflected by cathode rays, the
fact that a metal plate placed in the path of cathode rays cast a shadow, and the fact that
electric fields are also deflected by cathode rays.
6-4
These observations can be explained by assuming that the atoms or molecules in the gas in
a cathode-ray tube lose one or more electrons when excited by the voltage applied to the
electrodes of the tube. The negatively charged electrons flow towards the anode. The
positive ions formed when the atoms or molecules lose electrons flow in the opposite
direction.
6-5
Scientists imagination was captured by the observation that x-rays could pass through solid
objects placed between the cathode-ray tube and the detector and that x-rays could be used
to produce images of the bones inside a hand.
6-6
The higher energy associated with X-rays allows them to pass through matter while the
lesser energetic visible and ultraviolet light are absorbed.
6-7
Alpha particles are the remains of helium atoms stripped of the associated two electrons.
Beta particles are the same as the particles of cathode rays, electrons. Gamma rays are
very high energy electromagnetic radiation.
6-8
The discovery of a form of radiation that could pass through matter and the fluorescence
associated with the emission of X-rays caused Becquerel to investigate more closely certain
substances which fluoresced. This study involved uranium salts which he found could
"expose" photographic plates in the absence of light. Hence the initial discovery of
radioactivity.
6-9
The H+ ion consists of one proton and no neutrons. All other ions have neutrons. These
neutrons add to the mass of the ion, but not to the charge of the ion, thereby reducing the
charge to mass ratio.
6-10
The charge/mass ratios for the particles in atom units.
(a) proton 1/1
(b) neutron 0/1
(c) alpha 2/4=1/2
(d) electron 1/0.0055
(e) X-rays 0/0
An electron has the largest charge to mass ratio.
6-11
The fundamental make up of matter is described with respect to electrons occupying space
about a compact massive nucleus. The nucleus is described most fundamentally as having
positively charged and neutral components of about equivalent mass. Nuclear physics has
greatly expanded on this model, but for the chemist, three fundamental components of
matter describe all. Negatively charged electrons are the basis of the reactivity of atoms,
positively charged protons determine the nature of the atom and coupled with neutrons
determine the mass related properties of the atom.
6-12
A hydrogen atom consists of a proton and an electron occupying a relatively large volume of
space around the proton. The mass is roughly equivalent to that of a proton (since electrons
have such a small mass) but the overall charge is neutral. A hydrogen atom has roughly the
same mass as a neutron but it consists of two charged particles and occupies a bigger
volume. A proton and a neutron have approximately the same mass, but the proton has a
positive charge while the neutron is neutral.
6-13
The present model for the atom describes a massive positive charged nucleus composed of
protons and neutrons. The nucleus is surrounded by a very spacious cloud of negative
charge (equal in magnitude to that of the nucleus). Based upon sound mathematical
constraints, we have elected to have energies calculable to a relative high degree of
agreement with experiment, but to leave the position of the electron highly uncertain. We
must speak of the probability that an electron exists in a given region of space about the
nucleus.
6-14
Sodium (Na) forms positive ions. For the Na+ ion the values for the atomic number, mass
number, number of protons, and number of neutrons all remain unchanged from those for
elemental sodium. The number of electrons for the Na+ ion is one less than the number of
electrons in elemental sodium. The Na+ ion has 11 protons and 10 electrons resulting in a
net +1 charge.
Chlorine (Cl) form negative ions. For the Cl- ion the atomic number, mass number, number
of protons, and number of neutrons all remain unchanged. The number of electrons for the
Cl- ion is one greater than for elemental Cl. The Cl- ion has 17 protons and 18 electrons
resulting in a net -1 charge.
6-15
24 protons = Chromium (Cr) = 24 atomic number
24 protons + 28 neutrons = 52 mass number
24 protons - 21 electrons = +3 net charge
52Cr3+
6-16
Potassium (K) = 19 protons = 19 atomic number
39 mass number - 19 protons = 20 neutrons
19 protons-(+1 net charge) = 18 electrons
atomic number = Z = 19
mass number = 39
6-17
Iodine (I) = 53 protons = 53 atomic number
127 mass number- 53 protons = 74 neutrons
53 protons-(-1 net charge) = 54 electrons
Z=53
mass number = 1.27
6-18
20 mass number - 11 neutrons = 9 protons
The element is 2OF
6-19
34 protons=Selenium (Se)
34 protons+45 neutrons=79 mass number
34 protons-36 electrons=-2 net charge
79Se2-
6-20
Ba = 56 protons
56 protons - (+2 net charge) = 54 electrons
6-21
1922 model gives 19 protons, 10 electrons in the nucleus, and 9 additional electrons outside
the nucleus for a neutral fluorine atom.
For the F- ion, the 1922 model gives 19 protons, 10 electrons in the nucleus, and 10
additional electrons outside the nucleus.
Assuming the nucleus is composed of protons and neutrons, gives 9 protons, 10 neutrons
and 9 electrons for F, and 9 protons, 10 neutrons and 10 electrons for F-.