Notes for Standard 1.2 and 1.3
Read about cathode ray tubes and the discoveries of Joseph John Thompson. (pages
72—73 of your text) What evidence led him to conclude that electrons are present
in atoms of all elements and that the electrons in one element are the same as the
electrons in another element? What evidence led him to conclude that electrons
have a negative charge?
Read about the discovery of the atomic nucleus on page 74. What was so
surprising about the gold foil experiment? What evidence led Rutherford to
conclude that the nucleus of an atom is very small compared to the total volume of
an atom? What evidence led Rutherford to conclude that the nucleus is positively
charged and very dense?
It would be difficult to build an actual scale model of an atom because of the
following problems:
If the nucleus were the size of a marble, the whole atom would be the size of a
football field and the marble would need to weigh millions of tons. Also, the
electrons move so fast that they form a cloud or electric field around the atom.
Electrons do not orbit neatly like planets in a solar system, but instead orbit in
strangely shaped three-dimensional regions around the nucleus in which there is a
probability of finding the electron.
Read about the composition of the atomic nucleus on page 75.
Read about Atomic Number on page 77. What gives atoms their identity?
Where are the electrons, protons, and neutrons in an atom?
What is the relative electric charge of each of these particles? What is the relative
mass of each of these particles? (See table one on page 76)
There are 6.022 X1023 atoms in one mole of any element. The mass in grams of one
mole of any element can be found on the periodic table.
The periodic table is organized by increasing atomic number (# of protons in the
nucleus). In a neutral atom, the number of protons(+) equals the number of
electrons (-). The mass number is the number of protons plus neutrons and is
often written after a hyphen (carbon-12 or oxygen-16). Atoms with the same
number of protons but different numbers of neutrons are called isotopes. For
example, carbon-12 has 6 protons and 6 neutrons, carbon-13 has 6 protons and 7
neutrons and carbon-14 has 6 protons and 8 neutrons. Carbon-12, carbon-13, and
carbon-14 are all isotopes. Most elements occur naturally as mixtures of isotopes.
The molar mass given on the periodic table for each element is actually the average
atomic mass of one mole of that element, which takes into account the % abundance
at which each isotope occurs in nature (read page 81).
On the periodic table, vertical columns are called groups and horizontal rows are
called periods. Elements in the same group have similar chemical behavior because
they have the same number of valence electrons. For example, group 1 (the alkali
metals) are all very soft, low-density metals that react violently with water to
produce very basic (alkali) solutions. They each have only one valence electron.
Group 2 (the alkaline earth metals) are less reactive than the group one metals and
are a little harder and more dense. Group two elements each have two valence
electrons.
Group 17 elements (the halogens) are very reactive non-metals. Group 17 elements
each have 7 valence electrons. Group 18 elements (the noble gases) are all
unreactive gases. They are unreactive because they each have a completely filled
outer main energy level of 8 electrons.
The stair-step line that begins between Al and Si divides the periodic table between
metals on the left and non-metals on the right. The elements that border the stairstep line have properties of both metals and non-metals and are known as
metalloids or semimetals or semiconductors. (Read pages 16—20)
The size of atoms generally increases as you move down a group. This is because as
you move down the table, the electrons have to fill main energy levels that are
farther from the nucleus.
Francium is the most reactive metal because its single electron is in the 7th main
energy level far removed from the pull of the protons in the nucleus. This makes it
very easy to give up that electron to another atom.
Flourine is the most reactive non-metal. It only has two occupied shells of
electrons so it is very easy for electrons orbiting other atoms to get close enough to
flourine to be attracted by the pull of the nine protons in flourine’s nucleus.