Download 2C1 Background OC39

JSSS
Teacher support material
Describe the structure of the atom, state the location, relative charge, and
atomic number of the sub-atomic particles, and define atomic number and
isotope
OC39
Prior Knowledge
Students should have studied the following LOs: OC1, OC3, OC4, OC8, OC9 and OP48.
Students are expected to learn about the structure of the atom or, more
correctly, to learn about a particular model of the structure of the atom.
The usual model of the structure of the atom met at Junior Certificate level
consists of the nucleus at the centre of one or more shells of electrons. The
electrons are usually shown as being placed in these circular shells. Here
is a typical representation of a sodium atom. This model is appropriate at this level.
Students will be familiar with the use of ‘nucleus’ in biology and may sometimes confuse atoms
and cells. They should be made aware that the atomic nucleus is a completely different entity to
the cell nucleus.
The lack of application of basic electrical ideas to the atom is reflected in the way students often
conceptualise the way the electrons are held in position around the nucleus.
According to accepted scientific principles:

all electrons in an atom are attracted to the nucleus

the force acting on an electron due to the nucleus depends on the magnitude of the
nuclear charge and the distance between the electron and the nucleus

the attractive force between an electron and the nucleus acts in both directions

each electron repels the others electrons with a force which depends on the distance
between them.
Students are often confused about the relative numbers of protons, neutrons and electrons in
atoms and may think that all three numbers must always be equal. It should be explained to
students that number of protons determines the identity of the element, the number of electrons
equals the number of protons in a neutral atom, and the number of neutrons determines the
particular isotope of the element.
Some students think that only certain elements have isotopes and/or all isotopes are radioactive.
It should be explained to students that most elements have naturally occurring isotopes, but not
all isotopes occur naturally or are stable. Emphazise, that all the isotopes of an element, behave
the same chemically. However those isotopes whose nuclei are unstable are radioactive.
Explain to students that, although the Bohr model of the atom is often presented, the well defined
orbits are not correct. Point out that, unlike the motion of the planets in our solar system,
electron motion is irregular and an electron may, in fact, be quite close to the nucleus at times.
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