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TOPIC 3: RADIOACTIVITY | PHYSICS FORM 4

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he Nucleus of an Atom

The Structure of the Nucleus of an Atom
Describe the structure of the nucleus of an atom
The word atom is derived from the Greek word atom which means indivisible.
The Greeks concluded that matter could be broken down into particles to small to be seen. These particles were called atoms.
Atoms are composed of three type of particles: protons, neutrons, and electron.Protons and neutrons are responsible for most of the atomic mass e.g in a 150 person 149 lbs, 15 oz are protons and neutrons while
only 1 oz. is electrons.The mass of an electron is very small (9.108 X
10-28 grams).
Both
the protons and neutrons reside in the nucleus. Protons have a positive
(+) charge, neutrons have no charge i.e they are neutral. Electrons
reside in orbitals around the nucleus. They have a negative charge (-).
It
is the number of protons that determines the atomic number, e.g., H =
1. The number of protons in an element is constant (e.g., H=1, Ur=92)
but neutron number may vary, so mass number (protons + neutrons) may
vary.
The same element may contain varying numbers of neutrons; these forms of an element are called isotopes. The chemical properties of isotopes are the same, although the physical properties of some isotopes may be different.
Some
isotopes are radioactive-meaning they “radiate” energy as they decay to
a more stable form, perhaps another element half-life: time required
for half of the atoms of an element to decay into stable form. Another
example is oxygen, with atomic number of 8 can have 8, 9, or 10
neutrons.
The Atomic Number, Mass Number and Isotopes of an Element and their Symbols
Explain the atomic number, mass number and isotopes of an element and their symbols
The
atomic number of a chemical element (also known as its proton number)
is the number of protons found in the nucleus of an atom of that
element.Therefore it is identical to the charge number of the nucleus.
It is conventionally represented by the symbol Z.
The
atomic number uniquely identifies a chemical element. In an uncharged
atom, the atomic number is also equal to the number of electrons.
The atomic number, Z, should not be confused with the mass number, A.
Mass
number is the number of nucleons, i. e the total number of protons and
neutrons in the nucleus of an atom. —The number of neutrons, N, is
known as the neutron number of the atom; thus, A = Z + N (these
quantities are always whole numbers).
Since
protons and neutrons have approximately the same mass (and the mass of
the electrons is negligible for many purposes) and the mass defect of
nucleon binding is always small compared to the nucleon mass, the atomic
mass of any atom, when expressed in unified atomic mass units (making a
quantity called the “relative isotopic mass”), is roughly (to within
1%) equal to the whole number A.
Isotopes
Isotopes are atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses.
A
little more than three-quarters of naturally occurring elements exist
as a mixture of isotopes (see monoisotopic elements), and the average
isotopic mass of an isotopic mixture for an element (called the relative
atomic mass) in a defined environment on Earth, determines the
element’s standard atomic weight.
Historically,
it was these atomic weights of elements (in comparison to hydrogen)
that were the quantities measurable by chemists in the 19th century.The
chemical properties of isotopes are the same, although the physical
properties of some isotopes may be different.
Some
isotopes are radioactive-meaning they “radiate” energy as they decay to
a more stable form, perhaps another element half-life: time required
for half of the atoms of an element to decay into stable form. Another
example is oxygen, with atomic number of 8 can have 8, 9, or 10
neutrons.
Forces Holding the Nucleus
Mention forces holding the nucleus
Stable and unstable atoms
There
are forces within the atom that account for the behavior of the
protons, neutrons, and electrons. Without these forces, an atom could
not stay together.
Recall
that protons have a positive charge, electrons a negative charge, and
neutrons are neutral. According to the laws of physics, like charges
repel each other and unlike charges attract each other. A force called
the strong force opposes and overcomes the force of repulsion between
the protons and holds the nucleus together.
The
net energy associated with the balance of the strong force and the
force of repulsion is called the binding energy. The electrons are kept
in orbit around the nucleus because there is an electromagnetic field of
attraction between the positive charge of the protons and the negative
charge of the electrons.
In
some atoms, the binding energy is great enough to hold the nucleus
together. The nucleus of this kind of atom is said to be stable. In some
atoms the binding energy is not strong enough to hold the nucleus
together, and the nuclei of these atoms are said to be unstable.
Unstable atoms will lose neutrons and protons as they attempt to become
stable.
  1. Binding
    energy is the net energy that is the result of the balance with the
    strong force and the repulsive force, and this is the amount of energy
    that holds the nucleus together.
  2. A stable atom is an atom that has enough binding energy to hold the nucleus together permanently.
  3. An unstable atom does not have enough binding energy to hold the nucleus together permanently and is called a radioactive atom.

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