TOPIC 5: ATOMIC STRUCTURE
The Atom
Sub-atomic Particles
Sub-atomic Particles in an Atom
Identify sub-atomic particles in an atom
Dalton
thought that atoms were solid, indivisible particles. But, as a result
of work done mainly by Lord Rutherford, the idea has been greatly
changed in recent years. According to Rutherford, the atom consists of 3
kinds of particles – protons, neutrons and electrons. These are called
sub-atomic particles.
thought that atoms were solid, indivisible particles. But, as a result
of work done mainly by Lord Rutherford, the idea has been greatly
changed in recent years. According to Rutherford, the atom consists of 3
kinds of particles – protons, neutrons and electrons. These are called
sub-atomic particles.
The
centre of the atom is called nucleus. The nucleus contains a cluster of
two sorts of particles, protons and neutrons. Thenucleus is very small,
occupying only about 1% of the volume of an atom. The rest of the atom
is mostly empty space, withelectrons spread out in it.
centre of the atom is called nucleus. The nucleus contains a cluster of
two sorts of particles, protons and neutrons. Thenucleus is very small,
occupying only about 1% of the volume of an atom. The rest of the atom
is mostly empty space, withelectrons spread out in it.
Electrons
move around the nucleus in special paths calledelectron shells
(orbits/or orbitals or energy levels). Protons andelectrons have
electric charges. Neutrons have no charges.All the particles in an atom
are very light. Their masses aremeasured in atomic mass units rather
than grams. The proton isa positively charged particle. Its mass is
about equal to that ofhydrogen atom. The neutron is has no charge, it is
neutral. Itsmass is about equal to that of hydrogen atom. The electron
isnegatively charged. Its charge is equal but opposite to the chargeon
the proton. It has a very small mass, about 1⁄1840 of the massof the
proton.
move around the nucleus in special paths calledelectron shells
(orbits/or orbitals or energy levels). Protons andelectrons have
electric charges. Neutrons have no charges.All the particles in an atom
are very light. Their masses aremeasured in atomic mass units rather
than grams. The proton isa positively charged particle. Its mass is
about equal to that ofhydrogen atom. The neutron is has no charge, it is
neutral. Itsmass is about equal to that of hydrogen atom. The electron
isnegatively charged. Its charge is equal but opposite to the chargeon
the proton. It has a very small mass, about 1⁄1840 of the massof the
proton.
The Properties of each Particle in an Atom
Explain the properties of each particle in an atom
The properties of these particles are summarizedin the table below:
Table 5.1: Properties of sub-atomic particles
A
single atom is electrically neutral (it has no electrical charge).This
means that in any atom there must be equal numbers ofprotons and
electrons. In this way, the total positive charge onthe protons is
balanced by the total negative charge on theelectrons orbiting the
nucleus. So, the charges must cancel.
single atom is electrically neutral (it has no electrical charge).This
means that in any atom there must be equal numbers ofprotons and
electrons. In this way, the total positive charge onthe protons is
balanced by the total negative charge on theelectrons orbiting the
nucleus. So, the charges must cancel.
Electronic Arrangements
Electronic
arrangement refers to the manner in which electronsare arranged in an
atom. An atom contains a central nucleuscontaining protons and neutrons,
and a cluster of electronsrevolving in orbits around the nucleus. These
electrons aregrouped in shells.
arrangement refers to the manner in which electronsare arranged in an
atom. An atom contains a central nucleuscontaining protons and neutrons,
and a cluster of electronsrevolving in orbits around the nucleus. These
electrons aregrouped in shells.
A Maximum Number of Electrons in the Shells
Determine a maximum number of electrons in the shells
Bohr
(1913) put forward a theory of electron positioning whichis still
generally accepted and used until now for chemicalpurposes. Bohr’s
Theory on the arrangement of electrons in anatom can be summarized as
follows:
(1913) put forward a theory of electron positioning whichis still
generally accepted and used until now for chemicalpurposes. Bohr’s
Theory on the arrangement of electrons in anatom can be summarized as
follows:
- Electrons are in orbit around the nucleus of the atom.
- The
electron orbits are grouped together in shells; a shellis a group of
orbits occupied by electrons withapproximately equal energy. - The electrons in shells distant from the nucleus havehigher energy than those in shells close to the nucleus.
- Electrons
fill the shells starting with the first shell, whichis closest to the
nucleus. Shells are numbered 1, 2, 3, 4,98etc. outwards from the
nucleus. The shells may berepresented by the letters K, L, M and N
respectivelystarting from the nucleus. - The maximum possible number of electrons in a shellnumbered n is 2 2n .
- The
first shell can only contain up to 2 electrons. Thesecond shell can
contain a maximum of 8 electrons. Thethird shell can contain up to 18
electrons. - In the outermost shell of any atom, the maximumnumber of electrons possible is 8.
- The outer electrons of some atoms can be removed fairlyeasily to form ions.
- Chemical bonding between atoms to form moleculesinvolves the electrons in the outer shell only.
Electronic arrangement of some typical atoms is shown in figure5.2.
The
arrangement of electrons around the nucleus is also knownas electronic
configuration. This arrangement depends on themaximum number of
electrons that can occupy a shell. An atomwith 13 electrons will have
the following electronicconfiguration: 2:8:3. This means that there are 2
electrons in thefirst shell, 8 electrons in the second shell and 3
electrons in the third shell.
arrangement of electrons around the nucleus is also knownas electronic
configuration. This arrangement depends on themaximum number of
electrons that can occupy a shell. An atomwith 13 electrons will have
the following electronicconfiguration: 2:8:3. This means that there are 2
electrons in thefirst shell, 8 electrons in the second shell and 3
electrons in the third shell.
The number and arrangement of electrons in the atoms of the first 20 elements are shown in table 5.2.
Table 5.2: The electron arrangements of the first 20 elements
After
the first 20 elements, the organization of the electrons becomes
increasingly complicated. The third shell (n = 3) can be occupied by a
maximum number of 18 electrons
the first 20 elements, the organization of the electrons becomes
increasingly complicated. The third shell (n = 3) can be occupied by a
maximum number of 18 electrons
At
this stage, you will not be asked to work out electronarrangements
beyond element 20 (calcium), but you should beable to understand the
electronic structures involving moreelectrons (for example bromine with
the arrangement 2:8:18:7).
this stage, you will not be asked to work out electronarrangements
beyond element 20 (calcium), but you should beable to understand the
electronic structures involving moreelectrons (for example bromine with
the arrangement 2:8:18:7).
Energy Shell Diagrams
Draw energy shell diagrams
Energy Shell Diagram
Atomic number, Mass number and Isotope
Relationship between Atomic Number and Number of Protons
Relate atomic number with number of protons
All
atoms of one element have the same number of protons. Thisis called the
atomic number (or proton number) of that element.It is given by the
symbol Z.
atoms of one element have the same number of protons. Thisis called the
atomic number (or proton number) of that element.It is given by the
symbol Z.
No
two elements can have the same atomic number. Sodiumatoms have 11
protons. This is what makes them different fromall other atoms. Only
sodium atoms have 11 protons, and anyatom with 11 protons must be sodium
atom.
two elements can have the same atomic number. Sodiumatoms have 11
protons. This is what makes them different fromall other atoms. Only
sodium atoms have 11 protons, and anyatom with 11 protons must be sodium
atom.
In
the same way, an atom with 6 protons must be carbon atom.Also any atom
with 7 protons must be nitrogen atom. So, youidentify an atom by the
number of protons in it. There are 109elements altogether. Of these,
hydrogen has the smallest atoms,with only 1 proton each. Helium atoms
have 2 protons each.Lithium atoms have 3 protons each, and so on up to
meitneriumatoms, which have 109 protons each. Table 5.3 shows the
first20 elements arranged according to the number of protons theyhave.
the same way, an atom with 6 protons must be carbon atom.Also any atom
with 7 protons must be nitrogen atom. So, youidentify an atom by the
number of protons in it. There are 109elements altogether. Of these,
hydrogen has the smallest atoms,with only 1 proton each. Helium atoms
have 2 protons each.Lithium atoms have 3 protons each, and so on up to
meitneriumatoms, which have 109 protons each. Table 5.3 shows the
first20 elements arranged according to the number of protons theyhave.
Every
atom has an equal number of protons and electrons, so theatomic number
also tells us the number of electrons in that atom.In any given atom of
an element, the number of neutrons has no effect on the identity and
properties of that particular element. It is the number of protons and
electrons that determine the identity and properties of any given
element. The number of neutrons only affects the mass, since each one of
them has the same mass as that of a proton.
atom has an equal number of protons and electrons, so theatomic number
also tells us the number of electrons in that atom.In any given atom of
an element, the number of neutrons has no effect on the identity and
properties of that particular element. It is the number of protons and
electrons that determine the identity and properties of any given
element. The number of neutrons only affects the mass, since each one of
them has the same mass as that of a proton.
Mass Number of an Atom from Numbers of Protons and Neutrons
Calculate mass number of an atom from numbers of protons and neutrons
Protons
alone do not make up all the mass of an atom. The neutrons in the
nucleus also contribute to the total mass. The mass of the electrons can
be regarded as so small that it can be ignored. As a proton and a
neutron have the same mass, the mass of a particular atom depends on the
total number of protons and neutrons present. This is called mass
number (or nucleon number). The mass number of an atom is found by
adding together the number of protons and neutrons. It is given by the
symbol A. Table 5.3 shows the mass number of the first 20 elements,
arranged in order of increasing atomic mass (mass number).
alone do not make up all the mass of an atom. The neutrons in the
nucleus also contribute to the total mass. The mass of the electrons can
be regarded as so small that it can be ignored. As a proton and a
neutron have the same mass, the mass of a particular atom depends on the
total number of protons and neutrons present. This is called mass
number (or nucleon number). The mass number of an atom is found by
adding together the number of protons and neutrons. It is given by the
symbol A. Table 5.3 shows the mass number of the first 20 elements,
arranged in order of increasing atomic mass (mass number).
If the mass number and atomic number for any given atom are known, then its sub-atomic composition can be worked out.
The
mass number = number of protons + neutrons in an atom. Sodium atom has
11 protons and 12 neutrons, so the mass number of sodium is 23. Since
the atomic number is the number of protons only, then:
mass number = number of protons + neutrons in an atom. Sodium atom has
11 protons and 12 neutrons, so the mass number of sodium is 23. Since
the atomic number is the number of protons only, then:
Mass
number – atomic number = number of neutrons. So, for sodium atom, the
number of neutrons = (23-11) =12. You can also take into account the
fact that, because the number of protons is always equal to the number
of electrons, then the number of electrons in sodium atom is simply 11.
The same rule can be applied to work out the sub-atomic composition of
any element.
number – atomic number = number of neutrons. So, for sodium atom, the
number of neutrons = (23-11) =12. You can also take into account the
fact that, because the number of protons is always equal to the number
of electrons, then the number of electrons in sodium atom is simply 11.
The same rule can be applied to work out the sub-atomic composition of
any element.
These two relationships are useful:
- Number of electrons = number of protons = atomic number
- Number of neutrons = mass number (A) – atomic number (Z).
The Concept of Isotope
Explain the concept of isotope
Atoms
of the same element may have different numbers of neutrons. In a normal
situation, atoms of the same element will have the same number of
neutrons. However, many cases occur in which two atoms of the same
element contain the same number of protons but different numbers of
neutrons. Having equal number of protons, these atoms must also have
equal numbers of electrons. However, the differing numbers of neutrons
cause the atoms to have different mass numbers. An element showing such
properties is said to show isotopy and the varieties of the atom are
called isotopes of the element.
of the same element may have different numbers of neutrons. In a normal
situation, atoms of the same element will have the same number of
neutrons. However, many cases occur in which two atoms of the same
element contain the same number of protons but different numbers of
neutrons. Having equal number of protons, these atoms must also have
equal numbers of electrons. However, the differing numbers of neutrons
cause the atoms to have different mass numbers. An element showing such
properties is said to show isotopy and the varieties of the atom are
called isotopes of the element.
Therefore,
isotopy can be defined as the tendency of atoms of one element to
posses the same atomic number but different mass numbers (atomic
masses). Isotopes can be defined as atoms of the same element with the
same number of protons but different numbers of neutrons, or as „atoms
of the same element with the same atomic number but different atomic
masses‟.
isotopy can be defined as the tendency of atoms of one element to
posses the same atomic number but different mass numbers (atomic
masses). Isotopes can be defined as atoms of the same element with the
same number of protons but different numbers of neutrons, or as „atoms
of the same element with the same atomic number but different atomic
masses‟.
The
isotopes of an element have the same chemical properties because they
contain the same number of electrons. It is the number of electrons in
an atom that decides the way in which it forms bonds and reacts with
other atoms. However, some physical properties of the isotopes are
different. The masses of the atoms differ, and therefore other
properties, such as density and rate of diffusion, also vary.
isotopes of an element have the same chemical properties because they
contain the same number of electrons. It is the number of electrons in
an atom that decides the way in which it forms bonds and reacts with
other atoms. However, some physical properties of the isotopes are
different. The masses of the atoms differ, and therefore other
properties, such as density and rate of diffusion, also vary.
Many
isotopes (like tritium) are unstable. The extra neutrons in their
nuclei cause them to be unstable so that nuclei break spontaneously
(that is, without any extra energy being supplied), emitting certain
types of radiation. They are known as radioisotopes.
isotopes (like tritium) are unstable. The extra neutrons in their
nuclei cause them to be unstable so that nuclei break spontaneously
(that is, without any extra energy being supplied), emitting certain
types of radiation. They are known as radioisotopes.
Notation for isotopes
In
order to distinguish between different isotopes of the same element in
writing symbols and formulae, a simple system is adopted. The isotope of
an element, say X will have the symbol X,AZ , where A is the mass
number of the isotope and Z is the atomic number of any atom of X. Thus,
for all isotopes of one element, Z is constant, and A varies because
there are different numbers of neutrons in the different isotopes of the
element. For example, the three isotopes of carbon are expressed as
12C6, 13C6,and 14C6. Chlorine has two isotopes: 35Cl17 and 37Cl17 .
Since A represents the total number of neutrons and protons in the
nucleus of an atom (mass number/atomic mass), and because Z is the
number of protons (atomic number), then the number ofneutrons in the
nucleus of a given isotope is given by:Number of neutrons in the nucleus
= A – Z
order to distinguish between different isotopes of the same element in
writing symbols and formulae, a simple system is adopted. The isotope of
an element, say X will have the symbol X,AZ , where A is the mass
number of the isotope and Z is the atomic number of any atom of X. Thus,
for all isotopes of one element, Z is constant, and A varies because
there are different numbers of neutrons in the different isotopes of the
element. For example, the three isotopes of carbon are expressed as
12C6, 13C6,and 14C6. Chlorine has two isotopes: 35Cl17 and 37Cl17 .
Since A represents the total number of neutrons and protons in the
nucleus of an atom (mass number/atomic mass), and because Z is the
number of protons (atomic number), then the number ofneutrons in the
nucleus of a given isotope is given by:Number of neutrons in the nucleus
= A – Z
Relative atomic masses
As
we have seen, most elements exist naturally as isotopes.Therefore, the
value we use for the atomic mass of an element isan average mass. This
takes into account the proportions(abundance) of all the naturally
occurring isotopes. If aparticular isotope is present in high
proportion, it will make alarge contribution to the average.
we have seen, most elements exist naturally as isotopes.Therefore, the
value we use for the atomic mass of an element isan average mass. This
takes into account the proportions(abundance) of all the naturally
occurring isotopes. If aparticular isotope is present in high
proportion, it will make alarge contribution to the average.
Example
A
sample of chlorine gas contains 75% of the isotope 35Cl17 and 25% of
the other isotope 37Cl17 . What is the relative atomic mass of chlorine?
sample of chlorine gas contains 75% of the isotope 35Cl17 and 25% of
the other isotope 37Cl17 . What is the relative atomic mass of chlorine?
Solution
To work out this problem, simply multiply the mass number ofeach isotope with the abundance and sum up the products thus:
This
average value for the masses of atoms of an element isknown as the
relative atomic mass (Ar).Therefore, the relative atomic mass of
chlorine is 35.5 (i.e., Ar =35.5).
average value for the masses of atoms of an element isknown as the
relative atomic mass (Ar).Therefore, the relative atomic mass of
chlorine is 35.5 (i.e., Ar =35.5).
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