Maris Stella High School
Structure of Atoms
Name: _________________________(
) Class: 3______ Date: __________
Topic 5: Structure of Atoms
Syllabus Objectives:
(a) State the relative charges and approximate relative masses of a proton, a
neutron and an electron.
(b) Describe, with the aid of diagrams, the structure of an atom as containing
protons and neutrons (nucleons) in the nucleus and electrons arranged in
shells (energy levels).
(c) Define proton (atomic) number and nucleon (mass) number.
(d) Use and interpret symbols such as
.
(e) Define the term isotopes.
(f) Deduce the number of protons, neutrons and electrons in atoms and ions
given proton and nucleon numbers
1. Particles in an Atom
 All elements are made up of atoms.
 An atom is made up of 3 subatomic particles: the proton, the neutron
and the electron.
Particle
proton
neutron
found in the
nucleus (centre
of the atom)
electron - goes round nucleus in orbits
Mass
(Relative to a hydrogen
atom)
Charge
1
1
1
1840
 Since an atom has no overall charge, it contains equal number of
protons and electrons.
i.e. for an atom: Number of protons = Number of electrons
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Maris Stella High School
Structure of Atoms
2. Definitions


Mass number (or nucleon number) - the sum of the number of
____________ and ____________ in an atom
[Note: ‘nucleons’ refer to the ______________________ in the nucleus]
 Atomic number (or proton number) - the number of __________ in an
atom
 The atomic number is fixed for each atom. An atom is identified by its
atomic number. Different elements will have different atomic number.
3. Representation of elements with symbols
Atomic number Mass number
Note:
Since the proton number of sodium is 11, it means that
o A sodium atom has 11 protons.
o Conversely, any atom with 11 protons is a sodium atom.
Mass number = number of protons + number of neutrons
Hence, number of neutrons = mass number - _______________
Examples:
o Derive the number of protons, electrons and neutrons in a sodium atom
o Malaysium(Ma), an imaginary element contains 111 protons and 141
neutrons. How would you represent the atom of Malaysium in symbolic
form?
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Maris Stella High School
Structure of Atoms
4. Isotopes

Isotopes are atoms of the same element with the same number of
protons but different number of neutrons;
i.e., Isotopes have the same atomic number but different mass numbers.
E.g.
and
are isotopes.
Number of protons
Number of electrons
Number of neutrons

Similarities and differences between isotopesIsotopes
Similarities
Differences
number of protons
number of electrons

number of neutrons


atomic numbers

mass numbers

chemical properties


physical properties
e.g. density.
5. Arrangement of electrons in the atom
 Electrons are arranged in energy levels (called electron shells) around the
nucleus. The nearer the shell to the nucleus, the __________ the energy
level.
 Each energy level can hold a certain number of electrons. For the first 20
elements in the Periodic Table,
first shell can hold up to 2 electrons
electrons
Second shell can hold up to 8 electrons
Third shell can hold up to 8 electrons
nucleus
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Maris Stella High School
Structure of Atoms
 To work out the arrangement of electrons:
(i) Find out the proton number of the atom or element;
then for a neutral atom, the no. of electrons = the proton number
(ii) Main rule -- electrons always go into the shell nearest to the nucleus ,if
there is room. Once the shell is filled up, the electrons go into the next
available shell and so on.
E.g. the electronic structure of aluminium:
Note: the simplified electronic structure of aluminium is written as 2,8,3
6. Valence Electrons and the Valence Shell

Outermost shell OR Valence shell - shell ____________ from the
nucleus

Valence electrons (Outermost electrons)
- electrons in the _____________ shell
- determines the __________ properties of an element
7. Arrangement of Electrons and the Periodic Table
 The position of an element in the Periodic Table is determined by its proton
number.
 All elements in the same group (vertical column) have the same number of
______________, which is the same as the Group number.
 All elements in the same period (horizontal row) have the same number of
_______________.
i.e.:
Elements in:
(a) the same ________  same number of outermost/valency
electrons
(b) the same ________  same number of electron shells
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Maris Stella High School
Structure of Atoms
Try these!
6 12
1. The symbol for a carbon atom is C.
(a) Draw the atomic structure of carbon;
(b) Write down its electronic structure.
2. The elements X, Y and Z have the following electronic configurations:
X : 2, 8, 7 Y : 2, 4
Z : 2, 8, 8, 2
Name the Groups to which these elements belong.
3. An element X has the atomic number 12. Without looking at the Periodic
Table, work out the Group to which this element belongs.
4. Answer the following questions without looking at the Periodic Table:
(a) An element has 14 protons. What is the atomic number of the element next
to it on the left?
(b) Sodium is in Group I of the Periodic Table and has an atomic number of 11.
What is (i) the number of protons; (ii) the number of electrons,
in the element Y, which is a group VI element in the same Period as
sodium.
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