Atomic Structure
Isotopes
- atoms of same element which have different number of neutrons
- same chemical properties because of same number of protons
- different physical properties because of different number of neutrons
Relative Atomic Mass
Relative isotopic masses - relative mass of a particular isotope of an element
Relative atomic masses - weighted average of the isotopic masses of its natural isotopes
Radioactivity
- Gamma-rays are not deflected by an electric field because they do not carry charges
- Alpha and Beta particles are deflected in different directions because of oppositely charged
- Beta particles are deflected much more than Alpha particles because Beta particles have smaller
mass and move slower.
- Uses of radioactive isotopes:
(1) Radiotherapy: Gamma rays from Co-60 are for the treatment of cancer (98/1(b)(i))
(2) Nuclear Power: Nuclear reactions release a vast amount of energy for generating energy
(3) Carbon-14-dating: Concentration of C-14 in archaeological specimen is compared with that of
similar materials at present time (98/1(b)(ii))
(4) Leak detection: Some radioactive sources are introduced into systems such as pipelines and a
detector is used to locate the position of leakage
(5) As tracer: Radioactive isotopes are used in studying metabolism in living organism
Hydrogen Spectrum
The Hydrogen spectrum is composed of discrete spectral lines which indicates the presence of discrete
energy level, and hence an electron in an atom can only exist in certain defined energy level.
Orbital model of atom
The probability description of electrons: The dot density represents the probability of finding the electron.
A boundary line is drawn round the region where the probability of finding an electron is high and the
space enclosed by the boundary is called the orbital.
Electronic Configuration in relation to Periodic Table
The reason for the s-, p-, d-, f- blocks that are so called is that the last sub-shell to be filled in the s-, p-,
d-, f- blocks are the s-, p-, d-, f- sub-shells respectively.
Atomic radius
- When passing down a group, the atomic radius increases. This is because electrons are being added to
the underlying shell and these provided strong shielding effect for outer electrons.
- When passing across a period, electrons are being added to same shell and these added electrons shield
each other only weakly from the extra nuclear charge.
Electro-negativity
- It is the ability of an atom in a covalent bond to attract electrons to itself. (98/5(a))
- Factors affecting electro-negativity include atomic radius, nuclear charge and shielding by inner
sub-shell.
- When passing down each group in the Periodic Table, electro-negativity decreases because electrons
are added to the underlying shell and these provided strong shielding effect for outside electrons.
- When passing across the period, electro-negativity increases. This is because the number of shells
remains unchanged, thus shielding effect is similar. However, nuclear charge increases along the
period. (98/5(b))
Ionization Energy
- It is the energy required to remove an electron from an isolated atom.
- Factors affecting Ionization energy include atomic radius, nuclear charge and shielding by inner
sub-shells.
Successive ionization energy of atom
There is a general increase in ionization energy as electrons are removed from Na atom since
(1) electrons being removed is nearer to the nucleus than the previous one, OR
(2) positive charge on the resulting ion increases and the electrostatic attraction between the ion and the
electron to be removed increases.
First Ionization Energy against atomic number
Discontinuities in the first ionization energy:
(1) between H and He: Removal of 1 electron from He results in destruction of fully filled 1s sub-shell.
(2) between Be and B: Removal of 1 electron from B results in the formation of stable fully-filled
s- sub-shell.
(3) between N and O: Removal of 1 electron from O result in formation of stable half-filled p- sub-shell.
(4) along peaks for the noble gases: Removal of 1 electrons from noble gases (except He) results in the
destruction of an octet structure which is stable.
(5) along the lowest points for the alkali metals: Removal of 1 electron from alkali metals results in the
formation of fully-filled s- or p- sub-shells which is stable.
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