Trends In the periodic table. Trends in Atomic Radii. The atomic radius of an atom id defined as half the distance between the nuclei of 2 atoms of the same element joined together by a single covalent bond. Look at the atomic radii in the following table – notice any patterns?.
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In general atomic radii decrease across the period and increase down the group
The first ionisation energy of an element is the minimum energy required to remove the most loosely bound electron from an isolated atom of that element in its gaseous state.
The second ionisation energy of an element would refer to the energy needed for the removal of a second electron from the positive ion
Na Na+ + e-
Na+ Na2+ + e-
Reason: Their atoms are very stable because of their electronic configuration [full outer (sub) level], so it is difficult to remove an electron.
Reason: Their atoms have only one electron in their outer level, so it is
easily removed (as when this is lost it will have noble gas configuration.) This is why group one are so reactive.
Reason: 1. Increase in nuclear charge.
(greater pull for electrons)
2. Decrease in atomic radius.
Reason: 1. Increase in atomic radius.
2. Screening effect.
(This is where the inner shell or shells of electrons help to shield the outer electrons from the positive charge in the nucleus.
There are two exceptions to this generalisation:
(a) Group two elements (e.g. Be, Mg) have abnormally high values. This is because the most loosely bound electron comes from a full s orbital.
(e.g. 1s2, 2s2, 2p6, 3s2 in Mg) which is a relatively stable state. When the next element in each case
(B, Al) is being ionised, the electron being removed is the single electron in the p – orbital
(e.g. 1s2, 2s2, 2p6, 3s2,3p1 in Al).
(b) Group five elements also show abnormally high values (e.g. N and P). The reason here is that the electrons being removed are from exactly half – filled p –orbitals, (e.g. 1s2, 2s2, 2p6, 3s2,3p3 in P) and the half filled orbitals are the next most stable state after that of completely filled orbitals.
2K + ½O2 K2 O
Lithium in air will oxidise completely to a white lithium oxide powder in hours, it only takes a few seconds for this to happen with caesium!
Na + H2O NaOH + ½H2
The more reactive the metal the more heat will be produced and the hydrogen will catch fire this is why you see flames ! The reaction between alkali metals and acid is so dangerous it must never be attempted as too much explosive hydrogen is released