Differences and similarities in the properties between the second and the third period elements
A diagonal
relationship is said to exist between certain pairs of diagonally adjacent
elements in the second and third periods of the periodic table. These pairs
(lithium (Li)
and magnesium (Mg), beryllium (Be) and aluminium (Al), boron (B) and silicon (Si) etc.) exhibit similar properties;
for example, boron and silicon are both semiconductors, forming halides that are hydrolysed in water and have acidic oxides.
and magnesium (Mg), beryllium (Be) and aluminium (Al), boron (B) and silicon (Si) etc.) exhibit similar properties;
for example, boron and silicon are both semiconductors, forming halides that are hydrolysed in water and have acidic oxides.
Such relationship
occurs because crossing rightward and descending the periodic table have
opposite effects.
On moving rightward a period of the periodic table, the size of the atoms decreases, and on moving down a group the size of the atoms increases.
Similarly, on moving rightward a period, the elements become progressively more covalent, less basic and more electronegative, whereas on moving down the group the elements become more ionic, more basic and less electronegative.
Thus, on both descending a group and crossing the period by one element, the changes "cancel" each other out, and elements with similar properties which have similar chemistry are often found – the atomic size, electronegativity, properties of compounds (and so forth) of the diagonal members are similar.
On moving rightward a period of the periodic table, the size of the atoms decreases, and on moving down a group the size of the atoms increases.
Similarly, on moving rightward a period, the elements become progressively more covalent, less basic and more electronegative, whereas on moving down the group the elements become more ionic, more basic and less electronegative.
Thus, on both descending a group and crossing the period by one element, the changes "cancel" each other out, and elements with similar properties which have similar chemistry are often found – the atomic size, electronegativity, properties of compounds (and so forth) of the diagonal members are similar.
It is found
that the chemistry of a first-row (second period) element often has
similarities to the chemistry of the second-row (third period) element being
one column to the right of it in the periodic table.
Thus, the chemistry of Li has similarities to that of Mg, the chemistry of Be has similarities to that of Al, and the chemistry of B has similarities to that of Si.
These are called diagonal relationships.
NOTE: It is not as noticeable after B and Si) The reasons for the existence of diagonal relationships are not fully understood, but charge density is a factor. For example, Li+ is a small cation with a +1 charge and Mg2+ is somewhat larger with a +2 charge, so the charge density on each ion is roughly the same. Using the Li–Mg pair:(under room temperature and pressure)
Thus, the chemistry of Li has similarities to that of Mg, the chemistry of Be has similarities to that of Al, and the chemistry of B has similarities to that of Si.
These are called diagonal relationships.
NOTE: It is not as noticeable after B and Si) The reasons for the existence of diagonal relationships are not fully understood, but charge density is a factor. For example, Li+ is a small cation with a +1 charge and Mg2+ is somewhat larger with a +2 charge, so the charge density on each ion is roughly the same. Using the Li–Mg pair:(under room temperature and pressure)
Li and Mg
form only normal oxides whereas Na forms peroxide and metals below Na, in
addition, form superoxides.
Li is the
only Group 1 element which forms a stable nitride, Li3N.
Mg, as well as
other Group 2 elements, also form nitrides.
Lithium
carbonate, phosphate and fluoride are sparingly soluble in water. The
corresponding Group 2 salts are insoluble. (Think lattice and solvation
energies).
Both Li and
Mg form covalent organometallic compounds. LiMe and MgMe2
(cf. Grignard
reagents) are both valuable synthetic reagents. The other Group 1 and Group 2
analogues are ionic and extremely reactive (and hence difficult to
manipulate).
Chlorides of
both Li and Mg are deliquescent (absorb moisture from surroundings) and soluble
in alcohol and pyridine. Lithium chloride, like magnesium chloride (MgCl2.6H2O)
separates out from hydrated crystal LiCl.2H2O.
SEE TABLE BELOW
SEE TABLE BELOW
A
period 2 elements
|
The
period 3 element
|
The second period contains the
elements lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and
neon
|
The third period contains eight
elements: sodium, magnesium, aluminium, silicon, phosphorus, sulfur,
chlorine, and argon
|
This period corresponds to the filling
of the 2s and 2p orbitals. Period 2 elements obey the octet rule in that they
need eight electrons to complete their valence shell. The maximum number of
electrons that these elements can accommodate is ten, two in the 1s orbital,
two in the 2s orbital and six in the 2p orbital
3Li Alkali
metal [He] 2s1
4Be Alkaline
earth metal [He] 2s2
5B Metalloid[He]
2s2 2p1
6C
Polyatomic nonmetal[He] 2s2 2p2
7 N Diatomic nonmetal[He] 2s2 2p3
8 O Diatomic nonmetal[He] 2s2 2p4
9 F Diatomic nonmetal[He] 2s2 2p5
10 Ne Noble gas [He] 2s2 2p6
|
The first two, sodium and magnesium,
are members of the s-block of the periodic table, while the others are
members of the p-block. Note that there is a 3d subshell, but it is not
filled until period4, such giving the periodic table its characteristic shape
of "two rows at a time".
11 Na Alkali
metal [Ne] 3s1
12 Mg Alkaline earth metal[Ne] 3s2
13 Al Post-transition metal[Ne] 3s2 3p1
14 Si Metalloid [Ne] 3s2 3p2
15 P Polyatomic nonmetal[Ne] 3s2 3p3
16 S Polyatomic nonmetal[Ne] 3s2 3p4
17 Cl Diatomic nonmetal [Ne] 3s2 3p5
18 Ar Noble gas [Ne] 3s2 3p6
|
As the atomic number increases, the atomic radius of
the elements decreases, the electronegativity increases, and the ionization
energy increases
|
As the atomic number of elements in Period 3
increases, the atomic radius decreases, electronegativity increases.
and the ionization energy increases
|
This period have less metals
|
This period have more metals
|
All of the elements in the period can form diatomic
molecules except beryllium and neon i.e
have more diatomic molecules
|
This period have more polyatomic molecules
|
This period have less stable isotopes
|
All of the elements have at least one stable
isotope, i.e have more stable isotopes
|
No comments