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.

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.

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)

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.


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

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