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a covalent bond is formed when there is not enough difference in the order of electronegativity

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms, or between atoms and other covalent bonds. In short, attraction-to-repulsion stability that forms between atoms when they share electrons is known as covalent bonding.

Covalent bonding includes many kinds of interaction, including σ-bonding, π-bonding, metal to non-metal bonding, agostic interactions, and three-center two-electron bonds. The term covalent bond dates from 1939. The prefix co- means jointly, associated in action, partnered to a lesser degree, etc.; thus a "co-valent bond", essentially, means that the atoms share "valence", such as is discussed in valence bond theory. In the molecule H2, the hydrogen atoms share the two electrons via covalent bonding

FOR MORE INFORMATION
http://en.wikipedia.org/wiki/Covalent_bonding

The covalent radius, rcov, is a measure of the size of atom that forms part of a covalent bond. It is measured either in picometres (pm) or Ã¥ngströms (Ã
), with 1 Ã
= 100 pm.

In principle, the sum of the two covalent radii should equal the covalent bond length between two atoms. This relationship does not hold exactly because the size of an atom is not constant but depends on its chemical environment. In particular, polar covalent bonds tend to be shorter than would be expected on the basis of the sum of covalent radii. Tabulated values of covalent radii are either average or idealized values, which nevertheless show a certain transferability between different situations.

Table of covalent radii
Z Symbol r (Ã
)
1 H 0.31(5)
2 He 0.28
3 Li 1.28(7)
4 Be 0.96(3)
5 B 0.84(3)
6 C (sp3) 0.76(1)
C (sp2) 0.73(2)
C (sp) 0.69(1)
7 N 0.71(1)
8 O 0.66(2)
9 F 0.57(3)
10 Ne 0.58
11 Na 1.66(9)
12 Mg 1.41(7)
13 Al 1.21(4)
14 Si 1.11(2)
15 P 1.07(3)
16 S 1.05(3)
17 Cl 1.02(4)
18 Ar 1.06(10)
19 K 2.03(12)
20 Ca 1.76(10)
21 Sc 1.70(7)
22 Ti 1.60(8)
23 V 1.53(8)
24 Cr 1.39(5)
25 Mn (low spin) 1.39(5)
Mn (high spin) 1.61(8)
26 Fe (low spin) 1.32(3)
Fe (high spin) 1.52(6)
27 Co (low spin) 1.26(3)
Co (high spin) 1.50(7)
28 Ni 1.24(4)
29 Cu 1.32(4)
30 Zn 1.22(4)
31 Ga 1.22(3)
32 Ge 1.20(4)
33 As 1.19(4)
34 Se 1.20(4)
35 Br 1.20(3)
36 Kr 1.16(4)
37 Rb 2.20(9)
38 Sr 1.95(10)
39 Y 1.90(7)
40 Zr 1.75(7)
41 Nb 1.64(6)
42 Mo 1.54(5)
43 Tc 1.47(7)
44 Ru 1.46(7)
45 Rh 1.42(7)
46 Pd 1.39(6)
47 Ag 1.45(5)
48 Cd 1.44(9)
49 In 1.42(5)
50 Sn 1.39(4)
51 Sb 1.39(5)
52 Te 1.38(4)
53 I 1.39(3)
54 Xe 1.40(9)
55 Cs 2.44(11)
56 Ba 2.15(11)
57 La 2.07(8)
58 Ce 2.04(9)
59 Pr 2.03(7)
60 Nd 2.01(6)
61 Pm 1.99
62 Sm 1.98(8)
63 Eu 1.98(6)
64 Gd 1.96(6)
65 Tb 1.94(5)
66 Dy 1.92(7)
67 Ho 1.92(7)
68 Er 1.89(6)
69 Tm 1.90(10)
70 Yb 1.87(8)
71 Lu 1.87(8)
72 Hf 1.75(10)
73 Ta 1.70(8)
74 W 1.62(7)
75 Re 1.51(7)
76 Os 1.44(4)
77 Ir 1.41(6)
78 Pt 1.36(5)
79 Au 1.36(6)
80 Hg 1.32(5)
81 Tl 1.45(7)
82 Pb 1.46(5)
83 Bi 1.48(4)
84 Po 1.40(4)
85 At 1.50
86 Rn 1.50
87 Fr 2.60
88 Ra 2.21(2)
89 Ac 2.15
90 Th 2.06(6)
91 Pa 2.00
92 U 1.96(7)
93 Np 1.90(1)
94 Pu 1.87(1)
95 Am 1.80(6)
96 Cm 1.69(3