Chemistry · Ch 7 — Elements of Groups 16, 17 and 18

Atomic properties of Group 16, 17 and 18 elements

7.4.1

Atomic properties of Group 16, 17 and 18 elements

These properties are given in Tables 7.2, 7.3 and 7.4.

i. Atomic and Ionic radii : In groups 16, 17 and 18 the atomic and ionic radii increase down the group, as a result of the increase in the number of quantum shells. Across a period, atomic or ionic radii decrease with increasing atomic number, consequent to the increase in effective nuclear charge ( $Z_{\text{eff}}$ ). Group 17 elements (halogens) have the smallest atomic radii in their respective periods.

ii. Ionisation enthalpy : The group 16, 17 and 18 elements have high ionisation enthalpy. The ionisation enthalpy decreases down the group due to the increase in atomic size. Across a period, ionisation enthalpy increases with increase of atomic number, because electrons are being added to the same shell. However, the elements of group 16 have lower ionisation enthalpy values compared to those of group 15 in the corresponding periods, owing to the extra-stable, half-filled electronic configuration of the p-orbitals in the elements of group 15.

iii. Electronegativity : In a group (16, 17 and 18) the electronegativity decreases down the group. Oxygen has the highest electronegativity next to fluorine amongst all the elements. Halogens have very high electronegativity; fluorine is the most electronegative element in the periodic table.

iv. Electron gain enthalpy : In groups 16 and 17 the electron gain enthalpy becomes less negative down the group. However, in group 16, oxygen has less negative electron gain enthalpy than sulfur, due to its small atomic size. In group 17, fluorine has less negative electron gain enthalpy than that of chlorine — this is due to the small size of the fluorine atom. Group 18 elements (noble gases) have no tendency to accept electrons because of their stable electronic configuration ( $\mathrm{ns^2np^6}$ ), and thus have large positive electron gain enthalpy.

Table 7.2 : Atomic and physical properties of group 16 elements.

Element	Atomic number	Atomic mass g/mol	Atomic radius (pm)	Ionic radius $\mathrm{E^{2-}}$ (pm)	Ionization enthalpy $(\Delta_i H_1)$ kJ/mol	Electronegativity	Electron gain enthalpy kJ/mol	Density g/cm³	M.P. (K)	B.P. (K)
O	8	16.00	66	140	1314	3.50	−141	1.32	55	90
S	16	32.06	104	184	1000	2.44	−200	2.06	393	718
Se	34	78.96	117	198	941	2.48	−195	4.19	490	958
Te	52	127.60	137	221	869	2.01	−190	6.25	725	1260
Po	84	210.00	146	230	813	1.76	−174	−	520	1235

Table 7.3 : Atomic and physical properties of group 17 elements

Element	Atomic number	Atomic mass g/mol	Atomic radius (pm)	Ionic radius $\mathrm{E^{-}}$ (pm)	Ionization enthalpy $(\Delta_i H_1)$ kJ/mol	Electronegativity	Electron gain enthalpy kJ/mol	Density g/cm³	M.P. (K)	B.P. (K)
F	9	19.00	64	133	1680	4.0	−333	1.5	54.4	84.9
Cl	17	35.45	99	184	1256	3.2	−349	1.66	172.0	239.0
Br	35	79.90	114	196	1142	3.0	−325	3.19	265.8	332.5
I	53	126.90	133	220	1008	2.7	−296	4.94	386.6	458.2
At	85	210	−	−	−	2.2	−	−	−	−

Table 7.4 : Atomic and physical properties of group 18 elements. …

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