Roman numerals, valence electrons

Elements in the A groups have the same number of valence electrons as the Roman numeral of their group. For example, magnesium in Group llA has two valence electrons, carbon in Group IVA has four valence electrons, oxygen in Group VIA has six valence electrons, and... 

Note A transition element has a partially filled c/-subshell either as the element or in any commonly occurring oxidation state. Thus, Zn, Cd, and Hg are not transition elements. The n- )d electrons of the c/-block elements are considered to be valence electrons. Groups 1,2, and 13-18 are alternatively labeled with Roman numerals 1-Vlll, which correspond to the number of valence electrons in the element. 

FeO4 (ferrate) and Fe(catecholate)3 the covalence of iron is six and for Fe(CO)5 the covalence of iron is eight. In each of these compounds the iron atom is uncharged and has eight valence electrons (3d 4s2 —> d sp -> d sp. For these examples, the traditionally used formal oxidation states of iron [II, III, IV, VI, and VIII (or 0), respectively] are the same as their covalences (number of covalent bonds). However, the iron in (porphyrin)Fe( "> (OH2) (d5sp2) has a covalence of three, a formal oxidation state of three, and a charge of 1-t- via the covalently bound H2O. In the present discussion Roman numeral superscripts associated with the metals in the formulas for their compounds and complexes indicate their covalence (number of covalent bonds), not their oxidation state or number. 

The outermost electrons in an atom are valence electrons. For representative elements the number of valence electrons in an atom corresponds to the group or family number (old numbering system using Roman numerals). Metals tend to have fewer valence electrons than nonmetals. 

There are important differences between the two concepts that should be appreciated. Oxidation state is an imaginary charge on an atom that is in combination with one of the very electronegative atoms, i.e. F or O, in which their oxidation states are deemed to be —1 and —2 respectively. For example, in Mgp2 and MgO the oxidation state of the magnesium is + 2 i.e. II in the Roman numerals conventionally used to indicate oxidation states, as Mg ) and equal to the charge on the Mg " ions in those compounds. In ionic compounds there are no discrete electron-pair bonds so the strict definition of valency, given above, does not apply. 

Here s something to keep in mind about the number of valence electrons and the Roman numeral column number The Ik family has 1 valence electron the Ilk family has 2 valence electrons the VHk family has 7 valence electrons and the V7J7A family has 8 valence electrons. So for the families labeled with a Roman numeral and an A, the Roman numeral gives the number of valence electrons. Pretty cool, eh ... 

The Roman numeral makes it very easy to determine that oxygen (O) has six valence electrons (it s in the VIA family), that silicon (Si) has four, and so on. You don t even have to write the electronic configuration or the energy diagram to determine the number of valence electrons. 

The Roman numerals at the top of the A families show the number of valence electrons (s and p electrons in the outermost energy level) in the particular element (see Chapter 4 for details). So sodium has 1 valence electron and 11 total electrons because its atomic number is 11. 

Table 5.2 shows the variations of valency, i.e. the number of single electron-pair bonds, of the elements of the s- and p-blocks. The oxidation states (Roman numerals) of the metallic elements are indicated instead of their valency. The elements of the 2nd period, Li to Ne, show the values expected from the strict application of the octet rule. The elements of the subsequent periods follow the rule, but there are many exceptions... 

Elements can be placed in rows and columns of the Periodic Table by knowing something about their properties. The numbers at the top of the chart, Roman numerals I-VIII (or alternatively 1-18), are used to identify groups and chemical properties. The element groups usually give the number of electrons in the outermost orbital of the atoms in each column. Remember, these outer electrons are known as valence electrons. For example, the atoms of the elements in column IV have four electrons available to create bonds. Elements in column II have two free electrons in the outermost orbit around the nucleus. 

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