# 2 Chemical patterns are to be found in the periodic table¶

## 2.1 Chemical periodicity¶

The chemistry of the elements is immensely varied. But amidst that variety there are patterns, and the best known and most useful is chemical periodicity: if the elements are laid out in order of atomic number, similar elements occur at regular intervals.

The discovery of chemical periodicity is particularly associated with the nineteenth-century Russian chemist Dmitri Ivanovich Mendeléev (Figure 16). The periodicity is represented graphically by Periodic Tables. Figure 17 shows the Periodic Table used in this course. Chemical periodicity is apparent from the appearance of similar elements in the same column. For example, the alkali metals appear in the first column on the left of the Table, and the noble gases in the last column on the right. Horizontal rows are called Periods; vertical columns are called Groups. The Table can be neatly divided up into blocks of elements (transition elements, lanthanides, actinides and typical elements), each with their own distinctive properties. Above each element is its atomic number. These numbers run from 1-118, 118 being the highest atomic number so far [2001] claimed for any observed atom.

Figure 16 The hypnotic face of Dmitri Mendeléev (1834-1907) has been likened to that of Svengali or Rasputin. Such comparisons are encouraged by his insistence on having just one haircut a year. His scientific fame rests mainly on his boldness in using his Periodic Law to predict the properties of undiscovered elements. For example, after Lecoq de Boisbaudron had announced the discovery of the new element gallium in 1875, he received a letter from Mendeléev. The letter informed him that Mendeléev had already predicted the properties of gallium, and that his experimental value for its density appeared to be wrong. de Boisbaudron then redetermined the density of gallium, and found that Mendeléev's assertion was indeed correct!


Figure 17 The complete Periodic Table used in this course. Note how the position of hydrogen has been left undecided. Some of its properties point to a position in Group I with the alkali metals; others to a position in Group VII with the halogens


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This course is largely concerned with the typical elements. These occur on the extreme left and extreme right of Figure 17. It is convenient, therefore, to create from Figure 17 a mini-Periodic Table that contains the typical elements alone. By removing the transition elements, the lanthanides and actinides, and by pushing the two separate blocks of typical elements together, we arrive at Figure 18. This mini-Periodic Table consists of seven Periods and eight Groups. The seven Periods are numbered from 1 to 7, but it is more difficult to settle on the best way of labelling the Groups.

In Figure 18, they are numbered in roman numerals from I to VIII. This is the principal Group numbering scheme used in this course, but other ways of numbering the Groups are mentioned in Sections 2.2 and 2.3.

Figure 18 A mini-Periodic Table containing the typical elements up to radium; it consists of eight columns or Groups, and seven rows or Periods. Hydrogen has been omitted for the reasons cited in the caption to Figure 17


Clear examples of chemical periodicity are revealed by Figure 18. Many involve the valencies of the elements. Here we use valency in the classical sense: a number that determines the ratios in which atoms combine. Table 1 shows the most important valencies of some common elements.

 Valency 1 2 3 4 hydrogen (H) oxygen (O) nitrogen (N) carbon (C) lithium (Li) sulfur (S) phosphorus (P) silicon (Si) sodium (Na) magnesium (Mg) aluminium (Al) tin (Sn) potassium (K) calcium (Ca) fluorine (F) barium (Ba) chlorine (Cl) bromine (Br) iodine (I)

### Question 4¶

#### Question¶

What does Table 1 suggest for the empirical formula of an oxide of tin?

SnO2; we start with the valencies of tin (4) and oxygen (2). Exchanging the numbers against the elements gives us tin (2) and oxygen (4). This tells us the combining ratio: two tin atoms combine with four oxygen atoms. To get the empirical formula, the ratio 2 : 4 is converted to the lowest possible whole numbers; the result is 1 : 2. So the predicted formula of the oxide of tin is SnO2.

We now list three instances of chemical periodicity in Figure 18 that you should be able to exploit:

1. As the colour coding of Figure 18 shows, metals lie to the left, and non-metals to the right, with semi-metals in between.

1. When an element in Figure 18 forms one or more hydrides, then across the eight columns of the Table, the valency of the element in the highest hydride (the hydride that contains most hydrogen) runs in the order 1, 2, 3, 4, 3, 2, 1, 0. Thus, nitrogen occurs in the fifth column, so its hydride is NH3 (ammonia).

1. The empirical formulae of fluorides and normal oxides provide an especially important example of chemical periodicity. Normal oxides are compounds in which single oxygen atoms are combined with atoms of other elements. For most of the elements in Figure 18, the highest observed valencies are equal to the Group number of the element. This allows the empirical formulae of the highest fluorides and highest normal oxides of the elements to be predicted. Thus, aluminium occurs in Group III, so the highest fluoride is AlF3, and the highest normal oxide is Al2O3.

These generalisations are not perfect. For example, the oxide trend does not work for the elements Po, F, Br, I, He, Ne, Ar, Kr and Rn; the fluoride trend does not work for N, O, Cl, Br, or for any of the noble gases. There is a further comment on this in the next section. Nevertheless, each generalisation is true enough to be very useful.

## 2.2 The Group number of the noble gases¶

In Figure 18, the Period numbers increase steadily from 1 to 7 down the columns. It obviously seems appropriate that the Group numbers should show a similar steady increase from I to VIII across the rows. However, this numbering scheme puts the noble gases in Group VIII. As Section 2.1 makes clear, almost none of these six elements then obeys generalisation (iii). For example, with this Group numbering, generalisation (iii) predicts the formula AO4 for the highest normal oxides of the noble gases, where A represents a noble gas atom. Only for xenon is such a compound known.

The situation is improved if one changes the Group number of the noble gases from VIII to zero. This is because there are no known oxides or binary fluorides of helium, neon or argon. In the case of the noble gases, generalisation (iii) then fails only at xenon when predicting oxide formulae, and at krypton, xenon and radon when predicting fluoride formulae. So in introducing chemical periodicity through generalisations (ii) and (iii), it makes sense to number the first 7 Groups from I to VII as in Figure 18, but to use zero for the noble gases (Group 0). This was also the Group numbering favoured by Mendeléev. In this course, however, we shall use the scheme of Figure 18 in which the noble gases are designated as Group VIII, and the Group numbers increase regularly across each row. The reasons for this change are given in Section 3.4.

## 2.3 Elements on parade: an audiovisual interlude¶

Here you have the opportunity of viewing seven video sequences which show both reactions and properties of some chemical elements. The seven sequences provide examples of the way in which Periodic Tables such as Figures 17 or 18 elicit similarities or patterns in chemical behaviour.

## 2.4 Summary of Section 2¶

1. The typical elements can be displayed in a mini-Periodic Table of eight Groups and seven Periods (Figure 18). The Periods are numbered from 1 to 7 and the Groups are labelled I-VIII.

2. Metals appear on the left of this table, non-metals on the right and semi-metals in between.

3. In their highest fluorides and normal oxides, the valencies of the typical elements are usually equal to their Group numbers in Figure 18. In their highest hydrides, their valencies usually follow the pattern 1, 2, 3, 4, 3, 2, 1, 0 across the Period.

### Question 5¶

#### Question¶

A typical element Z from Figure 18 is a semi-metal and forms oxides with empirical formulae ZO2 and ZO3, and a single hydride, ZH2. Identify the element, and state the Group and Period of Figure 18 in which it lies. What is the formula of the highest fluoride of the element?

Z is tellurium (Te). The highest normal oxide ZO3 suggests (point iii of Section 2.1) a highest valency of six and, therefore, a Group VI element. Point ii confirms that these elements form a hydride ZH2. The only Group VI element that is a semi-metal is tellurium. It lies in Period 5. Its highest fluoride (point iii again) should have the empirical formula TeF6, and in fact it does.