Classification of Elements - Part VI

Characteristics of periods
The first period starts with hydrogen (H) and ends with helium (He). It has just two elements H (Z=1) and He (Z = 2). H has one electron in the K-shell. He has 2 electrons in the K-shell. As we have seen in the chapter on the structure of atoms, the K-shell can hold only 2 electrons. Thus the first period is complete. It has to be borne in mind that the place of hydrogen is unique in the periodic table. It has been placed above the alkali elements starting with Li in group 1A. This is because H has valency 1 just as the other alkali elements. But the properties of hydrogen otherwise are very different from the other group 1A alkali elements Li, Na, K, Cs, etc.

Now let us see the next periods : periods 2 and 3. The second period starts with Li (Z=3), where the K-shell is filled and the next shell, the L-shell is starting to fill. After Li the next element is beryllium (Be, Z=4). Its K-shell is complete and it has 2 electrons in the L-shell. The maximum number of electrons held in the L-shell is 8. So the period has 8 elements, in which each element’s L-shell is getting filled. The last element in the period is neon (Ne, Z=10). Neon’s both K and the L shell are completely filled. A similarly situation occurs for the third period.  Here the next shell after L-shell, namely the M-shell is getting filled. The maximum number of electrons in the M-shell is 8. Thus across the period, starting with element sodium (Na, Z=11) the M-shell has 1 electron; and the period ends with argon (Ar, Z= 18) which has 2 electrons in the K-shell, 8 electrons in the L-shell and 8 electrons in the M-shell.

In the fourth period, the N-shell starts to fill. N shell can hold a maximum of 2 electrons. Hence K has 1 electron in the N-shell and the next element Ca contains 2 electrons in the N-shell. Now the N-shell is complete. After Ca, there are 10 elements starting from Scandium (Sc) to Zinc (Zn). Here the next shell of electrons is filling. This shell can contain a maximum of 10 electrons[1]. The next shell can have a maximum of 6 electrons. Hence the fourth period is long and has 18 elements. The same is true for the fifth period.

In the sixth and the seventh period, the electrons start to fill a new orbital (f-shell). This has been separated out as the Lanthanide and the Actinide series.

Now let us look at some of the chemical and physical properties in a particular period. What we will learn from one period, will hold true for all the other periods.

If we see the atomic radii across the period, we will notice that the size decreases across the period.

Now let us consider the metallic character of the elements in the third period.  Figure below shows the same.

We have proper metals in the first and the second places : sodium (Na) and magnesium (Mg) are alkali and alkaline-earth metals. They give up the electrons in the last shell very easily. They are shiny in colour and conduct electricity. After Mg comes aluminum (Al). Al has 3 electrons in its outermost shell and behaves like a metal. The next element is silicon (Si). It has 4 electrons in its outermost shell. It thus needs to borrow four electrons or give up all its four electrons to form a stable shell. Si does not do any of these, instead it binds tetrahedrally most of the time. Thus Si behaves neither like a metal nor like a non-metal. Hence it is called as a metalloid. After Si, come three elements : phosphorus (P), sulphur (S) and chlorine (Cl). All the three are non-metals.  Thus while moving from left to right in the period, the metallicity decreases. Also the chemical reactivity first decreases and then increases. As discussed before, the chemical reactivity depends on how easily the outermost orbit gives off or borrows electrons to make a stable orbit. The two extremes of the third period, namely Na and Cl are very reactive. But Na is very electro-positive in nature, where as Cl is very electro-negative in nature.

If we look at the nature of the oxides formed by the elements in period three, we see that sodium oxide is basic in nature. The next oxide, namely magnesium oxide is also basic in nature. At the other extreme, chlorine oxide, sulphur oxides and phosphorus oxides are acidic in nature. The mid-elements like Al, Si have their oxides behave in both acidic and basic manner, depending on the oxidation conditions. Such oxides are said to be amphoteric in nature.

Summary of  the characteristics of elements in a period :

• The atomic numbers are consecutive.

• The number of valence electrons in the elements increases incrementally from left to right.

• The elements of the same period have different valencies.

• The atomic radii decrease while going from left to right in a period.

• Metallic character reduces while going from left to right in a period.

• Chemical reactivity is highest at the two extremes and is the lowest in the centre. The reactivity on the left extreme is most electro-positive whereas on the extreme right it is most electro-negative.

• Oxides formed of elements on the left are basic and of elements on the right are acidic in nature. Oxides of elements in the centre are amphoteric.

[1] In advanced chemistry, you will study the Hund’s rule which gives you a scheme of filling electrons in different orbits. The electron configurations go as 1s² - 2s²2p⁶ - 3s²3p⁶ - 4s²- 3d¹⁰ - 4p⁶ - 5s²- 4d¹⁰ - 5p⁶- 6s² - 4f¹⁴ - 5d¹⁰ - 6p⁶- 7s² - 5f¹⁴ - 6d¹⁰ and so and so forth. The first number indicates the principle quantum number n, the small letters s,p,d,f,g, etc, show the orbital quantum number l,  namely s implies l = 0, p implies l =1. l can take integral values from 0,1,…up to (n-1). The superscripted number shows the maximum number of electrons that can be accommodated in that particular orbital.

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