Welcome to the exciting world of chemistry, where the elements come to life thanks to th remarkable periodic table. From its humble beginnings, where only 23 elements were known unt the end of the 18th century, to its development of 118 elements today. It is very difficult am impossible to remember information about the reactions, properties, and atomic masses elements. So we obviously need a way to organize our information about them. The periodic tab is one of the most important tools in chemistry. It is very useful for understanding and predictin the properties of elements. For example, if you know the physical and chemical properties of or element in a group, you can predict the physical and chemical properties of any other element the same group. The periodic table allows you to relate the reactivity tendencies of elements their atomic structure. You can also predict which elements can form ionic or covalent bonds.
One of the most important activities is the search for order. A large number of observations objects can be arranged into groups according to common features they share, it becomes easier to describe them. After the discovery of atomic number by Moseley in 1913, it was noticed the atomic number could serve as a base for systematic arrangement of elements. Thus elements a arranged in the order of increasing atomic number. A table showing systematic arrangement of elements is called periodic table. It is based on the Periodic law that states if the elements are arranged in the order of their increasing atomic numbers, their properties are repeated in a periodic manner.
The most commonly used form of the periodic table is shown in figure 4.1. Note that the elements are listed in order of increasing atomic numbers, from left to right and from top to bottom. Hydrogen (H) is in the top left corner. Helium (He), atomic number 2, is at the top right corner. Lithium (Li), atomic number 3, is at the left end of the second row.
The horizontal rows of the periodic table are called periods. There are varying number of elements in periods. How many periods you find in the periodic table? There are seven periods. The number of elements per period range from 2 in period 1 to 32 in period 6. First three periods are called short periods and the remaining periods are called long periods. The properties of elements within a period change gradually as you move from left to right in it. But when you move from one period to the next, the pattern of properties within a period repeats. This is in accordance to the periodic law.
International Union of Pure and Applied Chemistry (IUPAC) has recently renamed newly discovered elements and placed them on the periodic table.
Elements that have similar properties lie in the same column in the periodic table. Each vertical column of elements in the periodic table is called a group or family.
Two numbering systems are often used to designate groups. You should know both. In the traditional system and the old IUPAC, the letters A and B are used. The first two groups are IA and IIA, while the last six groups are IIIA to VIIIA and the middle groups are in group B. The International Union of Pure and Applied Chemistry (IUPAC) decided that the groups would be 1-18 from left to right.
The elements in the same group have same number of valence electrons. Group number indicates the number of valence electrons in an element. For example, Group1 and Group 2 elements have 1 and 2 valence electrons respectively. In Groups 13 elements have 3, Group 14 have 4, Group15 have 5 valence electrons and so on. It is important to note that in Groups 13 to 18 (p block elements), the number of valence electrons is equal to group number minus 10.
Group A elements are called normal or representative elements. They are also called main group elements. Group B elements are called transition elements.
Some groups of elements in the periodic table have been given group names. For example metallic elements in Group 1 are called alkali metals. Group 2 elements are called alkaline earth metals. The elements in Group 17 or VIIA are called halogens. The Group 18 or VIIIA elements are called noble gases because they do not readily undergo chemical reactions.
Recall that all elements have a unique identification number known as the atomic number or proton number. The atomic number of an element represents the number of electrons or protons present in the atom of the element. Aufbau's Principle helps in determining the order in which the electron orbitals get filled.
According to Aufbau's principle, the order in which the orbitals fill up is as follows:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p and so on.
Each orbital has a fixed capacity for the maximum number of electrons accommodated, s-orbitals have the capacity of 2 electrons, while p orbitals have the capacity for 6 electrons, d orbitals have the capacity for 10 electrons and f orbitals have the capacity for 14 electrons.
Using these concepts, we can determine the electronic configuration of the given element.
Block of an element: When you have filled all the electrons, the orbital in which the last electron is in, represents the block in which the element is placed.
Period of an element: Now, to determine the period in which the element is placed, you need to look at the principal quantum number(n) of the valence electron. This number repersents period number of element
Group of an element: To determine the group, we need to understand some rules:
(a) If the element is in s block, then the group number is equal to the number of valence electrons.
(b) If the element is in the p block, then the number of the group can be determined by the formula: (number of valence electrons + 10).
For example, the atomic number of sodium is 11. Hence its electronic configuration is: 1s, 2s, 2p, 3s. Since the valence electron is in the 3s subshell, sodium belongs to belongs to the s block. The principal quantum number of the valence electron of Na is 3. Hence, it belongs to the 3rd period. Since Na belongs to the s block, its group number is equal to a number of electrons in valence subshells. This is equal to 1. Hence, sodium belongs to the Group 1.
Note: we can start filling the orbitals in the order mentioned by the Aufbau principle. 1s, 2s, 2p, 3s, 3p, 4s
Identify the group, period, and block of following elements on the basis of electronic configuration.
1. Al (atomic number=13)
2. K (atomic number = 19)
Write electronic configuration of element. Identify its valence shell. Remember that n value of the valence shell indicates period. Total number of electrons in the valence shells represents group number if element belongs to s block. If it belongs to p block, then group number is equal to the total number of valence + 10.
1. Electronic configuration of Al (atomic no. 13): 1s², 2s², 2p⁶, 3s², 3p¹
• Valence sub-shells is 3p, so Al belongs to p block
• As n = 3, Al is present in the 3rd period
• Total number of electrons in the valence shell = 2+1=3
• Group number of Al = total number of electrons in the valence sub-shells + 10 = 3+10 = 13
Hence Al belongs to Group 13
2. Electronic configuration of K (atomic no. 19) = 1s², 2s², 2p⁶, 3s², 3p⁶, 4s¹
• Valence shells is 4s, hence K belongs to s block
• As n=4, K is present in the 4th period
• Total number of electrons in the valence shell = 1
• Group number of K = total number of electrons in the valence sub-shells = 1
Hence K belongs to Group 1
Electronic configuration of atoms of some elements are given below. Classify them in groups and periods.
A. 1s² 2s²
B. 1s² 2s² 2p³
C. 1s² 2s² 2p⁵
D. 1s² 2s² 2p⁶ 3s²
E. 1s² 2s² 2p⁶ 3s² 3p⁵
F. 1s² 2s² 2p⁶ 3s² 3p¹
Remember that:
1. The elements whose atoms have similar valence shell electronic configuration belong to the same group.
2. The n value of the valence shell indicates period.
3. The elements whose atoms have same value of n for the valence shell lie in the same period.
| Group | II A | V A | VII A |
|---|---|---|---|
| Period 2 | A: 2s² | B: 2s²2p³ | C: 2s²2p⁵ |
| Period 3 | D: 3s² | F: 3s²3p¹ | E: 3s²3p⁵ |
Group 1 and Group 2 elements contain their valence electrons in the s sub-shell. Therefore, these elements are called s-block elements. Elements in groups 13 to 18 (except He) are known as p-block elements because their valence electrons are located in the p sub-shell. Lanthanides and actinides are known as f-block elements since their valence electrons lie in f sub-shell. Figure 4.2 shows the blocks of the periodic table.
Li 1s², 2s¹, as valence electron is in s sub-shell, Li belongs to s-block.
C-1s², 2s², 2p², as valence electron is in sub-shell p, C belongs to p-block.