Periodic Table and Electronic configurations (10th chemistry notes)

The Periodic Table is a chart that organizes all known chemical elements based on their atomic number (number of protons), electron configuration, and recurring chemical properties. Elements are arranged in rows called periods and columns called 1  groups. Elements within the same group tend to exhibit similar chemical behavior because they have the same number of valence electrons (electrons in the outermost shell).   

Electronic configuration describes how electrons are arranged in the different energy levels and sublevels within an atom. Electrons first fill the lowest energy levels available. The order of filling follows the Aufbau principle, Hund's rule, and the Pauli exclusion principle. This arrangement dictates an element's chemical properties and its position in the periodic table. For example, elements in the same group have similar outer electron configurations, leading to their similar reactivity. The periodic table is structured in blocks (s, p, d, f) that correspond to the type of subshell being filled with the last electron, which is directly related to the electronic configuration of the elements.

What is the basis of classification of elements in modern periodic table? Atomic Number

The atomic number is a fundamental property of an element that plays a crucial role in the modern periodic table. It is defined as the number of protons found in the nucleus of an atom of that element. The atomic number is denoted by the symbol 'Z'.

Basis of Classification of Elements in the Modern Periodic Table

The primary basis for the classification of elements in the modern periodic table is the atomic number. Here are some key points regarding its significance:

• Unique Identifier: Each element has a unique atomic number, which distinguishes it from all other elements.

• Arrangement: Elements are arranged in order of increasing atomic number, which reflects their periodic properties.

• Periodic Law: The periodic law states that the properties of elements are a periodic function of their atomic numbers, leading to the formation of groups and periods in the table.

• Electron Configuration: The atomic number also determines the number of electrons in a neutral atom, which influences its chemical behavior and reactivity.
  

  The atomic number is central to the organization and understanding of the periodic table, providing a systematic way to classify and predict the properties of elements. If you know the atomic number of an element, you can determine its position and nature from the periodic table. Eg: Atomic number of sodium is 11. Electronic configuration : 2, 8, 1 (K=2, L=8, M=1) Group number : 1 (Valance electron) Period number : 3 (Total number of shells )
  

  What happens to an electron as it moves away from the nucleus?

• The energy of the electron increases.

• The attraction between the nucleus and the electrons decreases
  

  We are familiar with writing the shell wise electronic configuration of various elements . Examples are given below.
  

  
  

Elements	K	L	M	N
3Li	2	1	-	-
11Na	2	8	1	-
18Ar	2	8	8	-
19K	2	8	8	1

Even if the third shell (M) can accommodate a maximum of 18 electrons, the last shell cannot accommodate more than eight electrons.

According to The Bohr model of an atom , electrons are revolving round the nucleus through fixed circular paths called Orbits or shells. Since each electron is associated with a definite amount of energy, these orbits are also known as Main energy levels. In these main energy levels, different Sub energy levels (Sub shells )are assigned. Sub shells are named as s , p , d, f etc. (s- sharp. p -principal. d- diffuse. f- fundamental) Orbitals orbitals are regions in a sub shell where the probability of finding an electron is maximum. Shapes of orbitals

Orbitals are defined as regions within an atom's sub shells where there is a high probability of finding electrons. Each type of orbital has a distinct shape, which influences the arrangement of electrons around the nucleus. The main types of orbitals are:

• s Orbitals:

  The s orbital is spherical in shape. It has no angular nodes and is symmetrical around the nucleus. As the principal quantum number increases, the size of the s orbital increases, but its shape remains spherical.
  

• p Orbitals:
  

  The p orbitals have a dumbbell shape and are oriented along the three Cartesian axes (x, y, and z). There are three p orbitals in each energy level starting from the second level (2p, 3p, etc.), denoted as px, py, and pz.
  

• d Orbitals:
  

  The d orbitals have more complex shapes. There are five d orbitals in each energy level starting from the third level (3d, 4d, etc.). The shapes include a cloverleaf structure for four of the d orbitals and a more spherical shape for one of them (dz).
  

• f Orbitals:

  The f orbitals are even more complex, with seven f orbitals starting from the fourth energy level (4f, 5f, etc.). The shapes of f orbitals are intricate and can be visualized as having multiple lobes and varying orientations.
  

Understanding the shapes of these orbitals is crucial for predicting how electrons will behave in chemical bonding and interactions. The following table shows the maximum number of electrons that can be accommodated in various shells and sub shells.

What is the relation between the shell number and the number of sub shells The shell number and the total number of sub shells are same . For eg: The first shell(K) has only one sub shell (1s) , the second shell (L) has two sub shells (2s ,2p) and so on. S shell will be common in all the Subshells. Distribution of electrons in various sub shells Electrons occupy various sub shells according to the increasing order of their energies. This is known as sub shell electronic configuration. It can be understood from the following figure. 1s <2s <2p <3s <3p <4s <3d <4p <5s <4d< ...

Write the sub shell electronic configuration of the first 30 elements of the periodic tables.

When we write the subshell wise electronic configuration, the number on the left side of the subshell denotes the shell number and the number on the top right side denotes the number of electrons.

Chromium and Copper exhibit unique electronic configurations. Provide an explanation.

The d subshell can hold a maximum of 10 electrons. Stability is increased when it is either half-filled (3d5) or completely filled (3d10). Q. The sub shell electronic configuration of an element is 1s 2 2s 2 2p 6 3s 2 . Find .. Ans: The number of shells in the atom? 3 . (K , L, M) The number of sub shells in each shell? K =1(1s) L =2 ( 2s , 2p ) M= 1 (3s) To which sub shell , does the last electron enter? 3s The total number of electrons in the atom ? 12 Atomic number of the element? 12 The short form of sub shell electronic configuration? [Ne] 3s 2

Short Form of Zirconium's Electronic Configuration

The subshell electronic configuration of Zirconium (40Zr) can be expressed in a condensed format. Instead of writing it as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d² 5s², it can be represented as [Kr] 4d² 5s². Sub shell electronic configuration and Block Based on the sub shell electronic configurations , the elements are arranged in four different blocks (s, p, d and f). The block to which the element belongs will be the same asthe subshell to which the last electron is added.

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