Free Learning Resources
PW Books
Notes (Class 10-12)
PW Study Materials
Notes (Class 6-9)
Ncert Solutions
Govt Exams
Class 6th to 12th Online Courses
Govt Job Exams Courses
UPSC Coaching
Defence Exam Coaching
Gate Exam Coaching
Other Exams
Know about Physics Wallah
Physics Wallah is an Indian edtech platform that provides accessible & comprehensive learning experiences to students from Class 6th to postgraduate level. We also provide extensive NCERT solutions, sample paper, NEET, JEE Mains, BITSAT previous year papers & more such resources to students. Physics Wallah also caters to over 3.5 million registered students and over 78 lakh+ Youtube subscribers with 4.8 rating on its app.
We Stand Out because
We provide students with intensive courses with India’s qualified & experienced faculties & mentors. PW strives to make the learning experience comprehensive and accessible for students of all sections of society. We believe in empowering every single student who couldn't dream of a good career in engineering and medical field earlier.
Our Key Focus Areas
Physics Wallah's main focus is to make the learning experience as economical as possible for all students. With our affordable courses like Lakshya, Udaan and Arjuna and many others, we have been able to provide a platform for lakhs of aspirants. From providing Chemistry, Maths, Physics formula to giving e-books of eminent authors like RD Sharma, RS Aggarwal and Lakhmir Singh, PW focuses on every single student's need for preparation.
What Makes Us Different
Physics Wallah strives to develop a comprehensive pedagogical structure for students, where they get a state-of-the-art learning experience with study material and resources. Apart from catering students preparing for JEE Mains and NEET, PW also provides study material for each state board like Uttar Pradesh, Bihar, and others
Copyright © 2025 Physicswallah Limited All rights reserved.
Dobereiner's Triads were proposed by the German chemist Johann Wolfgang Dobereiner. He attempted to classify elements with similar properties into groups of three elements each, called "triads.
John Newlands, an English scientist, made significant contributions to the early development of the periodic table. In 1866, he arranged the 56 known elements in increasing order of atomic mass. Newlands observed a pattern where every eighth element exhibited properties similar to the first. This observation led to the formulation of Newland’s Law of Octaves. According to this law, when elements are arranged in increasing order of atomic mass, the properties of two elements with an interval of seven elements between them would be similar.
In 1913, English physicist Henry Moseley conducted groundbreaking research on the characteristic x-rays emitted by different metals. He discovered a direct correlation between the square root of the frequency of these x-rays and the atomic number of the elements. Based on his findings, Moseley formulated the modern periodic law, which states:
Along a period, the number of valence electrons increases from 1 to 8 as we move from left to right. However, when considering the valence in compounds relative to hydrogen or oxygen, it peaks at 4 before declining to 0. For instance, in Na2O, oxygen, being more electronegative, gains two electrons from each sodium atom, leading to an oxidation state of -2, while sodium loses one electron, resulting in an oxidation state of +1. Within a group, the number of valence electrons remains constant, resulting in elements within the group sharing the same valence. For instance, all alkali metals in group 1 have a valence of one, while alkaline earth metals in group 2 have a valence of two. Noble gases in group 18 have zero valence, as they are chemically inert and do not form bonds.
Consequently, the maximum covalency of the first member is limited to 4, while the subsequent members can expand their valence shell to accommodate more than four pairs of electrons.
