Chemistry · Periodic Classification Of Properties · NEET
A triad is a group of THREE elements that have similar chemical properties, arranged in order of increasing atomic weight. Dobereiner noticed that when you pick such a group of three, the middle element's atomic weight is about the average of the first and third. Example triad: lithium, sodium, potassium.
Add the atomic weights of the first and third elements, then divide by 2. This gives the mean (average). Example: for Cl (35.5), Br (80), I (127), the mean of Cl and I = (35.5 + 127)/2 = 81.25, which is very close to Br's real value of 80. So Cl, Br, I form a triad.
Three classic triads: (1) Li = 7, Na = 23, K = 39 -> mean of Li and K = (7+39)/2 = 23 = Na. (2) Ca = 40, Sr = 88, Ba = 137 -> mean = (40+137)/2 = 88.5 ~ Sr. (3) Cl = 35.5, Br = 80, I = 127 -> mean = 81.25 ~ Br. Learn these three; they are the ones examiners use.
It worked for only a FEW groups of elements. Most known elements at the time could not be arranged into triads, so scientists treated the pattern as a coincidence, not a real law. It also used atomic WEIGHT, not atomic number, which we now know is the true basis. So it was too limited to become a periodic law.
Atomic WEIGHT (atomic mass). Atomic number was not discovered yet in 1829 (Moseley found it much later). This is a common trap: every early classification (Dobereiner, Newlands, Mendeleev) used atomic weight. Only the MODERN periodic table uses atomic number.
Dobereiner was first (1829, triads). Newlands came later (1866, Law of Octaves). The order to remember for NEET is: Dobereiner -> de Chancourtois -> Newlands -> Lothar Meyer -> Mendeleev -> Moseley (modern law). Dobereiner started the whole idea of trends among elements.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
When three elements with similar properties are arranged by increasing atomic weight, the atomic weight of the middle element is approximately the mean (average) of the other two.
1829. Johann Dobereiner, a German chemist, noticed the pattern around this time. It was the first attempt to classify elements by their properties.
It applied to only a limited number of elements. Most elements could not be grouped into triads, so it could not become a general law and was set aside as coincidence.
Yes. Dobereiner observed that not only the atomic weight but also the physical and chemical properties of the middle element lie between those of the other two members of the triad.
He was the first to see that elements can be grouped by trends in properties. This idea directly led to later work by Newlands, Mendeleev and finally the modern periodic table, so NEET tests him as the starting point.
