Early Classification and Mendeleev's Periodic Table മെൻഡലീഫ് പീരിയോഡിക് ടേബിൾ
Today we know 118 elements. How do chemists keep track of all of them – and how can one table tell you how an element you have never seen will behave? On this page you see why we classify, meet the first attempts (triads and octaves), and then do what Dmitri Mendeleev did in 1869: sort element cards by atomic mass, discover the repeating pattern, leave gaps for missing elements – and find the places where his idea runs into trouble.
- An element മൂലകം is made of only one kind of atom; each has a symbol പ്രതീകം (Na, K, Fe…). Elements are metals, non-metals or metalloids ലോഹങ്ങൾ · അലോഹങ്ങൾ · ഉപലോഹങ്ങൾ (page 1).
- Atomic number അറ്റോമിക നമ്പർ Z = number of protons. Atomic mass അറ്റോമിക മാസ് is how heavy an atom is compared with others (in u). Isotopes are atoms of one element with different mass numbers (page 2).
1. Why classify the elements?
The chapter opens with four students building a periodic table പീരിയോഡിക് ടേബിൾ out of cardboard blocks – H, Li, Na, K, Rb, Cs, Fr stacked in one column, the long rows of La…Lu and Ac…Lr kept on a separate shelf below.
Think of a big textile shop in Thrissur or your school library. Nobody piles up thousands of saris or books at random: things that are alike go on the same shelf, and the shelves follow an order. Then you can find anything quickly – and if a shelf has an empty slot, you know exactly what kind of item is missing. Chemists wanted the same for the elements: put similar elements together, in a sensible order, so that knowing one element tells you about its neighbours.
Before 1800 the only classification was "metal" or "non-metal". Sort these eight familiar elements. (Data from this guide's element list.)
| Element | Your choice |
|---|
Two boxes are not enough: sodium and iron are both metals, yet sodium is soft enough to cut with a knife and reacts violently with water, while iron is hard and only rusts slowly. Chemists needed a finer way to group elements.
2. Early attempts: triads and octaves go further
The textbook simply says മൂലകങ്ങളുടെ ആദ്യകാല വർഗീകരണ ശ്രമങ്ങൾ – "the early attempts to classify the elements". Two of them are worth knowing, because they show the idea of a repeating pattern growing.
Döbereiner's triads (about 1829)
The German chemist Johann Döbereiner noticed groups of three similar elements (triads). In each triad, the atomic mass of the middle element is close to the average of the other two. Pick a triad:
Atomic masses from this guide's element data (rounded). The rule worked for only a few groups of three – most elements did not fit into any triad.
Newlands' law of octaves (1865)
The English chemist John Newlands arranged the elements known to him in increasing atomic mass and saw that every eighth element was like the first – just as in music the eighth note repeats the first (sa re ga ma pa dha ni sa). The noble gases were not yet known, so they are missing from his list.
Octaves worked up to calcium. After that iron landed beside oxygen and sulphur, and cobalt and nickel shared a place with chlorine – the pattern broke. Something better was needed.
3. Mendeleev's periodic law (1869)
The textbook reminds you of the periodic law പീരിയോഡിക് നിയമം put forward by Dmitri Ivanovich Mendeleev മെൻഡലീഫ് (ദിമിത്രി ഇവാനോവിച്ച്) on the basis of atomic mass അറ്റോമിക മാസ്:
The properties of elements are periodic functions ആവർത്തനഫലങ്ങൾ of their atomic masses.
"Periodic" means repeating at regular intervals, like the days of the week or the monsoon every June. Arrange the elements by atomic mass and similar properties come back again and again.
1869-ൽ മെൻഡലീഫ് പീരിയോഡിക് ടേബിൾ തയ്യാറാക്കുമ്പോൾ ആറ്റം ഘടനയെക്കുറിച്ചോ ആറ്റത്തിലെ കണങ്ങളെക്കുറിച്ചോ വ്യക്തമായ ധാരണ രൂപപ്പെട്ടിരുന്നില്ല. – In 1869, when Mendeleev prepared his table, nobody clearly understood the structure of the atom or the particles inside it (the electron was found only in 1897, page 2). Even so, his periodic table had many merits മേന്മകൾ. Step through how he did it:
4. Virtual lab: be Mendeleev – the card sort
Here are the first 20 elements on cards, each with its atomic mass. Tap the lightest card that is left; it goes into the next place of the table (rows of 8, with H and He on top). When all 20 are placed, look down each column.
Masses are rounded to one decimal (the textbook rounds argon to 40 and potassium to 39). Mendeleev did not know the noble gases – they were discovered from 1894 onwards – but when they were added, exactly this argon–potassium problem appeared.
5. Merits of Mendeleev's periodic table
Q (page 24). മെൻഡലീഫ് പീരിയോഡിക് ടേബിളിന്റെ മേന്മകൾ ലിസ്റ്റ് ചെയ്യൂ. List the merits of Mendeleev's periodic table.
Think about: the order he used, where similar elements went, the empty places, the atomic masses he changed, and what happened when new elements were found.
- He arranged all the elements known then (about 63) in the increasing order of atomic mass ആരോഹണക്രമം – the first scientific table of all elements.
- Elements with similar properties were placed in the same group (Li, Na, K…), so the table showed families.
- He left gaps for elements not yet discovered and predicted their properties – later gallium (eka-aluminium) and germanium (eka-silicon) were found with almost exactly those properties.
- He corrected the atomic masses of some elements whose measured masses did not fit their properties (e.g. beryllium, indium, uranium – go further).
- The noble gases, discovered later, fitted in as a separate group without disturbing the rest of the table.
Predicting the missing elements go further
Mendeleev called the missing element below aluminium eka-aluminium and the one below silicon eka-silicon ("eka" = one, in Sanskrit – one place further down). He estimated their atomic masses from the neighbours. Try it:
| Property | Mendeleev's eka-silicon | Germanium (Ge) |
|---|---|---|
| atomic mass | 72 | 72.6 |
| density (g/cm³) | 5.5 | 5.3 |
| formula of the oxide | EsO₂ | GeO₂ |
| its chloride | a liquid boiling below 100 °C | GeCl₄, a liquid boiling at about 86 °C |
6. Limitations of Mendeleev's periodic table
Q (page 24). മെൻഡലീഫ് പീരിയോഡിക് ടേബിളിന്റെ ചില പരിമിതികളെക്കുറിച്ചും നിങ്ങൾക്കറിയാമല്ലോ. അവ ഏതെല്ലാമാണ്? You also know some limitations പരിമിതികൾ of Mendeleev's periodic table. What are they?
- No separate places for isotopes. Isotopes of an element have different atomic masses, so a table based on mass should give each one its own place – but it could not.
- The increasing order of atomic mass is not followed everywhere – argon (Ar, 40) is placed before potassium (K, 39). (You saw this in the card sort. Tellurium 127.6 before iodine 126.9 is another case – go further.)
- Elements with different properties in the same group – the soft, very reactive metals sodium and potassium were put together with the hard, much less reactive copper and silver.
- The position of hydrogen was uncertain – it is like the alkali metals in some ways and like the halogens in others (go further).
Isotopes – the first limitation
ഐസോടോപ്പുകളെക്കുറിച്ചും നിങ്ങൾ മനസ്സിലാക്കിയിട്ടുണ്ട്. – You have also learnt about isotopes (page 2).
Q (page 24). ഒരേ മൂലകത്തിന്റെ ഐസോടോപ്പുകൾ തമ്മിൽ എങ്ങനെ വ്യത്യാസപ്പെട്ടിരിക്കുന്നു? How do the isotopes of an element differ from one another?
Mendeleev's table arranged elements by atomic mass. Isotopes have different atomic masses, so each one should need its own place – but they all behave chemically like hydrogen. മെൻഡലീഫ് പീരിയോഡിക് ടേബിൾ പ്രകാരം അറ്റോമിക മാസിന്റെ അടിസ്ഥാനത്തിൽ ഇവയിലോരോന്നിനും പ്രത്യേക സ്ഥാനം നൽകാൻ സാധ്യമല്ല.
All these problems came from using atomic mass. In 1913 Henry Moseley showed that the true order is the atomic number – the number of protons. Isotopes share one atomic number, and argon (18) really does come before potassium (19). That is the story of the next page.
- 📄 Dmitri Mendeleev Wikipedia
The life of the scientist who built the first periodic table in 1869. - 📄 Mendeleev's predicted elements Wikipedia
How the gaps in his table predicted gallium and germanium (eka-aluminium, eka-silicon). - 📄 Henry Moseley Wikipedia
The young physicist whose X-ray experiments showed that atomic number is the true order. - 📄 Moseley's law Wikipedia
Go further: the X-ray result behind the modern periodic law.
7. Check yourself
- Classifying elements puts similar ones together so that one element tells you about others. Early attempts: Döbereiner's triads and Newlands' octaves (go further) – both worked only for some elements.
- Mendeleev (1869): properties of elements are periodic functions of their atomic masses. He arranged elements in increasing atomic mass, put similar elements in groups, left gaps and predicted new elements (Ga, Ge), corrected some atomic masses; the noble gases later fitted in as a new group.
- Limitations: no places for isotopes; mass order broken (Ar 40 before K 39); unlike elements in one group (Na, K with Cu, Ag); hydrogen's place uncertain.
- Isotopes of an element differ in the number of neutrons (mass number), not in the number of protons.