The periodic table looks like a wall of symbols to learn. It is closer to a map: an element’s position tells you how many shells it has, how many outer electrons, whether it is a metal, and roughly how it will behave. Once you can read that map, most of the questions in this chapter become short.
This guide starts with how elements came to be sorted, moves on to the modern table, and then explains the trends in valency, atomic size and metallic character with the reason behind each one. It also states clearly where this chapter stands: it is part of the Karnataka SSLC syllabus, and CBSE has removed it from Class 10.
How Elements Got Sorted
Before anyone had a complete table, three scientists spotted patterns.
| Scientist | Idea | What worked | What failed |
| Döbereiner | Triads: groups of three elements with similar properties, where the middle atomic mass is roughly the average of the other two (for example, Li, Na, K, with masses 7, 23 and 39) | It showed that properties repeat | Only a few triads could be found |
| Newlands | Law of octaves: every eighth element resembles the first when elements are in order of atomic mass | Hinted at repeating cycles | It held only up to calcium, and it forced unlike elements together |
| Mendeleev | Elements arranged by increasing atomic mass, in groups with similar properties | Gaps left for undiscovered elements | Hydrogen had no fixed place, and isotopes could not be explained |
Mendeleev’s Bold Predictions
Mendeleev’s greatest strength was courage. Where an element did not fit, he left a gap and predicted what the missing element would be like.
| Predicted by Mendeleev | Predicted atomic mass | Found later | Atomic mass found |
| Eka-aluminium | About 68 | Gallium | About 69.7 |
| Eka-silicon | About 72 | Germanium | About 72.6 |
The predicted properties matched the real elements closely, and that was the proof that the table reflected something real.
The Modern Periodic Table
The table was later rearranged on the basis of atomic number, the number of protons, rather than atomic mass. The modern periodic law states that the properties of elements are a periodic function of their atomic numbers. This fixed the problem of elements that seemed out of order.
The modern table has 18 vertical columns called groups and 7 horizontal rows called periods.
- The period number equals the number of shells that contain electrons.
- Elements in the same group have the same number of valence electrons, so they behave alike.
Trends You Must Know
| Property | Across a period (left to right) | Down a group | Reason |
| Valency | Rises from 1 to 4, then falls from 4 to 0 | Stays the same | Valency depends on the number of valence electrons, which is the same in a group |
| Atomic size | Decreases | Increases | Across a period, more protons pull the same shells closer. Down a group, new shells are added |
| Metallic character | Decreases | Increases | Atoms lose electrons less easily across a period, and more easily as size grows down a group |
| Non-metallic character | Increases | Decreases | The opposite of metallic character |
| Tendency to lose electrons | Decreases | Increases | Follows the pull of the nucleus on the outer electrons |
Here is the first row of the trends in action, using period 3.
| Element | Na | Mg | Al | Si | P | S | Cl | Ar |
| Valence electrons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Valency | 1 | 2 | 3 | 4 | 3 | 2 | 1 | 0 |
| Character | Metal | Metal | Metal | Metalloid | Non-metal | Non-metal | Non-metal | Noble gas |
The valency rule explains the pattern. If an atom has four or fewer valence electrons, its valency equals the number of valence electrons. If it has more than four, the valency is 8 minus the number of valence electrons.
Reading a Position
Given an atomic number, you can work out almost everything.
- Atomic number 12. Electron arrangement 2, 8, 2. Three shells, so period 3. Two valence electrons, so group 2. Valency 2. It is a metal, magnesium.
- Atomic number 17. Electron arrangement 2, 8, 7. Three shells, so period 3. Seven valence electrons, so group 17. Valency 8 − 7 = 1. It is a non-metal, chlorine.
Two checks help in the exam: always write the electron arrangement first, and always count shells for the period and valence electrons for the group.
Putting the Trends to Work
The trends become useful when you use them to compare two elements. In every case, use the same two-step reasoning: find the positions, then apply the direction of the trend.
| Compare | Which is larger or more metallic? | Reasoning |
| Na and Mg | Sodium is larger and more metallic | Same period, so the one further left has fewer protons pulling the same shells, giving a larger atom that loses its electron more easily |
| Li and K | Potassium is larger and more metallic | Same group, so the lower element has more shells |
| Cl and F | Chlorine is larger | Same group, and chlorine has a third shell |
| Mg and Al | Magnesium is larger | Same period, and magnesium is further left |
| Be and Ca | Calcium is larger and more reactive as a metal | Same group, and calcium sits lower |
A useful habit is to say the reason aloud as a sentence: “Same period, so more protons pull the same number of shells closer.” Examiners reward the reasoning, and not just the answer.
The First Twenty Elements at a Glance
You are often expected to know the valency of the first twenty elements. Grouping them by valency makes them much easier to remember.
| Valency | Elements |
| 1 | Hydrogen, lithium, sodium, potassium, fluorine, chlorine |
| 2 | Beryllium, magnesium, calcium, oxygen, sulphur |
| 3 | Boron, aluminium, nitrogen, phosphorus |
| 4 | Carbon, silicon |
| 0 | Helium, neon, argon |
The pattern is the one you saw in period 3: valency rises to 4 in the middle of a period and falls back to 0 at the noble gas.
Why Noble Gases Do Not React
Helium has two electrons in its only shell, and neon and argon have eight electrons in their outermost shell. These arrangements are complete, so the atoms have no tendency to lose, gain or share electrons. That is why their valency is zero and why they are called inert. Every other element reacts in a way that tries to reach one of these arrangements, which is the underlying reason atoms form compounds at all.
Why Mendeleev’s Table Needed Fixing
Mendeleev’s table had real successes, but also some problems that the modern table fixed.
- Hydrogen has properties like the alkali metals and like the halogens, so it had no fixed position.
- Isotopes are atoms of the same element with different atomic masses. If elements were arranged by mass, isotopes would need separate places, which made no sense.
- Some pairs were out of order. Arranging strictly by atomic mass placed a few elements in groups where their properties did not match.
- Atomic mass does not increase regularly, so predicting where the next element should fall was not always possible.
Once the table was based on atomic number, which increases by exactly one from each element to the next, these problems disappeared. Atomic number is also the number of electrons in a neutral atom, which links the position directly to the electron arrangement, and that is what decides the chemistry.
Periodic Classification in Your Textbooks
Periodic Classification of Elements is Chapter 5 in the Karnataka SSLC Class 10 Science textbook, according to published syllabus listings. CBSE has removed the chapter from Class 10 for 2026-27, according to published deleted-syllabus lists, so CBSE students will meet the ideas first in Class 11. These chapter notes, the chapter MCQs and a page on the first 20 elements are useful for SSLC revision. In Class 11, the chapter on classification of elements and periodicity continues the story, with periodic trends in properties of elements and, for NEET, these periodic table trend shortcuts. Please confirm current portions with the latest official syllabus for your board and exam.
Frequently Asked Questions
Q1. Why does atomic size decrease across a period?
Across a period, electrons are added to the same shell while the number of protons increases. The stronger pull of the nucleus draws the shell closer, so the atom becomes smaller.
Q2. Why does atomic size increase down a group?
Each step down a group adds a new shell of electrons, so the outermost electrons are farther from the nucleus and the atom becomes larger.
Q3. What is the modern periodic law?
The properties of elements are a periodic function of their atomic numbers.
Q4. How do you find the group and period of an element?
Write the electron arrangement. The number of shells gives the period, and the number of valence electrons gives the group (for the main groups, group 1 and 2, and group number 10 plus the valence electrons for groups 13 to 18).
Q5. Why did Mendeleev leave gaps in his table?
He believed that elements with similar properties belonged in the same group, so when no known element fitted a place, he left a gap and predicted the properties of the missing element. Gallium and germanium later matched his predictions.
A Map That Predicts
The periodic table earns its place because it predicts. Know the position, and you know the shells, the valence electrons, the valency and the character. Learn that one chain of reasoning, and the trends stop being facts to memorise and become consequences you can work out.
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