Class 10 Science • Complete Chapter Notes
As the number of known elements increased, scientists needed a systematic way to organise them. Elements have different physical and chemical properties, but some elements also show similarities in their properties.
Classification helps us study a large number of elements by placing elements with similar characteristics in an organised pattern.
One of the earliest classifications divided the known elements broadly into metals and non-metals.
As more elements were discovered, scientists looked for better methods that could group elements according to similarities in their properties.
In 1817, Johann Wolfgang Döbereiner, a German chemist, identified groups of three elements having similar properties. These groups were called triads.
When the three elements are arranged in increasing order of atomic mass, the atomic mass of the middle element is approximately the average of the atomic masses of the other two elements.
| Element | Atomic Mass |
|---|---|
| Lithium (Li) | 6.9 |
| Sodium (Na) | 23.0 |
| Potassium (K) | 39.0 |
Some groups such as Ca-Sr-Ba and Cl-Br-I also illustrate the idea of triads.
John Newlands arranged elements in increasing order of atomic masses.
He observed that every eighth element had properties similar to the first element, similar to the repetition found in musical octaves.
Dmitri Ivanovich Mendeleev arranged elements mainly on the basis of increasing atomic masses and similarities in their chemical properties.
The properties of elements are a periodic function of their atomic masses.
Leaving gaps was an important strength of Mendeleev's table. It allowed him to predict that undiscovered elements should exist and estimate some of their properties.
The modern periodic table is based on atomic number rather than atomic mass.
Atomic number is the number of protons present in the nucleus of an atom.
The modern periodic table arranges elements according to increasing atomic number and their recurring chemical properties.
The vertical columns of the periodic table are called groups.
The modern periodic table has 18 groups.
The horizontal rows of the periodic table are called periods.
The modern periodic table has 7 periods.
The electronic configuration of an element helps us understand its position in the periodic table.
The number of occupied electron shells generally indicates the period of an element.
The electrons present in the outermost shell are called valence electrons.
For many main-group elements, the number of valence electrons is related to their group position and helps explain their chemical behaviour.
Valency is the combining capacity of an atom.
It is related to the number of electrons involved in achieving a stable outer electronic configuration.
For many main-group elements, valency tends to increase from one to four and then decrease from four to zero as we move across a period.
Atomic size refers broadly to the size of an atom and is commonly represented using its atomic radius.
Atomic size generally decreases from left to right across a period.
This is because the nuclear charge increases while the electrons are added to the same principal shell, increasing the effective attraction between the nucleus and electrons.
Atomic size generally increases from top to bottom in a group because new electron shells are added.
Metallic character describes the tendency of an element to show metallic properties and lose electrons to form positive ions.
Metallic character generally decreases from left to right.
Metallic character generally increases down a group.
Non-metallic character is associated with the tendency of an element to gain or share electrons rather than lose them easily.
Non-metallic character generally increases from left to right.
Non-metallic character generally decreases down a group.
The position of an element in the periodic table is related to its electronic configuration and therefore influences its chemical reactivity.
Metals generally react by losing electrons. Down many groups of metals, increasing atomic size can make removal of the outermost electron easier, so metallic reactivity can increase down the group.
Non-metals generally tend to gain or share electrons. Their tendency to attract electrons is influenced by atomic size and effective nuclear attraction.
| Property | Across a Period | Down a Group |
|---|---|---|
| Atomic size | Generally decreases | Generally increases |
| Valency | Generally increases to 4, then decreases to 0 | Similar valence-shell pattern within a main group |
| Metallic character | Generally decreases | Generally increases |
| Non-metallic character | Generally increases | Generally decreases |
| Scientist / System | Basis | Main Idea |
|---|---|---|
| Döbereiner | Atomic mass and similar properties | Triads |
| Newlands | Increasing atomic mass | Law of Octaves |
| Mendeleev | Atomic mass and chemical properties | Periodic table with gaps for undiscovered elements |
| Modern Periodic Table | Atomic number | Periodic relationship of properties |
For the most current curriculum, students should use the official CBSE Academic material and the official NCERT textbook resources.
CBSE Academic – Curriculum 2026-27 CBSE – Class X Science Reading Material 2026-27 CBSE – Science Learning Standards CBSE – Class 10 Science Competency-Based Questions NCERT – Official Textbooks