Johann Wolfgang Döbereiner, a German chemist, arranged elements with similar properties into groups. He identified some groups with three elements each, calling them triads. He mainly classified the groups based on increasing atomic masses. He showed that when the three elements were written in the order of increasing atomic masses, the middle element’s atomic mass was the average of the atomic masses of the other two. For example, the triad of lithium (Li), sodium (Na), and potassium (K) has atomic masses of 6.9, 23.0, and 39.0, respectively. The average atomic mass of lithium and potassium is the actual atomic mass of sodium.

The characteristics of Döbereiner’s triads are:

• All elements in the triads have similar chemical properties.
• The elements in the triads have specific trends in their physical properties.
• The atomic mass of the middle element is roughly the average atomic mass of the other two elements.

Class 10 solutions chapter 5 covers all three of Döbereiner’s triads:

• Triad 1: Lithium (Li), sodium (Na), and potassium (K).
• Triad 2: Calcium (Ca), strontium (Sr), and barium (Ba).
• Triad 3: Chlorine (Cl), bromine (Br), and iodine (I).

The limitation of Döbereiner’s triads was that he could only identify these three triads.

This law states that when elements are placed in the order of increasing atomic masses, the physical and chemical properties of every eighth element are a repetition of the properties of the first element. The characteristics of Newland’s law of octaves include:

• It comprises elements from hydrogen (H) to thorium (Th).
• The properties of every eighth element are similar to those of the first element.

Limitations of the law of octaves:

• The law applies only up to calcium (only for lighter elements).
• Two elements with different properties, cobalt (Co) and nickel (Ni), were adjusted in the same slot.
• Newland stated that only 56 elements existed in nature, and no new elements would be discovered.

Mendeléev’s periodic table is based on elements’ chemical and physical properties and atomic masses. The law states that “the physical and chemical properties of elements are the periodic function of their atomic masses”. There are a few elements whose properties repeat after regular intervals; this is called the periodicity of properties.

The characteristics of Mendeléev’s periodic table include:

• All the 63 known elements are arranged in increasing order by their atomic masses.
• The table comprises vertical columns called groups and horizontal rows called periods.
• Elements with the same physical and chemical properties come under the same groups.

The merits of Mendeléev’s periodic table are:

• Various vacant spots provided hints for the discovery of new elements. For example, eka-aluminium, eka-silicon, and eka-boron.
• The properties of various undiscovered elements have been predicted based on their position in Mendeléev's periodic table.
• The periodic table helps in correcting doubtful atomic masses of various elements.
• After discovery, noble gases could easily be accommodated in the periodic table.

Limitations:

• There is no fixed position of hydrogen; while it is sometimes positioned with the alkali metals (first group), at other times, it is positioned with the halogens (17th group).
• The position of isotopes, for example, CI-35 and CI-37, is not decided.
• There is no trend in the atomic masses; a few elements with lower atomic masses are placed before those with higher atomic masses, for example, Ni-58.7 before Co-58.9.

The modern periodic table was developed by Henry Moseley in 1913. According to the modern periodic table, an element’s atomic number is a more fundamental property than its atomic mass. The modern periodic law states that an element’s physical and chemical properties are a periodic function of the atomic number.

• The modern periodic table emphasises the atomic number of elements.
• The atomic number equals the number of protons present in the nucleus of an element’s atom. 
• There are 18 vertical columns called groups and seven horizontal rows called periods.
• When moving from left to right in a period, the valence electrons increase from one to eight in the elements present.
• When moving from left to right in a period, the number of shells remains the same.
• The number of valence electrons is the same in all the elements of a group in the periodic table. 

Position of elements in the modern periodic table:

• The modern periodic table comprises 18 groups and seven periods.
• Elements belonging to a similar group have the same number of valence electrons. The shell number increases as we go down in the group.
• Elements belonging to one period have the same number of shells. The atomic number increases by one unit as they move from left to right, and the valence shell electrons increase by one unit. 
• Each period fills with a new electronic shell.

Trends in the modern periodic table

• Valency: Valency is determined by the number of valence electrons in the atom’s outermost shell. 
• Atomic size: It refers to the radius of an atom. It is the distance between the centre of the nucleus and the outermost shell of an isolated atom.
• Metallic character: It is the tendency of an atom to lose electrons.
• Non-metallic character: It is the tendency of an atom to gain electrons.
• Chemical reactivity: The chemical reactivity in metals increases because the tendency to lose electrons increases. Meanwhile, in non-metals, chemical reactivity decreases as the tendency to gain electrons increases.
• Electronegativity refers to an element’s tendency to attract a shared pair of electrons towards the covalently bonded molecules.
• Nature of oxides: Non-metallic oxides are acidic, while metallic oxides are basic. In metal reactivity, they increase down the group because they lose electrons quickly. Non-metallic reactivity decreases as we move down the group because they gain electrons slowly.

Metallic and non-metallic properties

• The metallic character of an element means the tendency to lose electrons from an atom’s outermost shell. 
• The non-metallic character is described by the tendency to gain electrons from an atom’s outermost shell.
• The metallic character in a period decreases while it increases in a group.
• The non-metallic character in a period increases while it decreases in a group.

1. Do Döbereiner's triads also exist in the columns of Newland’s octaves?

Yes, Döbereiner’s triads exist in the columns of Newland’s octaves. For example, potassium (K), lithium (Li), and sodium (Na) are part of Döbereiner's triads and are also found in the second column of Newland’s octaves.

2. What are the limitations of Döbereiner’s triads?

Some limitations of Döbereiner's triads are:

• They are not applicable for high or low-mass elements.
• Not every element fits into the triads.
• With the introduction of different methods to calculate atomic masses, the validity of Döbereiner’s triads decreases. For example, in the triad of fluorine (F), chlorine (Cl), and bromine (Br), the arithmetic mean of the atomic masses of fluorine and bromine are not equal to the atomic mass of chlorine

3. Which elements of the triads are among the first ten in the modern periodic table?

Lithium (Li) and beryllium (Be).

4. What criteria did Mendeléev use in creating his periodic table?

Mendeléev focused on the compounds formed by the elements of hydrogen (H) and oxygen (O2). While creating his periodic table, he observed a relationship between the atomic masses of many elements.

5. What were the limitations of Newland’s law of octaves?

The limitations of Newland’s law of octaves include:

• It applies to calcium (Ca) elements only.
• He assumed that there were only 56 elements in nature and no further elements would be discovered.
• Newland placed two elements in one slot to ensure that all elements fit in the table. This meant that unlike elements with different properties were placed in one column.
• Iron (Fe) was not placed into the column that resembles its properties.