10th Ncert Chemistry Notes,Ncert 10th English Textbook Pdf,Math Ka Basic Formula - You Shoud Know
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According to this, when elements are arranged in order of increasing atomic masses, groups of three elements, having similar properties are obtained. The atomic mass of middle element of the triad being nearly equal to the average of the atomic masses of the other two elements.
For Example Li 6. Limitation: It fails to arrange all the known elements in the form of triads, even having similar properties. Periodicity of Properties : The repetition of properties of elements after certain regular intervals is known as Periodicity of Properties.
Example: Position of hydrogen with alkali metals and halogens 17th group. Example: Cl and Cl Example: Ni The Modern Periodic Table: In , Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass. Modern Period Law: The physical and chemical properties of elements are the periodic function of their atomic number.
Modern periodic table is based on atomic number of elements. Atomic number Z is equal to the number of protons present in the nucleus of an atom of an element. Modern periodic table contains 18 vertical column known as group and seven horizontal rows known as periods. On moving from left to right in a period, the number of valence electrons increases from 1 to 8 in the elements present.
On moving from left to right in a period, number of shell remains same. All the elements of a group of the periodic table have the same number of valence electrons. In Period: On moving from left to right in a period, the valency first increases from 1 to 4 and then decreases to zero 0.
In Groups: On moving from top to bottom in a group, the valency remains same because the number of valence electrons remains the same.
It is a distance between the centre of the nucleus and the outermost shell of an isolated atom. In Period : On moving from left to right in a period, atomic size decreases because nuclear charge increases. In Group: Atomic size increases down the group because new shells are being added in spite of the increase in nuclear charge. In Period: Along the period from left to right, metallic characters decreases because a tendency to lose electron decreases due to the increase in nuclear charge.
In Period: Along the period from left to right, non-metallic character increases because tendency to gain electrons increases due to increase in nucleus charge. It increases with increase of nuclear charge or decrease in atomic size. Along the period electronegativity increases. Down the group electronegativity decreases. Na2O, MgO etc. Non-metal oxides are acidic in nature. In the case of metal reactivity, it increases down the group because of the tendency to lose electrons increases.
In the case of non-metal reactivity, decreases down the group because of the tendency to gain electrons decreases.
Atomic size: The atomic size may be visualised as the distance between the centre of the nucleus and the outermost shell of an isolated atom. The trend of atomic size radius in moving down a group: Ongoing down in a group of the Periodic Table, the atomic size increases because a new shell of electrons is added to the atoms at every step.
There is an increase in distance between the outermost shell electrons and the nucleus of the atom. The trend of atomic size radius in moving from left to right in a period: On moving from left to right along a period, the size of atoms decreases because on moving from left to right, the atomic number of elements increases which means that the number of protons and electrons in the atoms increases.
Due to the large positive charge on the nucleus, the electrons are pulled in more closely to the nucleus and the size of the atom decreases. This atomic mass is the average of the atomic masses of Li and K which have atomic masses 7 and 39 respectively. Triads as formed by Dobereiner. So Mendeleev boldly predicted the existence of some more elements.
He even predicted the properties of some of these elements and named them as Eka-boron, Eka-aluminium and Eka-silicon respectively. Later on the elements were discovered, for example, gallium replaced Eka-aluminium and it showed properties similar to that of aluminium.
He placed them in a new group. As atomic number determines the distribution of electrons in the orbits, and electrons of the outermost orbit determine the properties of an element.
Groups and periods in the Modem long form Periodic Table: There are 18 groups vertical columns and 7 periods horizontal lines in the Modern or long form Periodic Table. The number of the period is equal to the number of shells in the atoms of the elements belonging to that period.
Trends in Modem Periodic Table i Valency: Elements belonging to the same group have the same number of valence electrons and thus the same valency. Valency in a particular period from left to right first increases as positive valency and then decreases as negative valency.
Atomic size increases down a group due to the increase in the number of shells. Atomic size decreases along a period due to an increase in the nuclear charge which tends to pull the Ncert 10th Chemistry Notes Free electrons closer to the nucleus and reduces the size of the atom. A zig-zag line of metalloids separates metals from non-metals. Need for classification of elements: Increase in the discovery of different elements made it difficult to organise all that was known about the elements.
To study a large number of elements with ease, various attempts were made. The attempts resulted in the classification of elements into metals and non-metals. These groups were called triads. He was not able to prepare triads of all the known elements. Modem Periodic Table: Henry Moseley, gave a new!
Newland adjusted two elements in the same slot e. Co and Ni , having different properties. According to Newland, only 56 elements existed in nature and no more elements would be discovered in future. Example: Eka-boron, Eka-aluminium and Eka-silicon.
It is useful in correcting the doubtful atomic masses of some elements. Cl2O7, SO3, P2O5, In the case of metal reactivity, it increases down the group because of the tendency to lose electrons increases. There are 18 groups and 7 seven periods in the Modern Periodic Table.
Characteristics of triads of J. Elements of a triad show similar chemical properties. These elements of a triad show specific trends in their physical properties. The atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.
If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the Periodic Table.
The atomic masses do not increase in a regular manner in going from one element to the next. He could not assign a correct position to hydrogen in his table because hydrogen has some properties similar to alkali metals and some properties similar to halogens.
It has 8 groups. No place could be assigned to isotopes of an element. There were three gaps left by Mendeleev in his Periodic Table. No fixed position was given to hydrogen in this Periodic Table.
No distinction was made between metals and non-metals. Inert gases were not known at the time of Mendeleev. Metallic characters decrease from left to right in a period and increase while going down in a group. Non-metallic characters increase from left to right in a period due to increase in the electronegativity and these characters decrease from top to bottom in a group due to the decrease in the electronegativity of atoms while going down in a group.
Atomic mass of the middle element was roughly the average of the atomic masses of the other two elements. Properties of every eighth element were similar to that of the first element. It contained only 56 elements. The elements with similar physical and chemical properties came under the same groups. Mendeleev adjusted few elements with a slightly greater atomic mass before the elements with slightly lower atomic mass, so that elements with similar properties could be grouped together.
For example, aluminium appeared before silicon, cobalt appeared before nickel. Mendeleev left some gaps in his periodic table. He predicted the existence of some elements that had not been discovered at that time. His predictions were quite true as elements like scandium, gallium and germanium were discovered later. The gases like helium, neon and argon, which were discovered later, were placed in a new group without disturbing the existing order.
Atomic masses did not increase in a regular manner in going from one element to the next. Elements present in any one group have the same number of valence electrons. Also, the number of shells increases as we go down the group. Elements present in any one period, contain the same number of shells.
Also, with increase in atomic number by one unit on moving from left to right, the valence shell electrons increases by one unit. Each period marks a new electronic shell getting filled. Valency of elements in a particular group is same. Valency of elements in a period first increases from one to four and then decreases to zero.
Atomic Size: Atomic size refers to the radius of an atom. In a period, atomic size and radii decreases from left to right. In a group, atomic size and radii increases from top to bottom. Metallic and Non-metallic Properties: The tendency to lose electrons from the outermost shell of an atom, is called metallic character of an element.
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