Mendeleev realized that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table.
Gaps and predictions Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered. He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right. For example, he predicted the properties of an undiscovered element that should fit below aluminum in his table. When this element, called gallium, was discovered in 1875 its properties were found to be close to Mendeleev's predictions. Two other predicted elements were later discovered, lending further credit to Mendeleev's table.
Answer: it reacts with the alkali metals (M) to form a salt MX, where X is the halogen.
Explanation: Group 7A elements are halogen and they react with alkali metals like Sodium or potassium to form a salt like NaCl
Explanation :
Isotope : It is defined as the element that have the same number of protons but have the different number of neutrons of each of the atom.
Atomic number is defined as the number of protons or number of electrons.
Atomic number = number of protons = number of electrons
Mass number is defined as the sum of number of protons and number of neutrons.
Number of neutrons = Mass number - Number of protons
For the given options:
(a) ![^{10}_{5}B](https://tex.z-dn.net/?f=%5E%7B10%7D_%7B5%7DB)
Atomic number = 5
Number of proton = 5
Number of electron = 5
Atomic mass = 10
Number of neutrons = Atomic number - Number of protons
Number of neutrons = 10 - 5 = 5
(b) ![^{199}_{80}Hg](https://tex.z-dn.net/?f=%5E%7B199%7D_%7B80%7DHg)
Atomic number = 80
Number of proton = 80
Number of electron = 80
Atomic mass = 199
Number of neutrons = Atomic number - Number of protons
Number of neutrons = 199 - 80 = 119
(c) ![^{63}_{29}Cu](https://tex.z-dn.net/?f=%5E%7B63%7D_%7B29%7DCu)
Atomic number = 29
Number of proton = 29
Number of electron = 29
Atomic mass = 63
Number of neutrons = Atomic number - Number of protons
Number of neutrons = 63 - 29 = 34
(d) ![^{13}_{6}C](https://tex.z-dn.net/?f=%5E%7B13%7D_%7B6%7DC)
Atomic number = 6
Number of proton = 6
Number of electron = 6
Atomic mass = 13
Number of neutrons = Atomic number - Number of protons
Number of neutrons = 13 - 6 = 7
(e) ![^{77}_{34}Se](https://tex.z-dn.net/?f=%5E%7B77%7D_%7B34%7DSe)
Atomic number = 34
Number of proton = 34
Number of electron = 34
Atomic mass = 77
Number of neutrons = Atomic number - Number of protons
Number of neutrons = 77 - 34 = 43
Answer:
By absorbing energy electron is jump into higher energy level. This is called excitation.
Explanation:
The electron is jumped into higher level and back into lower level by absorbing and releasing the energy.
The process is called excitation and de-excitation.
Excitation:
When the energy is provided to the atom the electrons by absorbing the energy jump to the higher energy levels. This process is called excitation. The amount of energy absorbed by the electron is exactly equal to the energy difference of orbits. For example if electron jumped from K to L it must absorbed the energy which is equal the energy difference of these two level. The excited electron thus move back to lower energy level which is K by releasing the energy.
De-excitation:
When the excited electron fall back to the lower energy levels the energy is released in the form of radiations. this energy is exactly equal to the energy difference between the orbits. The characteristics bright colors are due to the these emitted radiations. These emitted radiations can be seen if they are fall in the visible region of spectrum.
It leads up to the climax.