John Alexander Newlands in 1865 profounded the Law of Octaves. He arranged the elements in increasing order of their atomic weights and noted that every eighth element had properties similar to the first element .The relationship was just like every eighth note that resembles the first in octaves of music.

The physical and chemical properties of the elements are periodic functions of their atomic numbers.So modern periodic table is based on atomic number.

The few defects in Mendeleev's periodic table were corrected by Moseley, and thus was formed the modern periodic table, based on the modern periodic law. Moseley changed the basis of arrangement of elements in Mendeleev's periodic table from atomic mass to atomic number and noticed that all defects of the periodic table were cleared by this change.

Noble gases are also known as inert gases and do not take part in chemical reactions.

The elements of the 3^rd period are representative elements [Na (Z = 11) to Ar (Z = 18)] as each of them is a representative of its group.

At any given moment, an electron can be found at any distance from the nucleus and in any direction according to the Heisenberg Uncertainty Principle. The s orbital is a spherically-shaped region describing where an electron can be found, within a certain degree of probability. The shape of the orbital depends on the quantum numbers associated with an energy state. All s orbitals have l = m = 0, but the value of n can vary.

The shortest period of the periodic table is the 1^st period .It contains only 2 elements which are Hydrogen and Helium.

Groups are the set of elements occupying the vertical columns in the periodic table and having similar properties.

The group number is the number assigned to the vertical columns of the structured list of all known elements in the periodic table. Elements within the same group have the same number of electrons in their outer electron shells.

With an increase in atomic size, the ionisation potential is reduced, since the distance of the outermost electron from the nucleus increases and hence the force of attraction decreases.

The noble gases, also known as the inert gases, are located in Group VIII of the periodic table. Group VIII is sometimes called Group O. The noble gases are helium, neon, argon, krypton, xenon, radon, and ununoctium.The noble gases are relatively nonreactive. This is because they have a complete valence shell. The noble gases have low boiling points and  all are gases at room temperature.The noble gases are on the extreme right of the periodic table.

Alkaline Earth Metals are metals that are found in Group II of the periodic table. Six elements such as Beryllium, Magnesium, Calcium, Strontium, Barium and Radium are placed in this group.

The distribution of electrons in different orbitals is known as its electronic configuration. This characterizes each electron in an atom. The electronic configuration is expressed by indicating the principal quantum number and its respective orbital along with the number of electrons present in it. For example, the notation 3p_x¹ indicates that in the third principal shell there is one electron in the 'p_x' orbital.

The number of shells signifies the period number of an element.So, the element having two shells must belong to the 2^nd period.

Chlorine gas is  yellowish green in colour  having melting point  -101 ̊ C  and boiling point  -35 ̊ C.

There are two blocks of rare earths, the lanthanide series and the actinide series. Lanthanum and actinium are both located in group IIIB of the table. When one looks at the periodic table, one notices that the atomic numbers make a jump from lanthanum (57) to hafnium (72) and from actinium (89) to rutherfordium (104). If one skips down to the bottom of the table, one can follow the atomic numbers from lanthanum to cerium and from actinium to thorium, and then back up to the main body of the table. Some chemists exclude lanthanum and actinium from the rare earths, considering the lanthanides to start following lanthanum and the actinides to start following actinium.

 The recurrence of characteristic properties of elements, arranged in the periodic table at regular intervals, is called periodicity. Elements having similar electronic configuration have similar properties. Thus the cause of periodicity is the repetition of similar electronic configuration.

Bromine is the only  nonmetallic liquid element. It is a heavy, mobile, reddish-brown liquid, volatilizing readily at room temperature to a red vapour with a strong disagreeable odour, resembling chlorine, and having a very irritating effect on the eyes and throat; it is readily soluble in water or carbon disulfide, forming a red solution, is less active than chlorine but more so than iodine; it unites readily with many elements and has a bleaching action; when spilled on the skin it produces painful sores. It presents a serious health hazard, and maximum safety precautions should be taken when handling it.

The elements in the 7^th  group have seven valence electron in their valence shell.

Alkali metals are very reactive in nature. As they have one electron in their valence shell, they react very vigorously with air, oxygen, water and other substances in order to complete their octet.

The extreme right elements have 8 electrons in their valence shell (except Helium which has 2, a duplet). Their valence shell being completely filled, their octet is satisfied and they do not take part in chemical reactions. Hence their valency or combining power is zero.

 Valence shell electrons are the electrons in the outermost shell .

In a particular group, all the members have the same number of valence electrons and hence they have the same properties.

Atomic density is the number of atoms of a given type per unit volume of the material.   

The electron affinity (EA) of an element is defined as the energy given off when a neutral atom in the gas phase gains an extra electron to form a negatively charged ion. Electron affinities are more difficult to measure than ionisation energies and are usually less accurately known. Electron affinities are large and negative for elements such as fluorine and oxygen, and small and positive for metals.