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1. (a) Orbital, in chemistry and physics, a mathematical expression, called a wave function, that describes properties characteristic of no more than two electrons in the vicinity of an atomic nucleus or of a system of nuclei as in a molecule.

3. (a) Electronegativity is a measure of an atom's ability to attract shared electrons to itself. On the periodic table, electronegativity generally increases as you move from left to right across a period and decreases as you move down a group.

(b) Electropositivity can be defined as the tendency of an atom to donate electrons and form positively charged cations. This property is primarily exhibited by metallic elements, especially the alkali metals and the alkaline earth metals.

(c) Ionization energy is simple terms can be described as a measure of the difficulty in removing an electron from an atom or ion or the tendency of an atom or ion to surrender an electron. The loss of electron usually happens in the ground state of the chemical species.

(d) The ionic strength is an expression of the effect of ions in solution on the electrostatic potential. 

(e) Atomic radius is the distance from the centre of the nucleus to the outermost shell containing electrons. In other words, it is the distance from the center of the nucleus to the point up to which the density of the electron cloud is maximum.

(f) Metallic character refers to the level of reactivity of a metal. Metals tend to lose electrons in chemical reactions, as indicated by their low ionization energies. Within a compound, metal atoms have relatively low attraction for electrons, as indicated by their low electronegativities.

(g) Lattice energy is defined as the energy required to separate a mole of an ionic solid into gaseous ions. Lattice energy cannot be measured empirically, but it can be calculated using electrostatics or estimated using the Born-Haber cycle.

(h) Inert pair effect cause the variable valency of elements. For example, both tin and lead show +2 and +4 oxidation state due to inert pair effect. 'Tl' element shows +3 and +1 oxidation state for same cause. For example, +5 oxidation state of Bi is less stable than +3 oxidation state of bismuth element.

(i) Bond energy (E) is defined as the amount of energy required to break apart a mole of molecules into its component atoms. It is a measure of the strength of a chemical bond. Bond energy is also known as bond enthalpy (H) or simply as bond strength.

(j) Oxidation state indicates the degree of oxidation for an atom in a chemical compound; it is the hypothetical charge that an atom would have if all bonds to atoms of different elements were completely ionic. Oxidation states are typically represented by integers, which can be positive, negative, or zero.

(i) "The angle of reflection equals to the angle of incidence".

(ii) "The incident ray, reflected ray and the normal to the reflecting surface at the point of incidence lie in the same plane".

The frequencies in which the standing waves can be formed are called harmonics.

or

The integral multiple of fundamental frequencies are called harmonics.

Green house effect : Temperature of the earth increases due to the radiation emitted by the earth is trapped by atmospheric gases like CO₂, CH₄, N₂, Chlorofluoro carbons etc is called green house effect.

(i) Radiation from the sun enters the atmosphere and heat the objects on the earth. These heated objects emit infrared rays.

(ii) These rays are reflected back to Earth's surface and trapped in the Earth's atmosphere. Due to this temperature of the earth increases.

(iii) The layers of carbon dioxide (CO₂) and low lying clouds prevent infrared rays to escape Earth's atmosphere.

(iv) Since day–by–day the amount of carbondioxide in the atmosphere increases, more infrared rays are entrapped in the atmosphere.

(v) Hence the temperature of the Earth's surface increases day by day.

The atomic hydrogen emits a line spectrum consisting of five series.

(1) Lyman series : υ = Rc(1/1² - 1/n²) where n = 2, 3, 4, ........

(2) Balmer series : υ = Rc(1/2² - 1/n²) where n = 3, 4, 5, ........

(3) Paschen series : υ = Rc(1/3² - 1/n²) where n = 4, 5, 6, ........

(4) Brackett series : υ = Rc(1/4² - 1/n²)  where n = 5, 6, 7, ........

(5) Pfund series : υ= Rc(1/5² - 1/n²) where n = 6, 7, 8, ........