Why do elements in the same group have similar physical and chemical properties?
The physical and chemical properties of elements depend on the number of valence electrons. Elements present in the same group have the same number of valence electrons. Therefore, elements present in the same group have similar physical and chemical properties.
pure substances are made up of same type of particles or atoms. all the elements and compounds are pure substances because they contain the same kinds of particles. examples: hydrogen, carbon dioxide element b will be nearer to the top of the activity series of metals, if in nature, metal a is found in a free state while metal b is found in the form of its compounds. Answer  
Atoms prefer to have a filled outermost shell because this makes them more stable.  
Atoms react with each other to get the most stable state possible. A filled outermost shell (a  
complete octet for s- and p-elements) is very stable because all orbitals are filled with  
electrons. Stability of an atom is connected with its energy. Atoms with greater stability have  
less energy. We can see this in chemical reactions: a reaction that increases the stability of the  
atoms will release energy in the form of heat or light. a) he helium (he) is an element by itself. he atom can only exhibit dispersion forces.b) hclh          1a                   1 × 1 e– = 1 e–cl         7a                   1 × 7 e– = 7 e–   total: 8 valence e–[readmore]cl is more electronegative than h, so hcl is a polar covalent compound. hcl exhibits dipole-dipole interaction.c) h2oh          1a                   1 × 2 e– = 2 e–o          6a                   1 × 6 e– = 6 e–   total: 8 valence e– since hydrogen is bonded directly to oxygen.h2o exhibits hydrogen bonding.d) cl2cl         7a                   1 × 7 e– = 14 valence e–composed of the same atom. there's no electronegativity difference.cl2 can only exhibit dispersion forces. magnesium atom loses 2 electrons to attain a stable electronic configuration and becomes a cation. example: a non-metallic atom like chlorine gains 1 electron to attain a stable electronic configuration and becomes an anion. example: cations and anions are oppositely charged particles which attract one another to form an electrovalent bond leading to the formation of an electrovalent compound here magnesium donates one electron each with two chlorine atoms resulting in the formation of magnesium chloride. an alloy is combination of a metal and a non metal. so it has partial characteristics of metal and non metal. being analloy the ions it it are randomly arranged. it does not have a specific arrangement of ions. hence the flow of electrons in this is more. the this feature of alloy leads to increase in resistivity due to free crystal lattice. we know that metals have high melting point but an alloy has low melting point beacause the presence of a non metal in it behaves as an impurity and restricts in raising to high melting point. due to high melting point the it takes a lot of current to melt and leads in slow movement of electrons which makes the device less efficient. therefore electronic device requires the metal of low melting point. an alloy plays the best role in the elctronic device like electric iron and heater. is this a good answer?

when the electrons in the element are excited they jump to higher energy level and as the electrons fall back down it leave the excited state energy which is re-emitted, the wavelength of which is the emission spectrum. 1. according to kinetic theory of ideal gases, molecules of a gas are in incessant random motion, colliding against each other and with the walls of the container. all these collisions are perfectly elastic collision.2. in perfectly elastic collision, total kinetic energy of gas is conserved.3. here, we have a mixture of oxygen and hydrogen gas. thereforek.e of oxygen + k.e of hydrogen=constant4. so, if kinetic energy of oxygen molecules increases, then kinetic energy of hydrogen molecules decreases or vice-versa so that, sum of both kinetic energies remains constant.5. therefore, both option (c) and (d) are correct. hint: at first think about the structure of ester so that you are able to draw the structure for a given molecular formula. the structure of ester is $r - co - o - r'$.complete answer:an ester is a chemical compound derived from an acid in which at least one –oh group is replaced by an $ - o - $ alkyl group. esters are generally derived from substitution reactions of a carboxylic acid and an alcohol which is also known as an esterification reaction. from the given molecular formula we can draw only two structures of esters. they are ethyl formate and methyl acetate. their structures are $h - co - o - c{h\_2} - c{h\_3}$ and $c{h\_3} - co - o - c{h\_3}$.additional information:esterification is the process of combining carboxylic acid and alcohol to give at least one ester and water.$r - co - oh + r - oh \rightleftharpoons r - co - o - r' + {h\_2}o$in the above reaction, the forward reaction gives us esterification and the backward reaction gives us hydrolysis of the ester. the hydrolysis of esters in the presence of alkalis such as potassium hydroxide (lye) or sodium, a reaction called saponification which is utilized in the preparation of soaps from fats and oils and is also used for the quantitative estimation of esters.note:esters are common in organic chemistry and biological materials, and often have a pleasant characteristic, fruity odor. this leads to their extensive use in the fragrance and flavor industry. ester bonds are also found in many polymers. It's not a strange coincidence. It's just the nature of the game!