Science [2 Mark Questions]

Guard cells protect the stomata. The guard cells swell when water flows into them, causing the stomatal pore to open. Similarly the pore closes if the guard cells shrink. Thus guard cells swell and shrink to regulate the opening and closing of stomatal pore.

• As moving from left to right in a period, the atomic number of elements increases by 1.
• As atomic number increases, nuclear charge also increases.
• Due to this increased nuclear charge, the electrons are attracted strongly towards the nucleus and hence the atomic size decreases.

• Electronic configuration of hydrogen resembles with alkali metals.
• Like alkali metals, hydrogen combines with halogens, oxygen and sulphur to form compounds having similar formulae.
• Hydrogen also resembles halogens as it exists in the form of diatomic molecules.
• Hydrogen combines with metals and non-metals to form ionic and covalent bonds respectively.
• Thus, hydrogen cannot be assigned fixed position in the periodic table.

• In Dobereiner’s triads, three elements were arranged in the order of increasing atomic masses of elements in which the atomic mass of the middle element is the average of the atomic masses of the other two elements.
• Dobereiner could identify triads from the elements known at that time was only a coincidence.
• Thus, this classification of elements into triads could not be applied to all the elements known at that time.

• Scientists classified the different elements according to their properties and obtained an orderly arrangement out of chaos.
• Scientists made the early attempts to classify the elements in metals and non-metals.
• Thereafter, further attempts of the classification of elements was carried out as our knowledge of elements and their properties were further studied.

• Electronegative elements possesses non-metallic character.
• Non-metals have a tendency of gaining or sharing the electrons during bond formation process.
• Trend of the non-metallic character in a group :
• On moving down the any group, the non-metallic character decreases.
• Trend of the non-metallic character in a period :
• On moving from left to right in a period, the effective nuclear charge experienced by valence electrons increases.
• Therefore, the tendency to attract electrons will increase.
• Thus, non-metallic character increases in a period on moving from left to right.

• Those properties which are determined by the electronic configuration of elements or which depend on electronic configuration of elements are known as periodic properties.
• OR
• The properties which show a recurring gradation within the same group or along a period are known as periodic properties.
• For example, valency, atomic radii (atomic size), metallic property, electronegativity, etc.

• At the time, when Mendeleev classified elements and gave his periodic table, he left some gaps for the undiscovered elements.

• Thus, many scientists were inspired to discover the unknown elements.
• Atomic masses of several elements were corrected on the basis of periodic table.
• Inert gases such as helium (He), neon (Ne) and argon (Ar) had been used in many context.
• These gases were discovered very late because they are very inert and present in extremely low concentrations in atmosphere.
• Another strength of the periodic table was that, when these gases were discovered later, they could be placed in a new group named zero group without disturbing the existing order.

• On moving down in a group, the atomic number of element increases.
• A new shell of electrons is added with increase in atomic numbers of elements.
• Thus, the distance between the valence shell and nucleus increases and atomic size increases down the group inspite of the increase in nuclear charge.