Olympiad Test: Structure of Atom- 1 - Class 9 MCQ

Number of electron = Number of proton = Number of atomic Number

i.e., Number of proton = 15

Number of neutron = 16

Therefore mass number = Number proton + Number of neutron

= 15 + 16 = 31

Mass number Symbol of element i.e., ³¹₁₅ X Atomic number.

Number of electron = Mass Number – neutron

number.

= 27 – 14 = 13

But an element has 3 positive charges; it means it loses 3 electrons. So the no. of electrons is 10.

Cation means it loses electrons but electronic configuration of ₁₂A is 2, 8,2 , ₁₃B is 2,8,3 ₁₅C is 2, 8, 5 and ₃ D is 2, 1.

Elements A, B, and D easily lose electrons and acquire positive charge but element C for acquiring stable configuration gains 3 electrons instead of losing five electrons and from an anion.

Electronic configuration of ₁₇Cl atom is = 2, 8, 7

Electronic configuration of Cl– ion is = 2, 8, 8 because negative charge means it gains one electron.

Electronic configuration of ₁₂Mg atom is = 2, 8, 2

Electronic configuration of Mg²⁺ ion is = 2, 8, because 2 positive charges means it loses two electrons.

The atomic number of S = 6 In sulfide ion S^2– means it acquires this configuration by gaining two electrons. Means the valence electron is (8).

Number of neutron = mass number – proton number.

= 23 – 11 = 12

Isobar means the same mass number but different atomic numbers.

Cathode rays (also called an electron beam or an e-beam) are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite the negative electrode is observed to glow from electrons emitted from the cathode.

An atom is electrically neutral because the number of protons is always equal to a number of electrons. Hence option (d) is right.

Atomic number = Number of proton(p) = Number of electron(e)