Atomic Number = the number of protons= 14 p+
In a neutral atom the number of protons = the number of electrons = 14 e-
Mass Number = number of protons + number of neutrons
The number of neutrons = Mass Number – Atomic Number
The number of neutrons (n0) = 25 - 14 = 11
14 p+, 14 e-, 11 no - so the element X is unstable silicon isotope with the half-life 220 milliseconds. Isotopes – atoms of the same element with different masses, which resulted from different number of neutrons. The isotopes of an element have similar chemical and physical properties. The most stable known isotope Silicon-28 is a metalloid, intermediate between metals and non-metals, with the four valence electrons. It may react with other elements to form covalent bond.