Electrons in an atom can be classified as core electrons and valence electrons. Valence electrons are those electrons which are present in valence shell and participates in bond formation. While, Core electrons are all remaining electrons which are not present in valence shell, hence not take part in bonding.
Atomic number of Selenium (Se) is 34 hence it has 34 electrons with following electronic configuration;
1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁴
From electronic configuration it is found that the valence shell is 4, and the number of electrons present in valence shell are 6. So,
Core Electrons = Total Electrons - Valence Electrons
Core Electrons = 34 - 6
Core Electrons = 28
Result:
There are 28 core electrons in Selenium.
Balanced equation:
Mg + 2 HNO3 —> Mg(NO3)2 + H2
This is a metal + acid reaction giving salt and hydrogen (not water).
The number of neutrons is variable, resulting in isotopes, which are different forms of the same atom that vary only in the number of neutrons they possess. Together, the number of protons and the number of neutrons determine an element's mass number.
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Answer:
exothermic entropy is increased
Explanation:
An exothermic process is one whose rate increases when the temperature is decreased. Hence if a decrease in temperature favours the dissolution of more solute at equilibrium, then the process is exothermic.
Similarly, the dissolution of a solute in a solvent increases the disorderliness (entropy) of the system because of the increase in the number of particles present. Hence once a solute in dissolved, the entropy of the system increases.
The number of protons must differ between two different elements
