Description in biology, cell theory is the historic scientific theory, now universally accepted, that living organisms are made up of cells, that they are the basic structural/organizational unit of all organisms, and that all cells come from pre-existing cells.
Covalent compounds are composed of atoms that are linked via covalent bonds i.e. bonds formed by mutual sharing of electrons. This is in complete contrast to ionic compounds which are held together by ionic bonds, i.e. bonds formed by complete transfer of electrons from one atom to the other.
In the given examples we have:
Barium nitrate: Ba(NO3)2 - Ionic
Dinitrogen tetroxide: N2O4- Covalent
Boron trifluoride: BF3-Covalent
Ammonium sulfate: (NH4)2SO4- Ionic
Carbon tetrachloride: CCl4- Covalent
Barium chloride: BaCl2 - Ionic
Answer:

Explanation:
By the First Law of Thermodynamics, the piece of metal and water reaches thermal equilibrium when water receives heat from the piece of metal. Then:




There are 30 protons and 39 neutrons in the nucleus.
This must me the isotope of an element with an atomic mass close to 69 u.
The only candidates are Zn and Ga.
Zn has a zinc-69 isotope with mass 68.926 u.
Ga has a gallium -69 isotope with mass 68.925 u.
The isotope is probably

.
It has 30 protons and 39 neutrons.