Bond breaking is an endothermic change and bond forming is an exothermic change. Bond energy is the amount of energy in kJ/mol needed to break one mole of the specified bor
H
H
H
H
H
H
H
C
.C
H
CI
H
0 0 0 0
HCI
H
H
H
H
H
H
Use the following bond energies to determine whether this reaction is exothermic or
endothermic. You must show your reasoning.
bond
bond energies in
KJ/mol
CCI
338
CH
412
CH
CI-CI
242
H-CI
431
348
I

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.

the answer is c.the valence of the atoms combine .

they are called fossil fuels