Which molecules are pure elements?

the molecules formed by one kind of atom are pure elements

Explanation:

1) The definition of molecules is the covalent bonding of two or more atoms.

2) Under that definition you must have at least two atoms to be considered a molecule.

3) Pure elements are the 118 elements that you find in the periodic table: each contain only one kind of atom.

4) Then, molecules of pure elements, are two or more atoms of the same kind linked by covalent bonds.

5) With that these some examples of molecules that are pure elements:

i) O₂

ii) N₂

iii) F₂

iv) Cl₂

v) Br₂

vi) O₃

6) And here some examples of molecules that are not pure elements (there are many, many):

i) CH₄

ii) CH₃CH₂CH₃

ii) CH3CH₂CH₂CH₂CH₂CH₂CH₂CH₃

7) Note that ionic compounds are excluded of the definition of molecules, since they are not formed by covalent bonds but by ionic bonds. For example, NaCl is not a molecule.

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.