Answer:
One independent variable
Explanation:
The other variables are called controlled variables because the experiment is designed to keep them at an unchanging value, known as "controlling" a variable. A valid experiment should have only one independent variable.
If an unknown substance CANNOT be broken down into simpler substances, it is AN ELEMENT made of one kind of atom.
Answer:
Covalent bond between identical atoms
Covalent bonds occur between identical atoms or between different atoms whose difference in electronegativity is insufficient to allow transfer of electrons to form ions. ... The two hydrogen atoms are attracted to the same pair of electrons in the covalent bond.
Answer: 
Explanation:
Electronic configuration represents the total number of electrons that a neutral element contains. We add all the superscripts to know the number of electrons in an atom.
The electrons are filled according to Afbau's rule in order of increasing energies and thus the electronic configuration for Hafnium with atomic number of 72 and containing 72 electrons is:
The electronic configuration for Hafnium in terms of nearest noble gas Is
![[Xe]6s^24f^1{14}5d^2](https://tex.z-dn.net/?f=%5BXe%5D6s%5E24f%5E1%7B14%7D5d%5E2)
