A) An ionic bond is formed when an element completely transfers its valence electron to another element.
B) Hydrogen bonds are special type of dipole dipole forces which are formed when hydrogen bonds with an electronegative element.
DNA strands are held together by hydrogen bonds between the bases on one strand and those on the other. Adenine and thymine have two hydrogen bonds between them while cytosine and guanine have three hydrogen bonds between them.
C) Non-polar covalent bond is defined as the bond which is formed when there is no difference of electronegativities between the atoms.
D) A polar covalent bond is defined as the bond which is formed when there is a difference of electronegativities between the atoms.
Even the most powerful light-focusing microscopes can't visualise single atoms. What makes an object visible is the way it deflects visible light waves. Atoms are so much smaller than the wavelength of visible light that the two don't really interact. To put it another way, atoms are invisible to light itself.
In a chemical formula, the elements in a compound are represented by their chemical symbols, and the ratio of different elements is represented by subscripts.
