What subatomic particle is specifically responsible for combining the atoms together to form a new substance?

Electron

Atoms, compounds, and molecules undergo chemical reactions to form new substances. In general, reactions proceed to produce products which are have more stability.

These chemical reactions are possible because of the ability of substances to form chemical bonds with other substances. These formation of bonds are primarily attributed to electronic interactions (i.e. the valence or outer electrons of the atoms). of substances. 

Atoms are able to form new substances by gaining, losing, or sharing electrons with other atoms. 

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water