Snowflake chem­istry

How do these del­i­cate frosty flakes form, and are they truly unique?

How It Works - - SCIENCE -

Snowflakes form around tiny par­ti­cles of dust or pollen float­ing through the at­mos­phere; as a par­ti­cle passes through clouds of water mol­e­cules, they stick to its sur­face to form a droplet. At freez­ing tem­per­a­tures high in the at­mos­phere, this droplet be­gins to freeze and form crys­tal faces. These crys­tals be­gin the for­ma­tion of the snowflake’s shape. The rea­son for their sym­met­ri­cal shape is due to the struc­ture of the water mol­e­cules. Hy­dro­gen and oxy­gen atoms bond to­gether at a 104.5-de­gree an­gle, creat­ing a V-shaped struc­ture with two hy­dro­gen atoms at­tached to one oxy­gen atom in the mid­dle. As water mol­e­cules bond to one an­other, six of these ‘V’ struc­tures form a hexag­o­nal shape. This process con­tin­ues and the crys­tals be­gin to fall as more water mol­e­cules join the frozen par­ti­cle party – as many as 1 bil­lion bil­lion (1018) water mol­e­cules can be present in the av­er­age flake! The sur­round­ing tem­per­a­ture of a fall­ing snowflake will in­crease as it nears the ground. This lim­its the amount of freeze and the num­ber of water mol­e­cules that can join onto it, creat­ing spik­ing sym­met­ri­cal struc­tures, al­though with many vari­a­tions. As the old say­ing goes, no two snowflakes are ever the same. Dur­ing its de­scent sev­eral fac­tors af­fect the even­tual shape and size of a snowflake. Hu­mid­ity, wind, tem­per­a­ture and even the vari­ant of hy­dro­gen atoms present – all these fac­tors have an ef­fect on flake for­ma­tion. Un­less each in­di­vid­ual water mol­e­cule and form­ing crys­tals are ex­posed to the ex­act same con­di­tions they will not form in the ex­act same way, which ex­plains the count­less va­ri­eties of snowflake.

Stel­lar snowflakes form when water mol­e­cules col­lect at the out­er­most points of the snowflake, creat­ing branches

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