Snowflakes
When you hear the word winter, snow is one of the first things that come to mind. For those of you who live in colder climates I’m sure you’re thinking of treacherous driving conditions and your back aching after shoveling the driveway. But when you see snowflakes up close, you realize just how beautiful snow can be (see the photo above)! Snowflakes are tiny crystals of ice that form when the atmospheric temperatures dip below the freezing point for water (32°F/0°C). Snowflakes have intricate designs and patterns that reflect sunlight giving them a shimmery appearance. While snow may have lost its luster for those who dream of escaping winter every year, I’m about to tell you why snowflakes are so cool; no pun intended.
Snowflakes start as molecules of water high up in the atmosphere. To understand how snowflakes form let’s first look at the molecular formula for water: H₂O. In the covalent bond between Oxygen (O) and Hydrogen (H), Oxygen hogs the shared electrons between the two elements. This gives Oxygen a negative charge and consequently gives both Hydrogen atoms a positive charge. Because each element has a charge, individual water molecules are attracted to one another. A Hydrogen from one water molecule is attracted to an Oxygen from another water molecule and so on and so forth. This kind of attraction is called Hydrogen bonding.
Hydrogen bonding is the force that keeps water molecules in close proximity with each other and is why water exists as a liquid. In the liquid phase, water molecules move quickly and have lots of kinetic energy which allow them to flow past each other. At colder temperatures, water molecules lose their kinetic energy and the hydrogen bonding forces are strong enough to lock the water molecules in place. The water molecules will begin to arrange themselves in a stationary, 3D structure known as ice. This phase change from a liquid to a solid is given the fancy name nucleation. As water molecules begin to freeze, tiny particulate in the air such as pollen or dust will act as a nucleation site for a snowflake. Water molecules will begin to form crystals of ice using the particulate as a kind of scaffolding, which eventually becomes the snowflake we observe.
Now that we know how snowflakes begin to form, let’s look at how snowflakes get their shape. When water begins to freeze, the molecules will orient themselves to maximize attractive forces (Hydrogen bonding) and minimize repulsion forces found between two like-charged atoms (i.e., two Oxygen atoms). To achieve this, six water molecules form a hexagon with an Oxygen atom at each vertex as shown below.
Image sourced from: http://mubirupepu.blogspot.com/2017/04/structure-of-water-and-ice.html
Water molecules will continue to arrange themselves into countless different patterns while attempting to optimize the attractive and repulsive forces at play. Furthermore, depending on environmental conditions like the temperature, relative humidity and windspeed, water molecules will freeze at different rates and therefore arrange themselves in different patterns. This is what leads to the beautiful diversity of snowflakes and why people say “No two snowflakes are the same.”