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THE WATER MOLECULE Notes Page 15 of INB GET TO KNOW H 20 - 2 parts Hydrogen - 1 part Oxygen - Hydrogens have a positive charge - Oxygen has a negative charge ATOMS AND ELECTRON SHELLS - Atoms are made up of protons, neutrons, and electrons - Protons and neutrons can be found in the nucleus - Electrons can be found around the nucleus, in shells - Each shell layer has a different number of electrons it can hold - In order to be stable (happy), the atom must have a full outermost shell. ELECTRON DOT STRUCTURES: H AND O Hydrogen has 1 electron floating around its nucleus, but it needs 2 to be stable. - Oxygen has 8 electrons floating around its nucleus. - 6 of those electrons are in its outermost electron shell. It needs 8 in its outermost electron shell to be stable. COVALENT BONDS - Hydrogen and Oxygen connect with one another through a covalent bond. This means they share their electrons to make each atom have a full outermost shell and be stable (happy) POLAR MOLECULE - The angle of the water molecule’s bonds is approximately 105 degrees – this makes water a “bent molecule” or “angular molecule.” - Because of where the electrons lay within the bent molecule, water is polar. This means it has a negative side and a positive side. - All of the electrons (which have a negative charge) are gathered around the oxygen, so the oxygen side is negative. - The hydrogen side is free of electrons, so it is positive. WHY DOES IT MATTER? - Because water has a positive end and a negative end, it is drawn to other water molecules. - This means that water sticks together through weak positive-negative connections. These connections are called hydrogen bonds. - It also means water sticks to other molecules and can pull them apart. This is why many things dissolve so easily in water – it is a universal solvent (a liquid things called solutes dissolve in) COHESION, ADHESION, AND SOLUBILITY Cohesion – the ability of water molecules to stick to one another Ex. water in the glass Adhesion – the ability of water molecules to stick to other molecules Ex. condensation on the glass Solubility – the ability of water (or any other solvent) to dissolve another material (called a solute) Ex. sugar dissolved in the ice tea SURFACE TENSION - Water molecules want to cling to each other (cohesion). - At the surface, however, there are fewer water molecules to cling to since there is air above. - This results in a stronger bond between those molecules that actually do come in contact with one another, and a layer of strongly bonded water This surface layer is held together by surface tension and creates a considerable barrier between the atmosphere and the water. - Other than mercury, water has the greatest surface tension of any liquid. BUOYANCY IS DIFFERENT THAN SURFACE TENSION - Buoyancy or the ability of something to float in water is different than surface tension. - Buoyancy results from the upward force of the water opposing the downward force of the weight of the object attempting to float. - You will learn more about buoyancy in Marine Science II. - Hint: it has to do with displacing water! KEY POINTS: THE WATER MOLECULE - Water is a covalent molecule made of two hydrogens and one oxygen sharing electrons. - Because of the uneven distribution of the electrons, water molecules have a negative side and a positive side, making water a polar molecule. - Polar molecules stick to one another through hydrogen bonding. When water sticks to itself, it is called cohesion. Cohesion at the surface creates surface tension, which allows some things to float at the top of the water. - Polar molecules also pull apart other molecules. This is why salt, sugar, and other materials dissolve so well in water. Water is called a universal solvent.