Nursing Entrance Exam

molecule and farther from the hydrogen. An atom’s proclivity to pull electrons towards itself is called its electronegativity, which creates covalent bonds that are polar. Because of this connection, these polar covalent bonds are the strongest molecular bond, which is why molecules like water are so prevalent in our solar system (and also likely throughout the galaxy). An interesting aspect of polar covalent bonds is the hydrogen bond. When a hydrogen atom bonds with another electronegative atom, the newly created molecule develops an intense polar attraction to all other electronegative atoms. This attraction works almost like a magnet – one end of the molecule exhibits a positive charge (due to the effects of the polar covalent bonds pulling all the electrons towards one end of the molecule) and the other end exhibits a negative charge. This phenomenon is responsible for, among other things, the way water molecules stick to one another so readily. This is the reason why a glass of water can be filled to a millimeter or so above the rim before it spills. Hydrogen bonds are also responsible for how hydrophilic and hydrophobic molecules react to being mixed with water. Hydrophilic molecules are molecules like NaCl (salt) that exhibit their own strong charge for reasons we will discuss in a moment. The charged salt molecules mix eagerly with the charged water molecules due to the extra pull of the hydrogen bond. Conversely, hydrophobic molecules, such as oil, will not mix with water because they are neutrally charged and do not like charged molecules. This is the reason you have to shake up an oil-based salad dressing each time you use it. The oil and the water never truly mix and given only a short time, they will separate. An ionic bond is a different type of bond between atoms, as they only occur between ions, which are either positively or negatively charged due to having an unequal number of protons and electrons. Ionic bonds always occur between metals and non-metals, such as the gas chlorine (Cl) and the alkaline metal sodium (Na). In their normal states, neither of these elements are ions, but when they approach one another, the sodium gives the chlorine one of its electrons forming Cl- and Na+ ions, which subsequently become attracted to one another. Since no electrons are actually lost, the molecule still technically has a neutral charge; it is only the atoms that are charged. In ionic bonds, it is always the metal which gives its electron to the non-metal. Additionally, in a diluted or liquid form, molecules created in this manner will always conduct electricity, which is why saltwater is a great conductor. Ions and salts An ion is a charged species, an atom or a molecule, that has lost or gained one or more electrons. Positively charged cations (e.g. sodium cation Na+) and negatively charged anions (e.g. chloride Cl−) can form a crystalline lattice of neutral salts (e.g. sodium chloride NaCl). Examples of polyatomic ions that do not split during acid-base reactions are hydroxide (OH−) and phosphate (PO43−). Ions in the gaseous phase are often known as plasma.

Achieve

Page 163

of 175

© 2018

Made with FlippingBook - professional solution for displaying marketing and sales documents online