Tuesday, August 17, 2010

Compounds and Molecules as Aggregates of Atoms

In addition to being an element, a substance can also be a compound. Compound is composed of atoms of two or more elements chemically united in fixed proportion. For example water has two H atoms and one O atom. This composition does not change that is a compound cannot be separated into its pure components, the atoms of the elements present, except by chemical methods. When atoms interact chemically, electrical forces called chemical bonds hold the atoms together and form molecules like H2, N2, H2O, CO2 etc.

Chemical Bonds:
There are three types of chemical bonds, Covalent, Hydrogen and Ionic which are responsible in making all types of molecules whether smaller or larger, organic or inorganic in nature.

Covalent bonds sharing electron pairs:
When atoms share outer shell electrons with other atoms, the chemical bond that forms is covalent bond. In covalent bonding electrons are always shared in pairs. When a pair of electron is shared from each atom, single bond forms as in hydrogen [H – H] molecule. When two pairs of electrons are shared, double bond forms as in oxygen [O O] molecule and when three pairs of electrons are shared, a triple bond forms e.g. nitrogen [N N] molecule. In a molecule like H2, the electrons spend as much time orbiting one nucleus as the other. The distribution of charges is symmetrical and the bond is called non polar covalent bond. Due to this equal sharing, the molecule is electrically balanced and the molecule as a whole is neutral.

Hydrogen Bonds:
In molecules in which hydrogen combines with certain other atoms like O, N or Fe, Hydrogen electron is drawn toward another atom leaving a proton behind. As a result, Hydrogen atom gains slight positive charge. Remaining proton is attracted to negatively charged atoms of oxygen in nearby molecules. When this happens a weak attraction called Hydrogen bond, forms. Hydrogen atom in one water molecule forms hydrogen bond with oxygen atom in another water molecule and so forth until many molecules bond.

Hydrogen bonds and water Droplets:
Hydrogen bonds form when oxygen atoms of different water molecules weakly join by the attraction of electronegative oxygen for the positively charged hydrogen. Because of the arrangement of electron orbital and the bonding angles between oxygen and hydrogen, the molecule as a whole is polar as it carries a slight negative charge at one end and slight positive charge at the other end. This polarity is responsible for many important properties of water. In other molecules such as H2O where two hydrogen atoms combine with one oxygen atom the electrons spend more time orbiting oxygen nucleus than Hydrogen nuclei. As the electrical charge from the cloud of moving electrons is asymmetrical, the bond is called Polar covalent bond. Such a bond leaves oxygen atom with slightly negative charge and Hydrogen with slightly positive charge, even though the entire molecule is electrically neutral. Shape of water molecule reflects this polarity; rather than the linear arrangement H – O – H, two hydrogen are at one end, a bit like the corners of a triangle. This shape and the polarity can lead to the formation of another kind of chemical bond – the hydrogen bond.

Ionic Bonds: When an atom either gains or loses electrons, it acquires electrical charge and is called an ion. When atom loses one or more electrons, it becomes positively charged because more positively charged protons are now in the nucleus than negatively charged electrons surrounding the nucleus. This positive charge is shown as ‘plus’ sign and is known as cation e.g. Sodium (Na+), Potassium (K+), Hydrogen (H+), Calcium (Ca2+) and iron (Fe3+).
When atom gains one more electrons it becomes negatively charged and is shown as one or more ‘minus’ signs and such charged particle is called anion e.g. chlorine (Cl-), Hydroxyl (OH-), Bicarbonate (HCO3-), Sulphate (So42-), Phosphate (Po43-) and Carboxyl (CO3-).

Ionic Bonds: Ionic bonds form when an atom or group of atoms develop on electrical charge due to loss or gain of electrons. Due to opposite charges these atoms or group of atoms attract each other. A molecule of sodium chloride, the common table salt is formed charge when sodium atom and chlorine atom come together, sodium atom donates an electron to chlorine atom. This electron transfer changes the balance between protons and electrons in each of the two atoms. Sodium atom ends up with one more proton than it has electrons and chlorine atom with one more electron than it has Protons. Sodium atom is left with net charge of +1 (Na+) and net charge of chlorine atoms is -1 (Cl-). These opposite charges attract each other and form ionic bonds.

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