Structure of an Atom
All the atoms that exist in the world consist of a nucleus (protons/neutrons) that is surrounded by one or more electrons. The electrons form a ‘cloud’ that surrounds the nucleus. Within this cloud are layers or shells of electrons. The first electron shell contains one orbit which can accommodate up to two electrons. When the shell is complete (contains two electrons), it is stable and less likely to react. The second electron shell can contain up to eight electrons – with four different orbits of two electrons each. Like the first shell, it is most stable when it is complete – in this case with eight electrons (octet rule). The number of electrons in the outer shell (valence shell) determine the chemical reactivity of atoms. Atoms are only “happy” if they have a full outer electron shell and they will try to gain or lose electrons in order to have a full outer shell. The number of protons (atomic number) determines what element an atom is. The number and location of electrons determines how an atom behaves.
As you can see from the diagram below, the hydrogen atom (on the left), being the first element on the periodic table, contains only one electron. This leaves one vacancy in its first and only electron shell. The proton (red) carries a positive charge and the electron (blue) carries a negative charge. Because the positive and negative charges cancel each other out, the entire hydrogen atom is neutral in charge. The oxygen atom (on the right) has a nucleus consisting of eight positively-charged protons (8+) and eight uncharged neutrons (8n) surrounded by a cloud of eight negatively-charged electrons spinning around its nucleus in two different shells. It has two electrons in its inner shell and six electrons in its outer shell. In order for the outer shell of the oxygen atom to be completed, it needs two more electrons for a total of eight.
Chemical Bonds
Atoms can combine to form molecules in order to have a stable outer (valence) shell. Remember that atoms follow the octet rule. These 2 types of bonds are ionic and covalent.
Ionic Bonds
Charged atoms, or ions, can form when atoms lose or gain electrons – remember that atoms will gain or lose electrons in order to have a full outer shell (octet rule). If an atom starts off with 17 protons (chlorine) and 17 electrons, the positive and negative charges are balanced out. However, this atom only has 7 electrons in its outer shell, so it wants 1 more electron to have 8 and be happy. But when the atom gains an extra negative electron, it now has 18 negative electrons and 17 positive protons. Therefore its overall charge is -1. If an atom has one electron in its outer shell (such as sodium), it will usually give that electron away and use the next lower shell as a “full” outer shell. When it gives a negative electron away, it becomes a positively charged ion. Positive and negative ions are attracted to one another and bond together in ionic bonds. For example table salt (sodium chloride) is a dry solid composed of atoms connected by ionic bonds.
Covalent Bonds
A covalent bond results when two atoms share electrons, thereby completing their valence shells. As seen in the next section, the water molecule is formed by covalent bonds between the oxygen and hydrogen atoms.
Structure of the Water Molecule structure of water
Water is a tiny bent molecule with the molecular formula H2O, consisting of two light hydrogen atoms attached to a 16-fold heavier oxygen atom. As seen in the diagram below, on forming the water molecule, the ten electrons pair up into five ‘orbitals‘, one pair closely associated with the oxygen atom in its inner shell, two pairs (lone pairs) associated with the oxygen atom as ‘outer’ electrons and two pairs forming each of the two identical O-H covalent bonds.
structure of water
structure of water
Each water molecule is electrically neutral but polar, where the center of positive and negative charges lie in different places. Due to the relatively large positive charge from the eight protons in the oxygen atom’s nucleus (8+) and the closeness of its electrons, the oxygen atom has a stronger attraction to all the electrons (i.e. is much more electronegative) than the hydrogen atoms with one proton (1+). This results in a charge transfer from the hydrogen atoms towards the oxygen atom and, hence, the polarity of the water molecule, making the oxygen end of the molecule partially negative and the hydrogen end partially positive.
Many of the physical and chemical properties of water are due to its structure. The atoms in the water molecule are arranged with the two HO bonds at an angle of about 105° rather than on directly opposite sides of the oxygen atom. The asymmetrical shape of the molecule arises from a tendency of the four electron pairs in the valence shell of oxygen to arrange themselves symmetrically at the vertices of a tetrahedron around the oxygen nucleus. The two pairs associated with covalent bonds holding the hydrogen atoms are drawn together slightly, resulting in the angle of 105° between these bonds.
structure of water
Hydrogen Bonding
The electric dipole gives rise to attractions between neighboring opposite ends of water molecules, with each oxygen being able to attract two nearby hydrogen atoms of two other water molecules. Such hydrogen bonding, as it is called, has also been observed in other hydrogen compounds. Although considerably weaker than the covalent bonds holding the water molecule together, hydrogen bonding is strong enough to keep water liquid at ordinary temperatures, even though its low molecular weight would normally tend to make it a gas at such temperatures.
Various other properties of water, such as its high specific heat, are due to these hydrogen bonds. As the temperature of water is lowered, clusters of molecules form through hydrogen bonding (as seen in diagram represented by dotted lines), with each molecule being linked to others by up to four hydrogen bonds and each oxygen atom tending to surround itself with four hydrogen atoms in a tetrahedral arrangement. Hexagonal rings of oxygen atoms are formed in this way, with alternate atoms in either a higher or lower plane than their neighbors to create a three-dimensional structure.
Because water molecules form these hydrogen bonds with neighboring water molecules, water has some very unusual anomalies that can be read about in “Properties of Water“.