An atom is the smallest unit into which
matter can be divided without the release of electrically
charged particles. It also is the smallest unit of matter
that has the characteristic properties of a chemical element.
As such, the atom is the basic building block of chemistry.
Atomic number is the number of protons
(positively charged particles) in an atom's nucleus. All
the atoms of an element have the same number of protons.
Some forms of an element have a different number of neutrons
than do other forms of that element.
Isotopes are atoms of the same element
having different mass number. Isotopes have different number
of neutrons in their nucleus.
Atomic weight of an element is its weight
relative to 1/12 th of that of the isotope of carbon whose
atomic mass number is 12. This carbon isotope has been assigned
an atomic weight of exactly 12 atomic mass units (a.m.u.)
Ion is an atom or a group of atoms that
has an electric charge. Atoms and molecules become charged
if they gain or lose electrons. Each atom has a cloud of
negatively charged electrons around a small, heavy nucleus.
The nucleus contains positively charged protons. If the
number of electrons around the nucleus equals the number
of protons inside the nucleus, the atom is neutral. The
process of removing electrons from atoms or molecules to
produce positive ions is called ionization. The electrons
removed may then join other atoms or groups of atoms, causing
them to become negative ions. The amount of electric charge
an ion has is determined by the number of electrons gained
or lost by the atom or molecule.
Many common substances contain ions. For example, table
salt consists of equal numbers of positively charged sodium
ions and negatively charged chloride ions. In forming table
salt from the elements sodium and chlorine, each sodium
atom loses an electron and becomes a positive sodium ion.
Likewise, each chlorine atom gains an electron and becomes
a negative chloride ion. Seawater contains many kinds of
ions. The most common ones, in order of their amount, are
chloride, sodium, sulphate, and magnesium. The earth's atmosphere
also contains ions. They are concentrated in a layer called
Ions in solids usually fit together in a regular, repeating,
three-dimensional arrangement. Such a substance is called
an ionic crystal. For example, sodium ions alternate with
chloride ions in a crystal of table salt. Ions in an ionic
crystal are held together by electrostatic attraction, the
attraction between opposite charges.
Ions in liquids can migrate throughout the liquid. In a
solution, each ion attracts one or more molecules of the
solvent (dissolving liquid). Ionic crystals, such as sodium
chloride, usually dissolve only in solvents that contain
polar molecules. Polar molecules have a positive end and
a negative end. Each ion on the surface of the crystal attracts
the oppositely charged end of polar molecules. This attraction
weakens the attraction between ions in the crystal. The
ions then break away from the crystal and enter the solution,
combined with polar molecules. Ions combined with polar
molecules of the solvent are said to be solvated.
Ions in gases are too far apart at normal pressures to
attract each other strongly. As a result, single ions in
a gas may drift for a long time before they combine with
other ions. A mixture consisting of ionized gas and electrons
is called a plasma.
Differences between Ions and Atoms
Besides They differ in physical properties. For example Copper
ion is bluish in colour wheras copper atom is pinkish red.
1. Electrically charged.
2. Capable of free existence
2. May/Maynot be capable of free
Sodium Atom |
Many particles in space are ions. Some of these ions are
trapped by the earth's magnetic field and make up part of
the Van Allen belt.
Behaviour of ions: All ionic solids and
liquids, and most ionized gases, are electrically neutral.
The total charge of all their positive ions equals the total
charge of all their negative ions. This general rule also
applies to all other kinds of matter, and is called the
principle of electroneutrality.
Like neutral atoms and molecules, ions in a liquid or a
gas are constantly moving. Each one changes its direction
of motion billions of times each second because of collisions
and the forces exerted on it by other particles. After each
change in direction, an ion usually is no more likely to
be moving in one direction than in any other. Such random
motion is called Brownian motion. When two oppositely charged
electrodes are placed in a liquid or gas, each ion loses
part of its random motion and starts to drift toward one
of the electrodes. Negative ions drift toward the anode
(negative electrode) and are called anions. Positive ions
drift toward the cathode (positive electrode) and are called
cations. The movement of the charges carried by the drifting
ions makes up an electric current.
The ability of a solution to conduct electric current depends
on the concentration of ions in the solution. For example,
drinking water drawn from a typical tap contains few ions
and therefore is a poor conductor of current. But seawater,
with significant amounts of dissolved sodium chloride, magnesium
sulphate, and other ionic compounds, is a good conductor.
Producing ions: Any process that can add
or remove electrons from an atom or a molecule can produce
ions. Radiation and chemical reactions are such processes.
Radiation can increase the energy of the electrons in an
atom or a group of atoms. If this energy is increased enough,
one or more electrons can overcome the attraction of the
nucleus and escape from the atom. The loss of negative charge
results in the atom becoming a positive ion. Radiation that
can produce ions includes light, X rays, gamma rays, atomic
nuclei, subnuclear particles, and electrons.
High-energy radiation absorbed by plant or animal tissues
produces unnatural ions in the tissues. These ions become
involved in potentially harmful chemical reactions. In human
beings and other animals, the symptoms of these reactions
are called radiation sickness. The amount of radiation absorbed
by tissues is measured in units called roentgens or radiation
units (rads). One roentgen or rad corresponds to the formation
of more than a billion pairs of ions in the tissues. Death
is likely to occur if a person's body absorbs 500 roentgens
of radiation over a short period.
Ions are formed in a chemical reaction if molecules split
into electrically charged parts. For example, molecules
of hydrogen chloride (HCl) split when added to water. They
form positive hydrogen ions and negative chloride ions.
1. Formed by transfer of electrons.
2. Atoms are held together by strong electrostatic
forces of attraction. True chemical bonding is lacking.
3. High melting/boiling points.
4.Generally soluble in water and insoluble in organic
5. Fast speed of chemical reactions.
1.Formed by sharing of electrons.
2.A true rigid chemical bond exists between the atoms.
3.Low melting/Boiling points.
4.Generally insoluble in water but soluble in organic
solvents like Carbon tetrachloride.
5.Slow speed of chemical reactions.