Question

explain the difference between a neutral atom an isotope and an ion. use a specific element and explain the three possible forms of the element

Answer 1

Atoms are made of three types of sub atomic particles, they are the electrons, neutrons and protons. 
Electrons are negatively charged and they are orbiting around the nucleus in energy shells. Protons are positively charged , neutrons are neutral and have no charge, both neutrons and protons are located in the nucleus.
In atoms the atomic number is the number of protons the atom consists of and mass number is the sum of protons and neutrons in the nucleus.

In a neutral atom, the number of electrons and protons are equal, this means that the number of negative charges and positive charges are equal.
lets take Cl as an example. Its atomic number is 17.
number of protons and charge - +17
number of electrons with charge - -17
overall chage sum of the charges = + 17 -17 = 0
therefore no charge, hence its neutral.

isotopes are the same element having the same atomic number but different mass numbers.
³⁵Cl -protons - 17 electrons - 17 neutrons 18 neutrons
³⁷Cl - protons - 17 electrons - 17 neutrons 20 neutrons
Both are same element with same number of protons and electrons, however the number of neutrons are different.
Since protons + neutrons = mass of the atom
and when the number of neutrons change - mass of atoms too change
hence, ³⁵Cl has a mass of 35 and ³⁷Cl  has a mass of 37. both of these are called isotopes

Ions are atoms which have either gained an electron or lost an electron and are charged.
metals such as K(atomic number - 19) have one valence electron in the outer shell, to gain the configuration of a complete octet, K needs to give out this one electron.
then number of electrons 19-1 = -18
number of protons                    = +19
overall charge                           = +1
K is one electron less therefore becomes positively charged and called a cation- K⁺
if we take Cl, atomic number - 17
Cl has 7 valence electrons, to gain a complete octet in the outer shell it needs to have  8 electrons, therefore it needs one more electron to complete the outer shell.
Therefore it gains one electron,
number of electrons  17+1  = -18
number of protons               = 17
overall charge                      = -1
with an extra electron, Cl becomes negatively charged, a negative ion called anion - Cl⁻