Atoms consist of electrons surrounding a nucleus that contains protons and neutrons.
Neutrons are neutral, but protons and electrons are electrically charged. 
Protons have a relative charge of +1, while electrons have a relative charge of -1.
The number of protons in an atom is called its atomic number. In the periodic table atoms are arranged 
in atomic number order.

The structure of an atom:
At the center of an atom is a nucleus containing protons and neutronsElectrons are arranged around 
the nucleus in energy levels or shells.

    The number of protons in an atom is called its atomic number - 
also called the proton number. 

Mass number is the total number of protons and neutrons in 
the nucleus of the atom.

           The Ion:

When an atom loses an electron(or more), it becomes an Ion with positive charge.

Electrons can be lost because 
Ionizing radiation(you will study this later in the chapter) forces the 

electron away from the atom.

A carbon atom  
 The Carbon atom in the diagram has 6 Neutrons, 6 Protons, 6 Electrons 
When an atom is hit by ionizing radiation it loses an electron.

This turns the atom into a Positively charged ion as it has more protons than electrons.


The discovery of radioactivity

Radioactivity was discovered accidentally by Becquerel. he left his sample of uranium in a closed drawer along with the photographic plate. He was surprised to find that the crystal had left a clear, strong image on the photographic plate.
How could this be? There was no source of energy to produce the image! What Becquerel had discovered was that a piece of mineral which contained uranium could  would fog a photographic plate even when it was wrapped up, so no light fell on the photographic plat. 
What he had discovered was radioactivity! He attributed this phenomenon to spontaneous emission from Uranium.
Becquerel had discovered a way of revealing the presence of invisible radiation, using photographic film. This model is still used today. 
 For a quicker measurement of  radiation we can use Geiger counter. When the radiation enters the tube of this counter it gives an electric pulse and transfer it to a click or beep for each pulse.  

Why are some atoms radioactive while others are not?

Radiation is emitted by the nucleus of an atom. We say that the nucleus is unstable.An unstable nucleus emits radiation in an attempt to become more stable. This is known as radioactive decay.

What are the types of radiation?

There are three types of radiation emitted by the radioactive substances. These are named:
1- An alpha particle (
α- Particle):
Is made up of 2 protons and 2 neutrons.

(this is the same as the nucleus of a helium atom)  

-It is positively charged because it contains protons..

2- A beta particle
(β- particle):

It is an electron.

It is not one of the electrons that orbit the nucleus but it comes from the nucleus.
-It is negatively  charged and its mass is much less than that of  an alpha particle.

3- A gamma ray (ɣ- ray):

Is a form of electromagnetic radiation. We can think of it as a wave with a very short wavelength.

Some materials are radioactive because the nucleus of each atom is unstable and can decay (split up) by giving out nuclear radiation in the form of alpha particles, beta particles or gamma rays. The nuclear radiation given off can be detected using a Geiger counter. 

some unstable nuclei with the atomic number greater than 83 emit alpha particles to be stable.

some unstable nuclei with a great ratio of neutrons emit beta particles to be stable.

some unstable nuclei with a high energy emit gamma rays to be stable without any change in 

the  protons or neutrons number.

Gamma rays is usually emitted at the same time as the alpha or beta, but it may emitted some time later.