When do they emit α, β+ or β- particle to become more stable?

[bsmm11]

When do they emit α, β+ or β- particle to become more stable?

From what I've seen in the textbook, β- is emitted when there is an excess neutrons and β+ is emitted when there is a short of neutrons. Then when does an α-particle emitted?

 

[technician]

An important graph to use in this explanation is Number of neutrons against number of protons for the nuclides. Are you familiar with it?
For light nuclei the number of neutrons is more or less equal to the number of protons and therefore forlight nuclei the line is more ore less at 45 degrees. The line that fits the points is known as the 'line of stable nuclides', nuclides on this 45degree line are stable.
Nuclides plotted away from this line have increased probability of being unstable and therefore radioactive. The 3 isotopes of Carbon... C12, C14 and C10 are perfect examples to illustrate stability (C12) beta- decay (C14) due to too many neutrons and beta+ decay (C10) due to too many protons... you have recognised this.
Higher up the curve more neutrons need to be in the nucleus as the number of protons increases and this means that the curve tends to become steeper.
It is now not so easy to determine how nuclides away from the line will behave but in general a heavy nuclide , by emitting 2 neutrons and 2 protons (an alpha particle) will become more stable ( the resulting nuclide will be 'closer to the line'
Not a perfect explanation but, depending on your knowledge, I hope this helps.