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2. Inside the nucleus page 9
Neutrons and isotopes
The number of protons in a nucleus is the same for all the atoms of a particular element. It corresponds to the atomic number, Z, of that element. For example, if a nucleus contains 3 protons, then its atom must be lithium (atomic number 3).

name symbol Z neutrons A
lithium-6 isotope 3 3 6
lithium-7 isotope 3 4 7
lithium-8 isotope 3 5 8
Table 3. Three of the isotopes of lithium. The atomic number is always 3. However, the atomic mass number, A, changes as the number of neutrons changes.
Number of neutrons
However, the number of neutrons in a nucleus does not affect the element. Nuclei of lithium can have 3, 4 or 5 neutrons. We call these different isotopes of lithium.

Since different isotopes of an element have different numbers of neutrons (but always the same number of protons) they have different mass numbers. The isotopes of lithium have mass numbers 6, 7 and 8. We call them lithium-6, lithium-7 and lithium-8 respectively.

Radioactivity
Some isotopes are unstable and are therefore radioactive. They emit radiation to become more stable.

Lithium-6 and lithium-7 are both stable isotopes of lithium. However, lithium-8 is radioactive and decays by b radiation.

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Question 8
Look at the symbols below. In each case, decide on the missing information and type it into the space.
a) i. number of protons
ii. number of particles in the nucleus
iii. number of neutrons
iv. number of electrons
(in neutral atom)
b) i. number of protons
ii. number of neutrons