Isotopes (Edexcel GCSE Physics: Combined Science)

• Although the number of protons in a particular element is always the same, the number of neutrons can be different
• Isotopes are atoms of the same element that have an equal number of protons but a different number of neutrons
• In the diagram below are three isotopes of Hydrogen:

Hydrogen has three isotopes, each with a different number of neutrons

• Isotopes occur naturally, but some are more rare than others
• For example, about 2 in every 10,000 Hydrogen atoms is Deuterium
  • Tritium is even more rare (about 1 in every billion billion hydrogen atoms)

Differences Between Isotopes

• The number of neutrons in an atom does not affect the chemical properties of an atom, such as its charge, but only its mass
  • This is because neutrons have no charge but do have mass

• The charge of the nucleus of a particular element is always the same
• In the periodic table, the mass number of Chlorine is often given as 35.5

This section of a periodic table shows Chlorine as having a mass number of 35.5, but other elements have an integer mass number

• The mass number of chlorine is given as 35.5 because it has 2 isotopes, one with a mass number of 35 and the other with a mass number of 37
• Chlorine-35 is about three times more abundant than chlorine-37, so the given mass number of chlorine is closer to 35 than 37
• The number of electrons and protons in different isotopes remains the same
• Some isotopes are unstable as they have an imbalance of protons and neutrons

ANSWER:  B

Step 1: Properties of isotopes

    Isotopes are nuclei with the same number of protons but different number of neutrons

    The nucleon number is the sum of the protons and neutron

    Therefore, an isotope has a different nucleon number too

Step 2: Calculate protons in the  first nucleus

Nucleon number: 37

Neutrons: 20

Protons = 37 − 20 = 17

Step 3: Calculate protons in the second nucleus

Nucleon number: 35

Neutrons: 18

Protons = 35 − 18 = 17

Step 4: Conclusion

Therefore, they have the same number of protons but different numbers of neutrons and are isotopes of each other

The correct answer is therefore option B