The isotopes of an element differ in
(1) The number of neutrons in the nucleus
(2) The charge on the nucleus
(3) The number of extra-nuclear electrons
( 4) Both the nuclear charge and the number of extranuclear electrons.

Explanation

To solve the question regarding the differences in isotopes of an element, we will analyze each of the provided options step by step.

### Step 1: Understanding Isotopes
Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This means that while they share the same atomic number (which is determined by the number of protons), they differ in their mass number (which is the sum of protons and neutrons).

**Hint:** Remember that isotopes have the same atomic number but different mass numbers.

### Step 2: Analyzing the Options
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