Introduction:
In a mass spectrum, the abundance pattern is a representation of the relative intensities of different isotopes of an element present in a compound. The abundance pattern helps in determining the number of isotopes and their respective abundances in the compound. In this case, the abundance pattern is given as 1:3:3:1.

Explanation:
To determine the number of bromine (Br) atoms in the compound, we need to analyze the abundance pattern and relate it to the isotopes of bromine.

Step 1: Identify the Isotopes of Bromine:
Bromine has two isotopes with atomic masses 79 and 81. These isotopes exist in nature in a certain proportion.

Step 2: Interpret the Abundance Pattern:
The abundance pattern given as 1:3:3:1 suggests that there are four isotopes of bromine present in the compound. The relative intensities of these isotopes can be expressed as follows:

- Isotope 79 with abundance 1
- Isotope 81 with abundance 3

Step 3: Calculate the Number of Br Atoms:
To calculate the number of bromine atoms, we need to consider the relative abundances of each isotope.

Assuming the compound contains x bromine atoms, the contribution of each isotope can be calculated as follows:

- Contribution of isotope 79: 79 * 1 * x
- Contribution of isotope 81: 81 * 3 * x

The total contribution of all isotopes should be equal to the molecular weight of the compound.

Step 4: Equate the Contributions:
The molecular weight of the compound can be calculated by multiplying the atomic mass of each isotope with its respective abundance and summing them up:

Molecular weight = (79 * 1 * x) + (81 * 3 * x)

Step 5: Solve for x:
Since the molecular weight of the compound is not provided, we cannot directly determine the value of x. However, if the molecular weight of the compound is given, we can solve the equation to find the value of x, which represents the number of bromine atoms in the compound.

Conclusion:
In order to determine the exact number of bromine atoms in the compound, we need to know the molecular weight of the compound. The abundance pattern provided only helps in identifying the relative intensities of different isotopes of bromine.