The atomic mass of a lithium nucleus can be calculated by adding the number of protons and neutrons in the nucleus. In this case, there are 3 protons and 4 neutrons, so the atomic mass is:

$$3 \text{ protons} + 4 \text{ neutrons} = 7 \text{ atomic mass units}$$

Therefore, the atomic mass of a lithium nucleus is 7 atomic mass units.