Answer:
3 electrons
Step-by-step explanation:
Lithium is the third element of the periodic table, so its atomic number is 3:
Z = 3
This means that an atom of lithium contains 3 protons. Moreover, for a neutral atom the number of protons in the nucleus is equal to the number of electrons: this means that an atom of lithium contains 3 electrons as well.
In this specific case, we have an isotope of Lithium-6 (6Li), which has a mass number of 6:
A = 6
This means that the atom contains 6 protons+neutrons in the nucleus, so it contains 6-3 = 3 neutrons.