Final answer:
Lithium is the third element in the periodic table with an atomic number of 3. It is commonly used in batteries and in applications like cardiac pacemakers.
Step-by-step explanation:
Lithium (Li) is the third element in the periodic table with an atomic number of 3, meaning it has 3 protons and 3 electrons in a neutral atom. The first two electrons are found in the first energy level, while the third electron is found in the second energy level. Lithium is commonly used in batteries, such as lithium-ion batteries. It is also used in other applications like cardiac pacemakers and medical implants.