Lithium has an atomic number of 3 and a mass number of 7.
Which of the following is the number of protons in a lithium atom?
A.
7.
B.
3.
C.
12.
D.
4.
The Correct Answer and Explanation is:
The correct answer is B. 3.
Explanation:
Lithium (Li) is an element with the atomic number 3 and the mass number 7. To understand the number of protons in a lithium atom, we need to distinguish between several key concepts: atomic number, mass number, protons, neutrons, and electrons.
- Atomic Number: The atomic number of an element is the number of protons found in the nucleus of its atom. For lithium, this number is 3. This means that every lithium atom has exactly 3 protons in its nucleus.
- Mass Number: The mass number is the total number of protons and neutrons in the nucleus of an atom. For lithium, the mass number is 7. Therefore, the mass number represents the sum of protons and neutrons.
- Neutrons: To find the number of neutrons in a lithium atom, subtract the atomic number from the mass number. The calculation is as follows:
[
\text{Number of Neutrons} = \text{Mass Number} – \text{Atomic Number}
]
For lithium:
[
\text{Number of Neutrons} = 7 – 3 = 4
]
So, lithium has 4 neutrons. - Electrons: In a neutral atom, the number of protons equals the number of electrons. Therefore, lithium also has 3 electrons.
To summarize, the number of protons in an atom of lithium is directly given by its atomic number. Thus, lithium, with an atomic number of 3, contains 3 protons. This fundamental property helps define the identity of the element and distinguishes it from others in the periodic table. The mass number and the number of neutrons are important for understanding the isotope and the atomic mass, but they do not change the number of protons, which remains constant for each element.