Lithium has an atomic number of 3 and a mass number of 7. Which of the following is the number of neutrons in a lithium atom?
A. 4
B. 7
C. 12
D. 3
The atom is identified by the atomic number in the periodic table. The atomic number is equal to the number of protons and mass number is the sum of the number of protons and neutrons in the nucleus of an atom. Thus, to find how many neutrons are in lithium atom we use the formula below.
Mass number = number of protons + number of neutrons.
Letting n be number of neutrons in the lithium atom, we have
7=3+n
Rearranging the above equation
3+n=7
n=7-3
n=4
Thus, lithium atom has 4 neutrons.
Therefore, the Correct Answer is A.
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Q #1: Which of these infectious agents do not have nucleic acid?
A. viroids
B. viruses
C. bacteria
D. prions
Answer Explanation
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Q #2: Nitrogen gas is an extremely stable molecule because its structure contains which of the following?
A. Resonance bonds
B. Ionic bonds
C. Triple covalent bonds
D. Hydrogen bonds
Answer Explanation
The Lewis structure of nitrogen gas is shown below. Triple bonds are stronger than double or single bonds. Therefore, we can infer that the triple bond between the two nitrogen atoms makes nitrogen gas more stable.
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Q #3: Which of the following sequences on a DNA molecule would be complementary to GCTTATAT?
A. TAGGCGCG
B. ATCCGCGC
C. CGAATATA
D. TGCCTCTC
Answer Explanation
