NY Regents Chemistry Exam June 2022
The two naturally occuring isotopes of lithium are Li-6 and Li-7. The table below shows the atomic mass and percent natural abundance for these isotopes.
51 State the number of electrons in an atom of Li-7.
In a neutral atom, the number of electrons is equal to the number of protons which is equal to the atomic number. Atomic number can be found on the Periodic Table and is 3 for lithium.
Answer: 3 electrons
52 Compare the energy of an electron in the first shell of a lithium atom to the energy of an electron in the second shell of the same atom. [1]
As the shell number increases, the energy of an electron increases
Answer: The energy of an electron in the first shell is lower than the energy of an electron in the second shell.
53 Show a numerical setup for calculating the atomic mass of the element lithium. [1]
To calculate the atomic mass of an element when the mass of isotopes and % abundance are given, we take each isotope mass and multiple by percent abundance divided by 100% and add all the isotopes together.
Answer: Atomic mass = ((7.59)(6.015) + (92.41)(7.016)) / 100
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