Q: Gallium has two naturally occurring isotopes: $$^{69}\mathrm{Ga} ($$mass = 68.9257 amu, 60.4%) and $$^{71}\mathrm{Ga} ($$mass = 70.9248 amu, 39.6%). What is the atomic weight of gallium?

$$68.9257 \times 0.604 + 70.9248 \times 0.396 = 69.72$$ amu

Question 1

Which of the following statements best describes the modern atomic theory as it relates to chemical reactions?

Accepted Answer

Chemical reactions change only the way atoms are combined in compounds.

Question 2

A neutral atom of phosphorus has an atomic number of 15 and a mass number of 31. How many neutrons does it contain?

Accepted Answer

$$31 - 15 = 16$$ neutrons

Question 3

Which of the following is the correct electron configuration for magnesium (Mg, atomic number 12)?

Accepted Answer

$$1s^2$$ $$2s^2$$ $$2p^6$$ $$3s^2$$

Question 4

Gallium has two naturally occurring isotopes: $$^{69}\mathrm{Ga} ($$mass = 68.9257 amu, 60.4%) and $$^{71}\mathrm{Ga} ($$mass = 70.9248 amu, 39.6%). What is the atomic weight of gallium?

Accepted Answer

$$68.9257 	imes 0.604 + 70.9248 	imes 0.396 = 69.72$$ amu

Question 5

Which group on the periodic table contains elements that are colorless, odorless, and extremely unreactive under normal conditions?

Accepted Answer

Group 8A (Noble gases)

Question 6

Which of the following best describes the difference between s and p orbitals?

Accepted Answer

s orbitals are spherical, while p orbitals are roughly dumbbell-shaped.

Question 7

How many electrons can the third shell of an atom hold, and how are they distributed among the subshells?

Accepted Answer

18 electrons: 2 in 3s, 6 in 3p, 10 in 3d

Question 8

Which element is represented by the symbol $$^{194}_{78}\mathrm{Pt}$$? How many protons, electrons, and neutrons does it have?

Accepted Answer

Platinum; 78 protons, 78 electrons, 116 neutrons

Question 9

Which of the following is the correct shorthand electron configuration for vanadium (V, atomic number 23)?

Accepted Answer

$$[Ar] 4s^2$ 3d^3$

Question 10

Which of the following statements about isotopes is correct?

Accepted Answer

Isotopes of an element have the same number of protons but different numbers of neutrons.

Question 11

Which of the following elements is a metalloid based on its position in the periodic table?

Accepted Answer

Silicon (Si)

Question 12

What is the general valence-shell electron configuration for group 6A elements?

Accepted Answer

$$ns^2$$ $$np^4$$

Question 13

Which of the following best explains why alkali metals are never found in nature in their pure state?

Accepted Answer

They are highly reactive and readily combine with other elements.

Question 14

How many valence electrons does a tin (Sn) atom have, and what is its valence-shell configuration?

Accepted Answer

4 valence electrons; $$5s^2$$ $$5p^2$$

Question 15

Which of the following is the correct Lewis electron-dot symbol for an element in group 5A?

Accepted Answer

The symbol with five dots, one on each side and one paired.

Question 16

Which of the following best describes the trend in atomic radius across a period from left to right?

Accepted Answer

Atomic radius decreases due to increasing nuclear charge.

Question 17

Which of the following elements has the electron configuration $$1s^2$$ $$2s^2$$ $$2p^6$$ $$3s^2$$ $$3p^5$$?

Accepted Answer

Chlorine (Cl)

Question 18

Which block of the periodic table contains the transition metals?

Accepted Answer

d block

Question 19

A patient is exposed to a radioactive isotope of iodine, $$^{131}\mathrm{I}. $$How does this isotope differ from the most common isotope, $$^{127}\mathrm{I}$$?

Accepted Answer

$$^{131}\mathrm{I}$$ has four more neutrons than $$^{127}\mathrm{I}.$$

Atoms and the Periodic Table: Structure, Properties, and Electron Configurations

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