A 53.8-mg sample of sodium perchlorate contains radioactive chlorine-36 (whose atomic mass is 36.0 amu). If 29.6% of the chlorine atoms in the sample are chlorine-36 and the remainder are naturally occurring nonradioactive chlorine atoms, how many disintegrations per second are produced by this sample? The half-life of chlorine-36 is $3.0 \times 10 ^ { 5 }$ yr.

It was shown that $N / V=2.68 \times 10^{25} \mathrm{m}^{-3}$ for gas at STP. (a) Show that this quantity is equivalent to $N / V=2.68 \times 10^{19} \mathrm{cm}^{-3},$ as stated. (b) About how many atoms are there in one $\mu \mathrm{m}^{3}$ (a cubic micrometer) at STP? (c) What does your answer to part (b) imply about the separation of atoms and molecules?

Suppose that you have $22.4 \mathrm{~L}$ of nitrogen gas, $\mathrm{N}_2(g)$, at STP.

How many nitrogen atoms are in the sample?

Suppose you have 50 grams of copper and 50 grams of gold. Which sample has more atoms? Explain your thinking.

Arrange the following atoms in order of electronegativity (lowest first):\

• $\mathrm{Cl}$
• $\mathrm{O}$
• $\mathrm{H}$
• $\mathrm{Br}$
• $\mathrm{Na}$

The methane molecule $\left(\mathrm{CH}_4\right)$ has four hydrogen atoms located at the vertices of a regular tetrahedron of edge length $0.18 \mathrm{~nm}$, with the carbon atom at the center of the tetrahedron (Figure 9-48). Find the moment of inertia of this molecule for rotation about an axis that passes through the centers of the carbon atom and one of the hydrogen atoms.

How many electrons, protons, and neutrons are found in isolated atoms having the following atomic symbols?

$${ }_2^4 \mathrm{He}$$

C. Radon -222 $\left(\frac{222}{86} \mathrm{Rn}\right)$ is a radioactive gas with a half-life of 3.82 days. A gas sample contains $4.0 \times 10^{8}$ radon atoms initially. a. Estimate how many radon atoms will remain after 12 days. b. Estimate how many radon nuclei will have decayed by this time.

For each of the following groups, place the atoms and ions in order of decreasing size. $\mathrm { Cu }, \mathrm { Cu } ^ { + }, \mathrm { Cu } ^ { 2 + }$

Which has more atoms: $8.0 \mathrm{~g}$ of helium, He, or $40.0 \mathrm{~g}$ of argon, Ar? Explain your reasoning.

9.0 $\mathrm{~g}$ of $\mathrm{Na}, 0.50 \mathrm{~mol}$ of $\mathrm{Na}$, or $1.2 \times 10^{22}$ atoms of $\mathrm{Na}$

Select the ions or atoms from the following set that are isoelectronic with each other:

$$\mathrm{Na}^{+}, \mathrm{Sr}^{2+}, \mathrm{Br}^{-}$$

How many moles of $\mathrm{O}$ atoms are in $4.25 \mathrm{~mol}$ of calcium carbonate, $\mathrm{CaCO}_3$, the chief constituent of seashells?

Determine which of the following contains the largest number of atoms:
(a) $7.68 g$ of He
(b) $112 g$ of Fe
(c) $389 g$ of Hg