Q: Given the following isotopes of magnesium: $$^{24}\mathrm{Mg} (78.99$$%, mass = 23.985 amu), $$^{25}\mathrm{Mg} (10.00$$%, mass = 24.986 amu), and $$^{26}\mathrm{Mg} (11.01$$%, mass = 25.983 amu), what is the average atomic mass of magnesium?

$$(0.7899 \times 23.985) + (0.1000 \times 24.986) + (0.1101 \times 25.983) = 24.31\ \mathrm{amu}$$

Q: How many atoms are present in a 12.0 g sample of carbon-12 ($$^{12}\mathrm{C}$$)? (Avogadro's number $$= 6.022 \times 10$$^{23}$$\ \mathrm{atoms/mol})$$

$$\frac{12.0\ \mathrm{g}}{12.0\ \mathrm{g/mol}} \times 6.022 \times 10$$^{23}$$\ \mathrm{atoms} = 6.022 \times 10$$^{23}$$\ \mathrm{atoms}$$

Q: A sample contains 3.00 g of silver ($$\mathrm{Ag}$$, atomic mass = 107.87 g/mol). How many atoms of silver are present?

$$\frac{3.00\ \mathrm{g}}{107.87\ \mathrm{g/mol}} \times 6.022 \times 10$$^{23}$$\ \mathrm{atoms} = 1.68 \times 10$$^{22}$$\ \mathrm{atoms}$$

Q: A sample of neon contains 90.48% $$^{20}\mathrm{Ne} ($$mass = 19.992 amu), 0.27% $$^{21}\mathrm{Ne} ($$mass = 20.994 amu), and 9.25% $$^{22}\mathrm{Ne} ($$mass = 21.991 amu). What is the average atomic mass of neon?

$$(0.9048 \times 19.992) + (0.0027 \times 20.994) + (0.0925 \times 21.991) = 20.18\ \mathrm{amu}$$

Question 1

Which subatomic particle is responsible for the identity of an element, and where is it located in the atom?

Accepted Answer

Proton; in the nucleus

Question 2

What is the chemical symbol for the element with 17 protons?

Accepted Answer

Cl

Question 3

Which of the following best describes a metalloid?

Accepted Answer

It is brittle and has properties intermediate between metals and nonmetals.

Question 4

An atom has an atomic number of 12 and a mass number of 25. How many neutrons does it have?

Accepted Answer

$$25 - 12 = 13$$

Question 5

Which isotope notation correctly represents an atom with 6 protons, 8 neutrons, and 6 electrons?

Accepted Answer

$$^{14}_{6}\mathrm{C}$$

Question 6

A neutral atom of sodium (Na) loses one electron. What is the resulting ion and its charge?

Accepted Answer

$$\mathrm{Na}^+$$

Question 7

Given the following isotopes of magnesium: $$^{24}\mathrm{Mg} (78.99$$%, mass = 23.985 amu), $$^{25}\mathrm{Mg} (10.00$$%, mass = 24.986 amu), and $$^{26}\mathrm{Mg} (11.01$$%, mass = 25.983 amu), what is the average atomic mass of magnesium?

Accepted Answer

$$(0.7899 	imes 23.985) + (0.1000 	imes 24.986) + (0.1101 	imes 25.983) = 24.31\ \mathrm{amu}$$

Question 8

How many atoms are present in a 12.0 g sample of carbon-12 ($$^{12}\mathrm{C}$$)? (Avogadro's number $$= 6.022 	imes 10$$^{23}$$\ \mathrm{atoms/mol})$$

Accepted Answer

$$\frac{12.0\ \mathrm{g}}{12.0\ \mathrm{g/mol}} 	imes 6.022 	imes 10$$^{23}$$\ \mathrm{atoms} = 6.022 	imes 10$$^{23}$$\ \mathrm{atoms}$$

Question 9

Which of the following statements about electrons is correct?

Accepted Answer

Electrons are found outside the nucleus and have a negative charge.

Question 10

A sample contains 3.00 g of silver ($$\mathrm{Ag}$$, atomic mass = 107.87 g/mol). How many atoms of silver are present?

Accepted Answer

$$\frac{3.00\ \mathrm{g}}{107.87\ \mathrm{g/mol}} 	imes 6.022 	imes 10$$^{23}$$\ \mathrm{atoms} = 1.68 	imes 10$$^{22}$$\ \mathrm{atoms}$$

Question 11

Which of the following elements is a nonmetal and is essential for respiration in humans?

Accepted Answer

Oxygen

Question 12

A chlorine atom has 17 protons and 18 neutrons. What is its isotope notation?

Accepted Answer

$$^{35}_{17}\mathrm{Cl}$$

Question 13

Which of the following is a property of metals?

Accepted Answer

They are good conductors of electricity.

Question 14

If an atom has 15 protons, 16 neutrons, and 18 electrons, what is its charge?

Accepted Answer

$$15 - 18 = -3$$, so the charge is $$3-$$

Question 15

Which of the following best describes the difference between atomic number and mass number?

Accepted Answer

Atomic number is the number of protons; mass number is the sum of protons and neutrons.

Question 16

A sample of neon contains 90.48% $$^{20}\mathrm{Ne} ($$mass = 19.992 amu), 0.27% $$^{21}\mathrm{Ne} ($$mass = 20.994 amu), and 9.25% $$^{22}\mathrm{Ne} ($$mass = 21.991 amu). What is the average atomic mass of neon?

Accepted Answer

$$(0.9048 	imes 19.992) + (0.0027 	imes 20.994) + (0.0925 	imes 21.991) = 20.18\ \mathrm{amu}$$

Atoms, Elements, and Isotopes: Structure and Calculations

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