Chemistry 120

Chapter 5: Atomic Theory: The Nuclear Model of the Atom

Fig. 5-CO, p. 123

Atomic Theory

• Elements (matter) is composed of small, indivisible particles called atoms.

• Atoms of a given element are identical in mass and behavior.

• Atoms of different elements differ in mass and behavior.

• Chemical combination of elements to make different substances occurs when atoms join together in small whole number ratios.

• Chemical reactions only rearrange the way the atoms are combined; the atoms themselves are not changed

Law of Multiple Proportions (John Dalton)

• When the same two elements combine to form more than one compound:

the ratios of the mass of one element in the first compound to its mass in the second compound, (as it combines with the same mass of the other element), can always be expressed as ratios of small whole numbers

• Carbon combines with oxygen to form CO and CO2 .

Mass of Carbon(g)

Mass of Oxygen(g)

Ratio of O in CO2 to O in CO

CO 12.01 16.00

CO2 12.01 32.00 2:1

Example of the Law of Multiple Proportions

• That of nitrogen and oxygen forming three different compounds that contain different relative amounts of nitrogen and oxygen.

Mass of Nitrogen thatcombines with 1 g of Oxygen

• Compound A 1.750 g• Compound B 0.8750 g• Compound C 0.4375 g

Example of the Law of Multiple Proportions continued

The ratios of the masses of nitrogen combining with 1 gram of oxygen in each pair of compound should be small whole integer number• A/B = 1.750/0.8750 = 2 for nitrogen 0.8750/0.8750= 1 for nitrogen, a ratio 2:1

• B/C= 0.8750/0.4375 = 2 for nitrogen 0.4375/ 0.4375= 1 for nitrogen, a ratio 2:1

• A/C = 1.750/0.4375 = 4 for nitrogen 0.4375/0.4375= 1 for nitrogen, a ratio 4:1

Sub Atomic Particles

Example – Chemical Symbols

• How many protons does a sulfur atom have?

Example – Chemical Symbols

• How many electrons does a sulfur atom have?

Example – Chemical Symbols

• How many electrons does a sulfide ion, S2-, have?

Example – Chemical Symbols

• How many protons does an aluminum ion, Al3+, have?

Example – Chemical Symbols

• How many electrons does an aluminum ion, Al3+, have?

Example – Chemical Symbols

• An atom has 5 protons, 5 electrons, and 6 neutrons. What is the atomic number?

Example – Chemical Symbols

• An atom has 5 protons, 5 electrons, and 6 neutrons. What is the name of the element?

Example – Chemical Symbols

• An atom has 5 protons, 5 electrons, and 6 neutrons. What is the mass number?

Example – Chemical Symbols

• An atom has 5 protons, 5 electrons, and 6 neutrons. What is the charge?

Example – Chemical Symbols

• An atom has 20 protons, 18 electrons, and 20 neutrons. What is the atomic number?

Example – Chemical Symbols

• An atom has 20 protons, 18 electrons, and 20 neutrons. What is the name of the element?

Example – Chemical Symbols

• An atom has 20 protons, 18 electrons, and 20 neutrons. What is the mass number?

Example – Chemical Symbols

• An atom has 20 protons, 18 electrons, and 20 neutrons. What is the charge?

Example – Chemical Symbols

• What is the atomic notation for an atom that has 17 protons, 20 neutrons, and 17 electrons?

A. 𝐶𝑙1737 B. 𝐶𝑙1734 C. 𝐶𝑙3717 D. 𝐶𝑙1737 E. 𝑅𝑏3717

Example – Chemical Symbols

• What is the atomic notation for an atom that has 15 protons, 16 neutrons, and 18 electrons?

A. 𝑃1531 B. 𝐺𝑎3115 C. 𝑃3−1531 D. 𝐺𝑎3−3115 E. 𝑃3+1531

Isotopes

• Atoms which differ only in the number of neutrons present in the nucleus.

Example – Atomic Mass

• Calculate the atomic mass of magnesium given

Mass (amu) %abundance

Mg-24 23.985 78.70

Mg-25 24.986 10.13

Mg-26 25.983 11.17

Example – Atomic Mass

• Calculate the atomic mass of chlorine given

Mass (amu) %abundance

Chlorine-35 34.969 75.78

Chlorine-37 36.966 24.22

Example – Atomic Mass

• Lithium has two isotopes: lithium-6 at 7.420% and lithium-7 at 92.58%. If the atomic mass of lithium is 6.941 amu and the mass of lithium-6 is 6.0151 amu, what the mass of lithium-7?

Example – Atomic Mass

• Bromine has two isotopes. Bromine-79 the most abundant isotope at 50.69% has a mass of 78.9183 amu. What is the mass of bromine-81?