Chapter 2 Atoms and Elements

2.4 The Atom

Dalton’s Atomic TheoryAtoms

♣ Are tiny particles of matter.

♣ Of an element are similar and different from other elements.

♣ Of two or more different elements combine to form compounds.

♣ Are rearranged to form new combinations in a chemical reaction.

Subatomic ParticlesAtoms contains subatomic particles.♣ Protons have a positive (+) charge.♣ Electrons have a negative (-) charge.♣ Like charges repel and unlike

charges attract.♣ Neutrons are neutral.

Copyright © 2007 by Pearson Education, Inc.Publishing as Benjamin Cummings

Rutherford’s Gold-Foil Experiment

Structure of the Atom

An atom consists

♣ Of a nucleus that contains protons and neutrons.

♣ Of electrons in a large, empty space around the nucleus.

Atomic Mass ScaleOn the atomic mass scale for subatomic particles♣ 1 atomic mass unit (amu) is defined as 1/12 of the

mass of the carbon-12 atom.♣ A proton has a mass of about 1 (1.007) amu.♣ A neutron has a mass of about 1 (1.008) amu.♣ An electron has a very small mass, 0.000549 amu.

Particles in the Atom

TABLE 2.6

Learning CheckIdentify each statement as describing a1) proton 2) neutron 3) electron

A. Found outside the nucleus.B. Has a positive charge.C. Is neutral.D. Found in the nucleus.

The atomic number ♣ Is specific for each element. ♣ Is the same for all atoms of an element.♣ Is equal to the number of protons in an atom.♣ Appears above the symbol of an element.

Atomic Number

11Na

Atomic Number

Symbol

Examples of atomic number and number of protons:

♣ Hydrogen has atomic number 1, every H atom has one proton.

♣ Carbon has atomic number 6, every C atom has six protons.

♣ Copper has atomic number 29, every Cu atom has 29 protons.

♣ Gold has atomic number 79, every Au atom has 79 protons.

Atomic Number and Protons

State the number of protons in each. A. A nitrogen atom

1) 5 protons 2) 7 protons 3) 14 protons

B. A sulfur atom1) 32 protons 2) 16 protons 3) 6 protons

C. A barium atom1) 137 protons 2) 81 protons 3) 56

protons

Learning Check

An atom ♣ Of an element is electrically neutral; the net

charge of an atom is zero.♣ Has an equal number of protons and electrons.

number of protons = number of electrons

♣ Of aluminum has 13 protons and 13 electrons. The net charge is zero.

13 protons (13+) + 13 electrons (13 -) = 0

Number of Electrons in An Atom

Mass Number

The mass number ♣ Represents the number of particles in the

nucleus.♣ Is equal to the

Number of protons + Number of neutronsTABLE 2.7

Atomic Models

An atom of zinc has a mass number of 65.A. How many protons are in this zinc atom?

1) 30 2) 35 3) 65

B. How many neutrons are in the zinc atom?1) 30 2) 35 3) 65

C. What is the mass number of a zinc atom that has 37 neutrons?1) 37 2) 65 3) 67

Learning Check

An atom has 14 protons and 20 neutrons.

A. Its atomic number is

1) 14 2) 16 3) 34

B. Its mass number is

1) 14 2) 16 3) 34

C. The element is

1) Si 2) Ca 3) Se

Learning Check

Isotopes

♣ Are atoms of the same element that have different mass numbers.

♣ Have the same number of protons, but different numbers of neutrons.

Isotopes

For an atom, the nuclear symbol gives the number of ♣ Protons (p+) ♣ Neutrons (n)♣ Electrons (e-)

16 31 65O P Zn

8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-

Nuclear Symbols

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of the following.

12C 13C 14C6 6 6

protons ______ ______ ______

neutrons ______ ______ ______

electrons ______ ______ ______

Learning Check

Write the nuclear symbols for atoms with the following subatomic particles:

A. 8 p+, 8n, 8e- ___________

B. 17p+, 20n, 17e- ___________

C. 47p+, 60 n, 47 e- ___________

Learning Check

16OA. 8p+, 8n, 8e- 8

B. 17p+, 20 n, 17e- 37Cl17

C. 47p+, 60n, 47e- 107Ag47

Solution

Learning Check1. Which of the following pairs are isotopes of the

same element?2. In which of the following pairs do both atoms

have 8 neutrons? A. 15X 15X

8 7

B. 12X 14X6 6

C. 15X 16X7 8

Solution

B. 12X 14X6 6

Both nuclear symbols represent isotopes of carbon with six protons each, but one has 6 neutrons and the other has 8.

C. 15X 16X7 8

An atom of nitrogen (7) and an atom of oxygen (8) each have 8 neutrons.

Isotopes of MagnesiumIn naturally occurring

magnesium,

there are three isotopes.

24Mg 25Mg 26Mg12 12 12

Isotopes of Mg

Atomic Mass 23.99 24.98 25.98

% abundance 78.70 10.13 11.17

Isotopes of SulfurA sample of naturallyoccurring sulfur containsseveral isotopes with thefollowing abundancesIsotope % abundance32S 95.02 33S 0.7534S 4.2136S 0.02

32S, 33S, 34S, 36S16 16 16 16

Atomic Mass

The atomic mass of an element

♣ Is listed below the symbol of each

element on the periodic table.

♣ Gives the mass of an “average” atom

of each element compared to 12C.

♣ Is not the same as the mass number.

Na22.99

Some Elements and Their Atomic Mass

Most elements have two or more isotopes that contribute to the atomic mass of that element.

TABLE 2.9

32.07 amu

Atomic Mass for ClThe atomic mass of chlorine

is

♣ Based on all naturally occurring Cl isotopes.

♣ Not a whole number.

♣ The weighted average

of 35Cl and 37Cl.

Using the periodic table, give the atomic mass ofeach element:

A. calcium __________

B. aluminum __________

C. lead __________

D. barium __________

E. iron __________

Learning Check

Using the periodic table, give the atomic mass ofeach element:

A. calcium 40.08 amu

B. aluminum 26.98 amu

C. lead 207.2 amu

D. barium 137.3 amu

E. iron 55.85 amu

Solution

Chapter 2 Atoms and Elements

2.7Electron Energy Levels

Copyright © 2007 by Pearson Education, Inc.Publishing as Benjamin Cummings

Electromagnetic Radiation

Electromagnetic radiation

♣ Is energy that travels as waves through space.

♣ Is described in terms of wavelength and

frequency.

♣ Moves at the speed of light in a vacuum.

speed of light = 3.0 x 108 m/s

Electromagnetic SpectrumThe electromagnetic spectrum♣ Arranges forms of energy from lower to higher.♣ Arranges energy from longer to shorter

wavelengths.♣ Shows visible light with wavelengths from 700-

400 nm.

Electron Energy LevelsElectrons are arranged inspecific energy levels that♣ Are labeled n = 1, n = 2, n

= 3, and so on.

♣ Increase in energy as nincreases.

♣ Have the electrons with the lowest energy in the first energy level (n=1)closest to the nucleus. Copyright © 2007 by Pearson Education, Inc.

Publishing as Benjamin Cummings

Energy Level Changes♣ An electron absorbs

energy to “jump” to a higher energy level.

♣ When an electron falls to a lower energy level, energy is emitted.

♣ In the visible range, the emitted energy appears as a color.

Copyright © 2007 by Pearson Education, Inc.Publishing as Benjamin Cummings

Energy Emitted

In each of the following energy level changes, indicate if energy is

1) absorbed 2) emitted 3) not changed

A. An electron moves from the first energy level (n =1 ) to the third energy level (n = 3).

B. An electron falls from the third energy level to the second energy level.

C. An electron moves within the third energy level.

Learning Check

A. An electron in an atom moves from the first energy level (n =1 ) to the third energy level (n = 3).

1) absorbed

B. An electron falls from the third energy level to the second energy level.

2) emittedC. An electron moves within the third energy level.

3) not changed

Solution

Energy Diagram for Sublevels