1

The Structure of the Atom

2

Objectives

• Compare and contrast the atomic models of Democritus and Dalton

• Define an atom.

• Distinguish between the sub atomic particles in terms of relative charge and mass.

• Describe the structure of the nuclear atom, including the locations of the subatomic particles.

3

Objectives

• Explain the role of atomic number in determining the identity of an atom.

• Define an isotope and explain why atomic masses are not whole numbers.

• Calculate the number of electrons, protons, and neutrons in an atom given its mass number and atomic number.

4

Dalton’s Atomic Theory

1. Each element consists of individual particles called atoms

2. Atoms can neither be created nor destroyed

3. All atoms of a given element are identical

4. Atoms combined chemically in definite whole-number ratios to form compounds

5. Atoms of different elements have different masses

5

History

• The electron was discovered in 1897 by Thomson. He imagined the atom as a “raisin pudding” with electrons stuck in a cake of positive charge

In 1912, Rutherford discovered the nucleus (proton). He concluded the atom was mostly empty space, with a large dense body at the center, and electrons which orbited the nucleus like planets orbit the Sun

6

History• Rutherford’sRutherford’s gold foil experiment was used to show that

the nucleus was small, dense, and positively charged

• ThomsonThomson used the cathode ray to discover electrons

• The neutron was then discovered in 1932 by Chadwick.Chadwick.

7

Subatomic Particles and the Nuclear Atom

8

Development of Atomic Structure

• About 1900 Sir J. J. Thomson and Earnest Rutherford established the modern model of the atom.

1.1. Electrically Electrically positivepositive (+1) (+1) protonsprotons..

•• The number of protons equals the number of The number of protons equals the number of electronselectrons..

• The atom is composed of three subatomic particles.

2.2. Electrically Electrically neutralneutral (0) (0) neutronsneutrons..

3.3. Electrically Electrically negativenegative (-1) (-1) electronselectrons..

• Atoms have a net charge of zeroAtoms have a net charge of zero.

9

Parts of the atom

10

Parts of an Atom

• Each element in the Periodic Table has a different number of protons in its nucleus

– Change the number of protons change elements

• The element also has the same number of electrons

– Change the number of electrons ionize the element

• Some elements also have neutrons

– They are in the nuclei of atoms

p

e

n

15

How Atoms Differ

Section 4.3

18

Ions• An atom has the same number

of electrons as protons, an atom has no net charge.

• If an atom loses or gains one or more electrons it has become ionized.

• With less electrons, the atom has a positive charge (Cation), more electrons it has a negative charge (Anion).

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Parts of the atom

Particle Charge Mass (g)Mass (amu)

Proton +1 1.672 x 10-24 1

Neutron 0 1.675 x 10-24 1

Electron -1 9.110x10-28 1/1839

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Isotopes Isotope - Atoms with the same atomic number but different mass numbers.

•• Isotopes have the same number of protons but differ in number Isotopes have the same number of protons but differ in number of neutrons.of neutrons.

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Isotopes Isotope AbundanceIsotope Abundance

•• Naturally occurring sample of most elements will contain a Naturally occurring sample of most elements will contain a mixture of isotopes.mixture of isotopes.

•• Percent abundancePercent abundance is used to quantify the amount of each is used to quantify the amount of each isotope in an elemental sample.isotope in an elemental sample.

100isotopes all atom, of number Total

isotope one of atoms of Number Abundance Percent

Example: OxygenExample: Oxygen

Isotope % Abundance # in 100,00016O 99.759 99,75917O 0.037 3718O 0.204 204

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Atomic Mass and Isotopes The atomic mass which appears on the periodic table is the weighted averageweighted average of the masses of all the isotopes.

weighted fractional isotopicaverage = isotopic X mass

abundance