Ch.4Atomic Structure

How do we know atoms exist?

Picture, in your mind, what

you think an atom looks like.

Until recently, scientists had never seen an atom.

Without actually seeing them, scientists theorized the existence of atoms based on their observations of the properties of matter.

Greek philosophers proposed the atomic theory.

Atomic theory- the theory that all matter is composed of atoms.

Experimental results supporting the existence of atoms did not appear until the eighteenth-century.

• Early investigators noticed certain characteristics shared by all chemical compounds.

• These observation about compounds, and the reactions they undergo, led to three important laws: the law of definite proportions, the law of conservation of mass, and the law of multiple proportions.

The law of definite proportions

• The law of definite proportions states that a given compound contains the same elements in exactly the same proportions by mass, regardless of the size of the sample or the source of the compound.

• This means that every molecule of a certain compound has the same number and types of atoms, regardless of how or where it was produced.

Law of conservation of mass

• The law of conservation of mass states that the mass of the products of a reaction equals the mass of the reactants.

• This law applies when two or more elements combine to produce a compound, when a compound decomposes, or when the atoms in a compound are rearranged. (Fig.3-3 p.76)

The law of multiple proportions

• The law of multiple proportions applies to different compounds formed from the same two elements.

• Example: there are three different compounds made from the elements nitrogen and oxygen. All three compounds are gases, but each has its own physical and chemical properties (Nitrogen dioxide, Nitrogen monoxide, Dinitrogen monoxide).

Dalton’s atomic theory

Now that the laws had been established, John Dalton used the Greek concept of the atom and the

3 laws to give the atomic theory a scientific basis.

Dalton believed that there were about a hundred different kinds of atoms from which all matter was formed.

• According to Dalton, elements were composed of only one kind of atom and compounds were built from two or more kinds of atoms.

• Dalton proposed that the properties of elements differ from one another because their atoms differ.

• Dalton recognized that the laws of definite composition, conservation, and multiple proportions could be explained by the existence of tiny particles that combine with one another.

• Based on this, he proposed a theory about atomic structure and matter.

A Summary of Dalton’s Atomic Theory

1. All matter is made up of indestructible atoms.

2. All atoms of a given element are identical.

3. Atoms of different elements are different.

4. Atoms of different elements combine in simple-whole number ratios.

5. Chemical reactions consist of combination, separation, or rearrangement of atoms.

What is the internal structure of atoms?

• Experiments by several scientists in the mid-1800s led to the first modification of Dalton’s theory.

• See Historical Perspective

• Atoms were found to be divisible after all. Scientists discovered that the atom was made of smaller particles, referred to as subatomic particles.

• While many subatomic particles have been discovered, only three are important to your study of chemistry.

• Can you guess which three ?

• These particles are electrons, protons, and neutrons

• Proton-particle with a positive charge found in atomic nuclei.

• Neutron- particle with no electric charge found in the nuclei.

• Both protons and neutrons are located in the nucleus.

• Nucleus- dense central portion of the atom that contains all of the atoms positive charge and nearly all of its mass.

• Electron- particle with a negative charge found outside the nuclei.

• Because protons and electrons have equal but opposite charges, a neutral atom must contain equal numbers of protons and electrons.

• The number of protons and atom contains is known as the atomic number.

• The number of electrons must equal the number of protons.

• Atomic numbers are always whole numbers.

• Atomic Mass number is equal to the total number of particles in the nucleus, which is the total number of protons and neutrons in the nucleus of the atom.

number of protons and neutrons (mass number) = 11

minus the number of protons (atomic number) = - 5 number of neutrons = 6

Periodic Table

8

OOxygen

15.9994

8 protons

8 neutrons

8 electrons

Atomic structures can be represented by symbols.

Atomic number and mass number are sometimes written with an element’s symbol. The atomic number always appears to the lower left of the symbol.

• Mass numbers are written at the upper left of the symbol.

• Both numbers may be written with the symbol.

He4

2

Mass number

Atomic number

Isotopes

• All atoms of an element have the same atomic number and therefore the same number of protons. However, for many elements, the number of neutrons can vary.

• Atoms of the same element with different numbers of neutrons are called isotopes.

3 p+

3 n02e– 1e–

6Li 7Li

3 p+

4 n02e– 1e–

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Review

1. What is atomic theory?2. State the laws of definite proportions,

conservation of mass, and multiple proportions.

3. What is the difference between an element and a compound?

4. What type of electric charge does an electron carry?

5. What three subatomic particles are important to chemistry?

Models of the Atom Models of the Atom

a Historical Perspective a Historical Perspective

Aristotle

Early Greek Theories• 400 B.C. - Democritus thought matter

could not be divided indefinitely.

• 350 B.C - Aristotle modified an earlier theory that matter was made of four “elements”: earth, fire, water, air.

Democritus

• Aristotle was wrong. However, his theory persisted for 2000 years.

fire

air

water

earth

• This led to the idea of atoms in a void.

John Dalton• 1800 -Dalton proposed a modern atomic model

based on experimentation not on pure reason.

• All matter is made of atoms.• Atoms of an element are identical.• Each element has different atoms.• Atoms of different elements combine

in constant ratios to form compounds.• Atoms are rearranged in reactions.

• His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

Adding Electrons to the Model

1) Dalton’s “Billiard ball” model (1800-1900)Atoms are solid and indivisible.

2) Thomson “Plum pudding” model (1900)Negative electrons in a positive framework.

3) The Rutherford model (around 1910)Atoms are mostly empty space.Negative electrons orbit a positive nucleus.

Materials, when rubbed, can develop a charge difference. This electricity is called “cathode rays” when passed through an evacuated tube (demos). These rays have a small mass and are negative.Thompson noted that these negative subatomic particles were a fundamental part of all atoms.

Ernest Rutherford (movie: 10 min.)

Most particles passed through. So, atoms are mostly empty.

Some positive -particles deflected or bounced back!

Thus, a “nucleus” is positive & holds most of an atom’s mass.

Radioactive substance path of invisible

-particles

• Rutherford shot alpha () particles at gold foil.

Lead block

Zinc sulfide screen Thin gold foil

Bohr’s model

There are 2 types of spectra: continuous spectra & line spectra. It’s when electrons fall back down that they release a photon. These jumps down from “shell” to “shell” account for the line spectra seen in gas discharge tubes (through spectroscopes).

• Electrons orbit the nucleus in “shells”•Electrons can be bumped up to a higher shell

if hit by an electron or a photon of light.