History of the AtomDalton’s Atomic Theory (1808)

1. Elements are composed of extremely small particles called atoms. All atoms of a given element are identical. The atoms of one element are different from the atoms of all other elements.

2. Compounds are composed of atoms of more than one element. The relative number of atoms of each element in a given compound is always the same.

3. Chemical reactions only involve the rearrangement of atoms. Atoms are not created or destroyed in chemical reactions.

2.1

8 X2Y16 X 8 Y+

2.1

HISTORY OF THE ATOMHISTORY OF THE ATOM

Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere

surrounded by a soup of positive charge to balance the

electron's charge

1904

like plums surrounded by pudding.

PLUM PUDDING

MODEL

Thompson found that atoms could sometimes eject a

far smaller negative particle which he called an

Electron.

J.J. Thomson, measured mass/charge of e-

(1906 Nobel Prize in Physics) 2.2

1.In 1897 JJ.Thomson devised an experiment with cathode rays that resulted in the discovery of the electron.2.Up to this time it was thought that the hydrogen atom was the smallest particle in existence. Thomson demonstrated that electrons comprising cathode rays were nearly 2,000 times smaller in mas than the lightest known particle the hydrogen ion.3.When a high voltage is placed across a pair of plates, they become charged relative to each other. The positively charged plate is the anode and the negatively charged plate the cathode.4.Electrons pass from the surface of the cathode and accelerate toward the oppositely charged anode. The anode absorb many electrons, but is the anode has slits some electrons will pass through.5.The electron travel into an evacuated tube, where they move in a straight line until striking a

fluorescent screen. this screen is coated with a chemical that glows when electrons strike it.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1910 Ernest RutherfordFired alpha particle at a piece of gold foil which was

only a few atoms thick.

they found that although most of them passed

through. About 1 in 10,000 hitRutherford’s new

evidence allowed him to propose a more detailed

model with a central nucleus.

He suggested that the positive charge was all in a

central nucleus. With this holding the electrons in

place by electrical attraction

1. atoms positive charge is concentrated in the nucleus2. proton (p) has opposite (+) charge of electron3. mass of p is 1840 x mass of e- (1.67 x 10-24 g)

particle velocity ~ 1.4 x 107 m/s(~5% speed of light)

(1908 Nobel Prize in Chemistry)

2.2

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ charge

- charge

No charge

1

1

nil

Mass

Atoms have no overall electrical charge so,

an atom must have as many electrons as there are protons

in its nucleus.

The atomic number of an element also equals the number of electrons

in a neutral atom of that element.

Atomic Number (Z): is the number of protons

in the nucleus of the atom.

Z=#p

The number of protons (atomic number) determine the identity of an element.

The atomic number of an element also equals

the number of electrons

in a neutral atom of that element.

Ex: Sodium

• What is the atomic number of Sodium?

• How many protons does sodium have?

• How many electrons does sodium have?

Mass Number (A):

• The sum of the protons and neutrons in the nucleus.

A=#p + #n

ATOMIC STRUCTUREATOMIC STRUCTURE

the number of protons in an atom

the number of protons and neutrons in an atom

HeHe22

44 Atomic mass

Atomic number

number of electrons = number of protons

14

Atomic Mass Unit

is a unit used to compare the masses of atoms

and has the symbol u or amu.

1 AMU is approximately equal to the mass of a single

proton or neutron.

Isotopesof an element have

different mass numbers

because

they have

different numbers of neutrons,

but they have

the same atomic number.

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

Isotopes are atoms of the same element (X) with different numbers of neutrons in the nucleus

XAZ

H11 H (D)2

1 H (T)31

U23592 U238

92

Mass Number

Atomic NumberElement Symbol

2.3

How many protons, neutrons, and electrons are in C146 ?

How many protons, neutrons, and electrons are in C116 ?

6 protons, 8 (14 - 6) neutrons, 6 electrons

6 protons, 5 (11 - 6) neutrons, 6 electrons

Do You Understand Isotopes?

2.3