EARLY MODELS OF THE ATOM

Models of Matter A model is a tentative description

of a system or theory that accounts for all of its known properties

Models are invented (usually) to interpret the invisible and relate experimental results

The Four Elements

An Ancient Greek model of the atom was based on logic

Matter is composed of four elements: air, fire, earth and water

An atom is a single indivisible particle

John Dalton’s Atomic Theory (1803)

Based on experimentation Experimental data showed: Elements combine in the same percent

by mass each time the same compound is formed

Atoms of different elements combine in fixed proportions

Dalton’s Theory

1. All matter is made of small spherical particles called atoms

2. Each element is made of its own type of atom

3. Atoms of different elements have different properties

4. Atoms of different substances can combine in constant ratios to form new substances

5. Atoms cannot be created or destroyed during chemical or physical changes

6. Atoms can neither be subdivided nor changed to one another

Dalton’s Theory Continued…

Law of Constant Composition: Atoms of different elements can be

chemically combined in a fixed whole number ratio to form compounds. Example: H2O

Law of Conservation of Mass: The mass of the reactants in a chemical

reaction is equal to the mass of the products

The Atom

The smallest indivisible particle of an element that has the properties of that element

Thomson’s Atomic Theory (1897)

Used a cathode ray tube to energize electrons

Experimental data showed: Charged particles originated from the

gas inside the tube Charged particles are deflected away

from a negatively charged object, but towards a positively charged object.

Passing an electric current makes a beam Passing an electric current makes a beam appear to move from the negative to the appear to move from the negative to the positive end.positive end.

Thomson’s ExperimentThomson’s Experiment

Voltage source

+-

Voltage source

Thomson’s ExperimentThomson’s Experiment

By adding an electric field, he found that By adding an electric field, he found that the moving pieces were negative the moving pieces were negative

+

-

Thomson’s Model

Found the electron. Couldn’t find

positive (for a while).

Said the atom was like plum pudding.

A bunch of positive stuff, with the electrons able to be removed.

Atoms are composed of smaller particles. These particles are the same for all

different types of atoms. These particles are negatively charged

and are called electrons. Electrons are embedded throughout the

uniform sphere of positive charge to make up a neutral atom.

Matter is naturally neutral except when it gains or loses electrons

“Plum Pudding Model”

Rutherford’s Gold Foil Experiment (1911)

Rutherford used very small positive particles (alpha particles) and a fluorescent screen detector to see the particles

Since the mass is evenly distributed in gold atoms alpha particles should go straight through.

Gold Foil Experiment

Experimental data showed: Alpha particles (+ve) were shot at

a thin sheet of gold Most alpha particles passed

through unaffected Some alpha particles were

deflected at large angles.  Some are even reflected back.

What he got

Atom is mostly empty space Small dense positive region at the

center (a nucleus?) Alpha particles are deflected by this

region if they get close enough. Electrons surround but do not fill

the rest of the atom

Rutherford’s Theory…

Modern View

The atom is mostly empty space.

Nucleus contains protons and neutrons.

Electron cloud- region where you might find an electron.

Symbolic Representation of Sub Atomic Particles

Name Symbol Charge Mass Location

proton p+ +1 1 a.m.u. nucleus

neutron n 0 1 a.m.u. nucleus

electron e- -1 1/1837 a.m.u.

energy levels around nucleus

Sub-atomic Particles

Z - atomic number = number of protons determines type of atom.

A - mass number = number of protons + neutrons.

Number of protons = number of electrons if neutral.

Number of neutrons = atomic mass – atomic number

Symbols

XZ

A

Na11

23

IONS Charged atoms formed by a neutral

atom to attain a stable structure on the last shell (octet or doublet)

Two types of ions: Positive ions (cations) are formed

when an atom loses electrons Negative ions (anions) are formed

when an atom gains electrons

Ionic symbols

Cation Anion

X+N X-N

X = Symbol X = Symbol(+) = losing (-)= gainingN= # of electrons lost N= # of electrons

gained

Subatomic particles for ions

Cations Anions

protons p+ = Z p+ = Z

electrons e- = Z - N e- = Z + N

neutrons n = A - Z n = A - Z

Practice!

Calculate the subatomic particles for the following particles:

Cu Fe+3 N-3