THE ATOM

HISTORY AND STRUCTURE

ATOMIC STRUCTURE

Atom-

The smallest unit of an element that maintains the

properties of that element.

Originated from the Greek word Atomos - meaning

indivisible particle.

ATOMIC STRUCTURE

Ancient views of the atom

Aristotle

Earth, air, fire, water

everything was made from

these four basic “elements”

Democritus Model

Made of small indivisible

particles

Different atom for every

substance

ATOMIC STRUCTURE Dalton Model - John Dalton’s

atomic theory, developed in the early 1800’s.

All elements are composed of atoms.

All atoms of the same element have the

same mass, and atoms of different

elements have different masses.

Compounds contain atoms of more than

one element.

In a particular compound, atoms of

different elements always combine in the

same way. Atoms still small indivisible spheres

ATOMIC STRUCTURE

J.J. Thomson’s model

Early 1900’s

Discovered particles in the

atom with a negative

charge and called them

electrons

Since atoms are neutral,

there must also be positive

charges

His model described the

atom as a positively

charged sphere with

negative particles

scattered throughout.

Like raisins in plum pudding

DISCOVERY OF

ELECTRONS

ELECTRON

CHARGE

ELECTRON

MASS

ATOMIC STRUCTURE

Discovery of the proton

ATOMIC STRUCTURE

Ernest Rutherford’s model

1911

Gold Foil Experiment.

Shot alpha particles at gold foil.

Most went through, but some bounced back

Like throwing marbles at a chain link fence.

The atom has a tiny positive core called a

nucleus

Electrons are whirling around the nucleus

The atom is mostly empty space

ATOMIC STRUCTURE

Rutherford’s gold foil experiment

ATOMIC STRUCTURE

Neils Bohr model

1913

Modified Rutherford’s model to explain energy release and absorption

Electrons move in paths called energy levels

Electrons in each level have a definite amount of energy

Adding energy moves an electron further from the nucleus

When electrons return to a lower level they give off energy. (light)

ATOMIC STRUCTURE

Bohr model of the atom

ATOMIC STRUCTURE

Schrodinger

Electron cloud model

Same as Bohr model but instead of paths for the

electrons there are regions of space called

electron clouds.

ATOMIC STRUCTURE

Atomic Number –

The number of protons in an atom’s nucleus

In a neutral atom this also equals the number of

electrons

Mass Number-

The number of protons plus neutrons

Not all atoms of an element contain the same

number of neutrons (isotopes)

Isotopes are identified by their mass number

Hydrogen-1, Hydrogen-2, Hydrogen-3

ATOMIC STRUCTURE

Parts of the atom

Nucleus

The part of the atom that

has most of the mass

Has a positive charge

Contains protons and

neutrons

ATOMIC STRUCTURE

Discovery of the Neutron

ATOMIC STRUCTURE

Electron orbital-

A region in an atom where there is a high probability of finding electrons

Electrons exist in energy levels

Each level holds a certain number of electrons

Level 1 2e-

Level 2 8e-

Level 3 18e-

Level 4 32e- , higher levels also hold 32e-

Valence electrons

Outer level electrons

Determine an atom’s chemical properties

No element has more than eight valence electrons (OCTET rule)

Neutral atoms have equal numbers of protons and electrons

ATOMIC STRUCTURE

Drawing Bohr Models

Determine the number

of Protons, neutrons,

and electrons.

Determine the number

of energy levels

available for electrons

(period)

Place electrons

according to the

periodic table map

Bohr model of Oxygen

Protons?

Atomic # 8 – 8 protons

Also means 8 electrons

Neutrons

Mass # - Atomic #

16 – 8 = 8 neutrons

Energy levels?

Period 2 – 2 energy

levels

ATOMIC STRUCTURE

Bohr model of Oxygen

8 – protons

8 – neutrons

8 – electrons / 2 levels

8p

8n

2e-

6e-

ATOMIC STRUCTURE

Bohr model of Tin (Sn)

Protons

Atomic number 50 so 50 p

Electrons

50e-

Neutrons

119 – 50 = 69 n

Energy levels

5th period – 5 levels

50p

69n

2e-

8e-

18e-

18e-

4e-

ATOMIC STRUCTURE

Bohr model of Uranium (U)

Protons

Atomic number 92 – 92 p

Electrons

Also 92 e-

Neutrons

Mass # - Atomic #

238 – 92 = 146 n

Energy Levels

Period 7 – 7 energy levels

92p

146n

2e-

8e-

18e-

32e-

21e-

9e-

2e-