Basic Structure of

the Atom

Evidence Supporting the Atomic Theory

Building the Atomic Model

Democritus (460-370 BC)

John Dalton (1803)

J. J. Thomson (1897)

Ernest Rutherford (1909)

Niels Bohr (1913)

James Chadwick (1932)

Democritus

Ancient Greek

First to propose matter was made up of

tiny particles called atomos

Stated atoms could not be

created, destroyed, or further

divided

Dalton

Proposed Atomic Theory in 1803

Half a Century until well accepted

100 years until proven

Each element :

Composed of indivisible and indestructible atoms

Atoms of different elements are different;

atoms of the same element are the same

Atoms of different elements combine to form

compound atoms (molecules)

J. J. Thomson

Discovered electrons in 1897

Plum Pudding Model – Positive and

negative charges scattered randomly

throughout atom

Instantly accepted

Rutherford

Gold foil experiment 1909 –

Mass concentrated in very small core at

the atom’s center (nucleus)

Nucleus positive, negative electrons moving

around it

Contributions:

Field of Nuclear Physics (1898, alpha and beta

particles)

Radioactive decay

Rectify Periodic Table

Bohr

Doctoral Thesis 1911– Theory of

Electrons

In 1913, theorized electrons revolve

around the nucleus in energy levels (Einstein

and Planck)

Energy levels closest to nucleus have low energy

Energy levels increase in energy with distance from

the nucleus

Electrons gain and lose energy by moving between

energy levels (quantum)

“This is an enormous achievement” Einstein

James Chadwick (1932)

Identified neutron

Modern Atomic Model

The atom consists of three main

(charged) particles:

Protons (positive)

Neutrons (neutral)

Electrons (negative)

Two main parts:

Nucleus

Electron cloud

Atomic Model (cont.)

Nucleus contains:

Protons (+)

Neutrons (0)

Nucleus surrounded by:

electron cloud Negative charge due to electrons

Mass of an atom measured in amu’s, or

atomic mass units.

Atomic Structure

Atomic Structure

Atomic Model (cont.)

Protons and neutrons are found in the

nucleus, so it has a positive charge

Electrons are found in the electron cloud,

so it has a negative charge

The mass of an atom is measured in

amu’s, or atomic mass units. It is a unit

of measurement of protons and neutrons

Atomic Mass Units

1 amu is defined as 1/12 the mass of a

carbon atom containing 6 protons and 6

neutrons

1 amu is also the mass of 1 proton or 1

neutron

An electron has a mass of 1/2000 amu

Key Terms

Atomic number: the number of protons in

the nucleus of an atom.

Mass number: the sum of the number of

protons AND the number of neutrons in

the nucleus.

Mass # = # protons + # neutrons

Atomic Symbol

Isotopes

Isotopes are atoms of the same element

with differing numbers of neutrons.

Isotopes have different masses

Isotopes

Isotopes of Carbon

How do you record the mass

of a group of isotopes?

Because most elements have more than

one isotope, each element is given an

average atomic mass

The average atomic mass is the average

mass of the mixtures of its isotopes

How do you calculate the

average atomic mass of an

atom?

The number of naturally occurring

isotopes, their masses, and their percent

abundances must be known.

Example: Lithium has 2 isotopes: Li-6

(mass 6.015 amu and 7.5% abundance),

and Li-7 (mass 7.017 amu and 92.5%

abundance). What is its average atomic

mass?

How do you calculate the

average atomic mass of an

atom?

Calculate the average atomic mass of

silicon. The three silicon atoms have

masses of 27.98 amu, 28.98 amu, and

29.97 amu with relative abundances of

92.23%, 4.67%, and 3.10%, respectively.

Charged Atoms

In a neutral atom, the number of protons

equals the number of electrons. The

positive and negative charges balance

out, leaving the atom with 0 net charge

In a charged atom or ion, there is an

uneven number of protons and electrons,

so the atom will have either a positive or

negative net charge

Ions