The Atom

Basic Atomic Theory

Atom = “indivisible” in Greek Atoms are indivisible and

indestructible Atoms of same element are

identical Compounds are formed by a

combination of two or more different kinds of atoms

Chemical reactions cannot change one element into another Atoms only rearrange

Atoms are Indivisible and Indestructible?

Not true!

Why? Nuclear reactions Subatomic particles

Electrons

Discovered by J.J. Thompson using a cathode ray tube

Negatively charged (-1) Electron’s mass is 1/1840 Free moving around the

nucleus

Protons Discovered by Moseley Positively charged (+1) Proton’s mass is 1 (About 2000 times more

than an electron) Atomic # = # protons

No 2 elements have the same # of protons

Found in the nucleus of an atom

Neutrons Discovered by Chadwick No charge Same mass as proton (1) Found in the nucleus of an atom

Atomic Number

Whole number Atomic # = # of proton Atomic # = # of electrons

(neutral) Atomic # = Z

Shorthand nuclear symbol

Unique for every element

Atomic Number

Mass Number

Whole number Mass # = # of protons + # of

neutrons Mass # = A

Shorthand nuclear symbol Isotopes of the same element

have different mass # Isotopes are the different kinds of

atoms within one element (C-12 and C-13)

Mass Number

Using Mass Number

Mass # = # of protons + # of neutrons

To find # of protons # protons = # electrons (neutral

atom) Need to know mass # and # of

neutrons To find # of neutrons:

Need to know mass # and # of protons

Nuclear Shorthand

Top number is A (mass #) Element symbol in the

middle Bottom number is Z (atomic

#)

Z

AElement Symbol

Atomic Mass

Listed on periodic table Does not equal mass # Given in atomic mass units

(amu) 1 amu = 1/12 the mass of C-12

atom Is not a whole number

Due to relative abundance of naturally occurring isotopes Weighted average

Finding Atomic Mass

To calculate the atomic mass of an element Multiply the mass of each isotope

by its percent abundance (expressed as a decimal) The quantity of a specific isotope that

occurs in a natural sample of an element Add the masses together

Also called average atomic mass

Very Funny… but what is wrong with this?

Isotopes Atoms of the same element

Have the same # of protons and electrons

Have a different number of neutrons Have a different mass number

Have the same physical and chemical properties and undergo the same chemical reactions

Have differing nuclear qualities Some isotopes of an element may be

radioactive Average atomic mass takes all of

the isotopes into consideration

Isotopes

Ions

Atoms are neutral Equal number of protons and

electrons Atoms can become charged by

gaining or losing an electron Called ions

Ions are indicated with a (+) or (-) sign Cations are positive Anions are negative

Cations and Anions