The Atom

What’s Inside an Atom

The Nucleus• The nucleus is the small dense, positively

charged center of the atom. It contains most of the atom's mass.

The Proton

Charge: positive

Mass: 1 amu

Location: nucleus

The Neutron

Charge: Neutral

Mass: 1 amu

Location: Nucleus

The Electron

Charge: negative

Mass: almost zero

Location: electron cloud (outside the nucleus)

The Atomic Number• The atomic number is the number of

protons in the nucleus of an atom

• The number of protons determines the element

Atomic Number

Mass Number• The mass number is the sum of the

protons and neutrons in an atom (measured in amu)

Mass Number

Isotopes• Isotopes are atoms that have the same

number of protons, but have different numbers of neutrons.

Calculating the number of Neutrons in an Atom.

How many neutrons are in a Carbon atom with an atomic weight of 14.

Atomic weight is the number of protons and neutrons.

The atomic number is the number of protons………….so,

14 – 6 = 8 neutrons

Average Atomic Mass

• The average atomic mass is the weighted average of the masses of all the naturally occurring isotopes of an element.

Calculating the Average Atomic Mass

What is the average atomic mass of Chlorine;

if 75% is chlorine-35, and

25% is chlorine-37?

(35 X .75) + (37 X .25) =

(26.25) + (9.25) = 35.5 amu