Atomic Structure

Protons and Neutrons are both located in the nucleus, and weigh about the same. The weight of the protons and neutrons make up the atomic mass.

Electrons are located in a cloud around the nucleus, floating in their orbits. Electrons weigh almost zero.

The number of protons (positive) and electrons (negative) are the same in a NEUTRAL atom. The number of protons is the atomic number, so if a proton is added, a new element would be formed.

If an atom has a charge, meaning the number of protons and electrons AREN'T the same, then it is called an ION. Atoms that have gained or lost electrons become ions.

ANION: a negatively charged ion, meaning it GAINED elections. (non-metals gain electrons)

CATION: a positively charged ion, meaning it LOST electrons. (metals tend to give away electrons)

The mass number is the total number of protons + neutrons.

The atomic mass of the periodic table is the average mass of an elements isotopes. The atomic mass should be close to the mass number.

If a NEUTRON is added to an elements nucleus, then a heavier version of the SAME element is formed. This is called an ISOTOPE.

So to review:

When a proton is added, a new ELEMENT is formed.

When a neutron is added, an ISOTOPE is formed.

Let's move on the natural mixtures. There can not be half of a proton/neutron, so when the periodic table states that the atomic mass of chlorine is 35.5, we know that it must be the average mass of the isotopes.

When given the percentage of each isotope in a mixture, and a precise mass for each of those isotopes, we can calculate the average mass of it. This can be done simply by multiplying the percentage and the mass of each, and adding the products together.

Ex/ Given the following information, calculate the average mass of this oxygen mixture.

Oxygen-16: 99.763%, 15.99491463g
Oxygen-17: 0.038%, 16.9991312g
Oxygen-18: 0.020%, 17.9991603g

0.99763 x 15.99491463  = 15.9570067 g
0.00038 x 16.9991312 = 0.00645967 g
0.00020 x 17.9991603 = 0.003599832 g

Now add all the products together to get your average mass:

15.9570067 + 0.00645947 + 0.004599832 = 15.967 g