It is the number of protons that defines an element. Any atom with 79 protons will be a gold atom, and one with ten protons would be neon.

If you added or subtracted one proton, you would have a different element.

(example: gold is #79, add a proton and you have Mercury #80)

The atomic number of an atom is the number of protons in the nucleus.

Hydrogen is atom #1 because it has only one proton, Uranium is atom #92 because it has 92 protons.

The atomic number is always a whole number, since it is impossible to have a fraction of a proton.

Since atoms are neutral, the atomic number is also equal to the number of electrons in an atom.

The number of electrons in an atom is always equal to the atomic number of the element.

3

Li

Lithium The Atomic Number tells you that Lithium has 3 protons in its nucleus

6.941 Since atoms are neutral, it also has 3 electrons orbiting the nucleus

If the number of electrons in an atom does not equal the number of protons, then the particle they form is called an ion.

An ion is an atom that has gained or lost electrons and is no longer neutral.

If you add or subtract electrons from an atom of carbon, it is no longer a carbon atom but a carbon ion.

Since electrons carry a negative charge, if an atom gains electrons, it has more negatives than positives and it is a negative ion, if it loses electrons it is a positive ion.

Lithium atom

(has no charge because it has 3 protons and 3 electrons)

Lithium ion

(has +1 charge because it has 3 protons and 2 electrons)

The Atomic Mass (also called the atomic weight ) is the number of nucleons (protons and neutrons)

in the nucleus. The electrons aren’t counted since they have such little mass.

You can change the number of neutrons in an atom and it will still be the same element. For example, most gold atoms have 118 neutrons, if you added one neutron it would still be gold because it still had 79 protons, but it would be heavier than normal (an atomic mass of 198 instead of 197)

We would call these two atoms different isotopes of gold.

Isotope is just a fancy word for weight. Different isotopes of an element just means different weights.

It is often useful to use isotope notation to describe atoms. In isotope notation the atomic weight number of the isotope is written as a superscript, and the atomic number is written as a subscript. The following example is the isotope notation for a Platinum atom (atom # 78) with an atomic weight of 193

The atomic weight of an element is a usually a decimal number. This is because the atomic weight number is an average of all the isotopes of that element

To determine the most common isotope of an element, round the atomic weight to the nearest whole number.

Example: the atomic weight of silver is 107.8682, so the most common isotope is Ag 108

To find the number of neutrons in an element, round the atomic weight to the nearest whole number then subtract the atomic number.

Example: Lithium’s atomic mass is 6.941, which rounds to 7, it has an atomic number of 3,

7 – 3 = 4 neutrons in the average Lithium isotope

3

Li

Lithium

6.941

Lithium-7 is the most common isotope (weight) of Lithium.

It has 3 protons, and 4 neutrons for a total mass of 7 (electrons don’t contribute to atomic mass)

Lithium-6 is a less common isotope (weight) of Lithium.

It has 3 protons and 3 neutrons for a total atomic mass of 6