THE ATOM

Four important pieces of information…

Atomic Number• Number of protons in an element• Defines the element

Chemical Symbol• 1 or 2 letter symbol• 1st letter is always capitalized• If there is a second letter is it

lowercase

Element Name

Atomic Mass - More on this later

Atomic NumberThe atomic number of an element = the number of protons

• The number of protons is what defines an element!

IF the element is neutrally charged (not an ion) then… the number of protons = the number of electrons

Mass NumberMass number = number of protons + number of neutrons

This is NOT the number shown on the periodic table, the periodic table shows the atomic mass – more on this later.

Determining an Atom’s Composition

If you know the atomic number and the mass number of an atom then you know how many protons, neutrons, & electrons it has.

Atomic Number = Number of protons & electrons

Mass Number = Protons + Neutrons

SO… Neutrons = Mass Number – Atomic Number

Practice – Complete the chart belowElement Protons Electrons Neutrons

Helium

Titanium

Sodium

Krypton

Practice – Complete the chart belowElement Protons Electrons Neutrons

Helium 2 2 2

Titanium 2 2 26

Sodium 11 11 12

Krypton 36 36 42

Isotopes• For many elements there are multiple types of that element,

called isotopes.• Isotopes have the same number of protons, but different

numbers of neutrons.• Because they have different numbers of neutrons their

mass numbers are different.• Isotopes have the same chemical properties because they

have the same number of protons and electrons• Isotopes can (but don’t always) have different physical

properties because of their difference in neutrons.

Isotopes are identified by the element name, followed by a dash with the mass number of that isotope.

For example: Carbon-12 and Carbon-13 are two isotopes of Carbon.• Because they are still Carbon, they both have 6 protons (where

did I find this?)• Carbon - 12 has a mass number of 12• Carbon - 13 has a mass number of 13

Since the Number of neutrons = Mass Number – Atomic Number

Carbon – 12 has 6 Neutrons (12 – 6)

Carbon – 13 has 7 Neutrons (13 - 6 )

HW due Wednesday

Side 1 (chart) of worksheet

Atomic Mass• The mass that is found on your periodic table is the

atomic mass.• Atomic mass is the average of the masses of an

element’s different isotopes adjusted for how frequently each one is found in nature.

So what does this mean? Let’s look at an example…

Atomic Mass Example 1

Carbon-12 is found in 99% of all Carbon atoms while Carbon -13 is found in only 1%

So the atomic mass of carbon is 99% of the Carbon-12 mass + 1% of the Carbon-13 mass

Atomic Mass Example 2

What is average atomic mass of Lithium if 7.42% exists as Lithium–6 and 92.58% exists as Lithium–7?

Average Atomic Mass = (6 * .0742) + (7 * .9258) = 6.9258

round to the thousands = 6.926

For these problems the mass will not be the exact same as the mass listed on the periodic table, other isotopes have been left out or percentages may be slightly changed… otherwise you could just look at your periodic table for the answers

Atomic Mass Example 3

Neon has two major isotopes, Neon-20 and Neon-22.

Out of every 250 neon atoms, 225 will be Neon-20 and 25 will be Neon-22.

What is the average atomic mass of Neon? 

= (20 * (225/250)) + (22 * (25/250)) OR (20*.9) + (22 *.1)

Atomic Mass Example 4

Without doing any math, are there more Bromine-79 atoms or more Bromine-80 atoms on earth? (Hint: look at the periodic table)

Answer:

Bromine on the periodic table has a atomic mass of 79.8, because the number is closer to 80 then it is to 79 we know that there is a greater percentage of Bromine-80 on Earth.

Atomic Mass Example 5

Magnesium has three naturally occurring isotopes.

78.70% of Magnesium atoms exist as Magnesium-24

10.03% exist as Magnesium-25

11.17% exist as Magnesium-26

What is the average atomic mass of Magnesium? 

= (24*.787)+(25*.1003)+(26*.1117) = 24.2997 or 24.300