Exploring Inner Space

TEKS 8.5A describe the structure of atoms including the masses, electrical charges and locations, of protons and neutrons in the nucleus and electrons in the electron cloud

TEKS 8.5B identify that protons determine an element’s identity and valence electrons determine its chemical properties, including reactivity

Each element on the Periodic Table is made of the same

kind of atom.

Each atom in an element has the same number of

protons.

The number of protons present in the atom is what determines the

element’s identity and is shown by the atomic

number.

Structure of the Atom

The Nucleus The center of the atom is called the nucleus .

proton

neutron

nucleus

3 Li

6.941 Lithium

The nucleus contains protons and neutrons.

Since each proton and neutron in the nucleus has a mass of 1 atomic mass unit (amu), we can add the number of protons and neutrons together to find the atomic mass.

On your Periodic Table, fill in this information.

# protons = element’s identity = atomic number

# protons + # neutrons = atomic mass

On your Periodic Table, fill in this information.

The Periodic Table shows the mass for each element as a decimal.

We will usually round the atomic mass number.

If the mass number is known, and you know the number of protons, how can you find the number of neutrons?

Mass # - # Protons = # Neutrons

fill in this information on your Periodic Table

mass - # protons = # neutrons

# protons = # electrons in a stable atom

electron

energy levels

3

Li

6.941

Lithium

electron cloud

The Electron Cloud and Energy Levels

Surrounding the nucleus is an area called the electron

cloud. Inside the electron cloud, electrons orbit around in different energy levels.

The electrons are so small that they basically have no

mass, so they are not included in finding the

atomic mass. electron

neutron

proton

first energy level The first energy level, which is closest to

the nucleus, holds up to 2 electrons.

second energy level

The second energy level holds up to 8 electrons.

third energy level The third energy level may hold 8 or more

electrons.

8 or more!

How many electrons are in each energy level of the element Lithium?

electron

energy levels

3

Li

6.941 Lithium

electron cloud

1st – 2, 2nd – 1

By numbering the periods (rows) on the Periodic Table, you can determine how many energy levels are in an atom. For example, Lithium is in period 2, therefore it has 2 energy levels.

Number the first three periods on your

Periodic Table.

How many energy levels does sodium contain?

Period 3, 3 energy levels

The most stable form of an atom is electrically neutral. This means that the number of protons and electrons is equal.

Atomic # = Protons = Electrons

Valence Electrons The numbers of electrons in an atom’s outermost energy level are called valence electrons.

These electrons are important because they

determine how reactive the element is with other

elements.

Valence electrons…..

valence electrons are found here!

For example, look at the element oxygen. Oxygen is atomic number 8, which means it has 8 protons (+).

Therefore, in a stable oxygen atom, there are 8 electrons (-). The positive and negative charges are balanced.

6

Look at your Periodic Table. Find the element Carbon.

How many positive

(protons) charges does carbon have?

How many negative charges (electrons) does carbon have?

6

What is the mass of carbon? 12.0112

How can the number of neutrons be determined?

(atomic mass - # protons = # neutrons)

Carbon has 6 neutrons.