Russian chemist Dmitri Mendeleev placed the known elements in order of increasing atomic mass.

When he did this he noticed that the elements’ properties repeated in a regular pattern, or a periodic pattern.

Mendeleev placed the known elements in a table, where he arranged elements into columns with similar properties.

Mendeleev predicted the properties of several elements that were unknown at the time.

Many of his predictions were correct and were well accepted by the scientific community.

The only changes to Mendeleev’s periodic table were the addition of newly found elements and that the table was organized by atomic number rather than atomic mass.

The horizontal rows on the periodic table are called periods. This is because the properties begin to repeat in each new row.

The columns on the periodic table are called groups (or family) and the elements in the groups have similar properties.

Groups are designated with a number and the letter A or B.

The main group elements can be found in Groups 1A through 8A.

The main group elements (Group A elements) are also called representative elements.

The group B elements are called the transition elements.

Elements can be divided into three main classes.

Those classes are:› Metals› Metalloids› Nonmetals

Metals have the following properties. They are generally:› Shiny solids› Good conductors of heat and electricity

Group 1A has the name alkali metals Group 2A has the name alkaline

earth metals.

Elements to the right of the heavy stair-step line on the periodic table are called nonmetals.

Nonmetals generally have the properties that they are generally gases or brittle solids at room temperature.

Group 7A are called halogens. Group 8A are called the noble gases.

Many of the elements that border the stair step are metalloids.

Metalloids share properties in between those of metals and nonmetals.

Column A1. Strontium2. Chromium3. Iodine4. Nitrogen5. Argon6. Rubidium7. Silicon

Column Ba. Halogenb. Noble gasc. Alkaline earth metald. Metalloide. Alkali metalf. Representative

elementg. Transition Element

Scientists now understand that the repetition of properties of elements occurs because the electron configurations of atoms repeat.

The arrangement of elements in the periodic table reflects the electron structures of atoms.

For representative elements, the Group Number in front of the A tells the number of valence electrons in the atoms in the column.

Also the period number (or row number) of a representative element tells the energy level of the valence electrons.

The periodic table is divided into blocks that correspond to the energy sublevel being filled as you move across a period.

Groups 1A and 2A are the s block. Groups 3A- 8a are the p block. The B elements represent the d block;

however remember to go down one for the first quantum number.

The rows that are removed from the table and placed at the bottom represent the f block. Go down 2 numbers for the row number.

Without using the periodic table, determine the group, period, and block in which an element with each of the following electron configurations is found.

1. [He] 2s2 2p5

2. [Ar] 4s2

3. [Kr]5s2 4d10 5p3

4. [Ar]4s2 3d3

Section 6.3

Periodic Trends

The electron structure of an atom determines many of its chemical and physical properties.

There are several trends that can be observed using the periodic table.

The atomic radius is a measure of the size of an atom. The larger the radius, the larger the atom.

As you move across a period the atom decreases in size.

This is due to the increasing positive charge of the nucleus while electrons are being added, but the orbitals are close in energy.

The increased nuclear charge pulls the outermost electrons closer to the nucleus, making the atom smaller.

As you move down a group, atomic radii increases.

This is due to the addition of a larger energy level each time.

Electrons in higher levels are located farther from the nucleus than those in the lower energy levels.

Trend

From each of the following pairs, predict which atom is larger.

1. Mg, Sr2. Sr, Sn3. Ge, Sn4. Ge, Br5. Cr, W

Ion Formation

The driving force that makes reactions happen is ion formation.

Ions form when atoms gain or lose electrons.

When electrons are gained or lost the resultant ion has a positive or negative charge.

Cations When atoms lose electrons and form

positively charged ions, they become smaller, and are called cations.

They become smaller because the loss of the electrons means that the number of protons is greater than the number of electrons.

Therefore, the electrons will be pulled more tightly to the nucleus, and the outer electrons will feel the pull of the nucleus more strongly than before.

Anions

When atoms gain electrons they form negatively charged ions, called anions.

The added electron makes the ionic radius increase, because of repulsion and the weaker pull of the nucleus on each electron.

Trend in Ionic Radius

As you move from left to right across the periodic table, your positively forming ions, Groups 1A-4A get smaller, between 4A and 5A it gets larger but Groups 5A- 8A get smaller.

As you move from top to bottom the ions get larger in each column because of the addition of energy levels.

Ionization Energy To form a positive ion, an electron must be

removed from a neutral atom. Removing the electron requires energy.

That energy must overcome the attraction between the positive charge in the nucleus and the negative charge of the electron.

This energy, known as ionization energy is defined as the energy required to remove an electron from an atom in the gaseous state.

First Ionization Energy The first ionization energy is the amount

of energy required to remove the first electron from the outer shell of the atom.

Remember that as you move across a period, you increase atomic number and therefore add more positive charge to the atom.

That addition makes it more difficult to remove the electron and therefore more energy is required.

Ionization Energy

Trend in Ionization Energy

For the first ionization energy, as you move across a period, from left to right, the ionization energy increases.

As you move down a group, the ionization energy generally decreases. Why?

Trend

Additional Ionization Energies

It is also possible to remove electrons after removing the first, or furthermost electron from the electron cloud.

However, once you remove the first electron, much more energy is required to remove the second, and third, and fourth, etc.

Octet Rule

Remember, when Newland tried to design his periodic table he came up with a law of octaves, which wasn’t accepted.

We see using ionization energies that his predictions were every correct.

When atoms/ions have eight electrons in their outer shell, they are much more stable.

Octet Rule

The octet rule has come to be one of the most important principles in chemistry.

It says that atoms tend to gain, lose or share electrons in order to acquire a full set of eight valence electrons.

Use this to predict what kinds of ions will form: elements on the left will form positive ions and elements on the right negative ions.

Electronegativity

The electronegativity of an element tells about its ability to attract electrons in a chemical bond.

The more electronegative an element, the more electron loving it is.

Noble gases form few compounds and so virtually lack electronegativity.

Trend

As you move from top to bottom down a group, electronegativity generally decreases.

As you move from left to right, electronegativity generally increases.

So what is the most electronegative element?

Trend