Bell Work

• Those of you who did not take your Test Friday need to sit in the back 2 rows. All others need to sit in the front 3 rows.

• How are elements arranged on the periodic table?

Physical Science – Lecture 57

The Periodic Table

Matter

• Matter is used to create atoms. • Atoms are used to create the elements. • Elements are used to create molecules.

Elements

• All elements are made of atoms. • The differ in weights and organization.• All atoms are built the same way.• Every element has electrons, protons, and

neutrons.

Elements on the Periodic Table

• The elements listed on the periodic table cannot be changed using any chemical method.

• Each element has a unique number of protons, giving each element its own atomic number.

Arrangement of Elements

• If you examine samples of iron and silver, you can't tell how many protons the atoms have.

• However, you can tell the elements apart because they have different properties.

• You might notice there are more similarities between iron and silver than between iron and oxygen. Could there be a way to organize the elements so you could tell at a glance which ones had similar properties?

The First Periodic Table

• Dmitri Mendeleev was the first scientist to create a periodic table of the elements similar to the one we use today.

• Mendeleev's original table was developed in 1869.

Mendeleev's Table

• When the elements were ordered by increasing atomic weight, a pattern appeared where properties of the elements repeated.

• This periodic table is a chart that groups the elements according to their similar properties.

Why is the Periodic Table Useful?

• Many elements remained to be discovered in Mendeleev's time.

• The periodic table helped predict the properties of new elements.

Mendeleev's Table

• The modern periodic table has over 100 elements.

• Mendeleev's table didn't have very many elements.

• He had question marks and spaces between elements, where he predicted undiscovered elements would fit.

Mendeleev’s Table

• Mendeleev found he could arrange the 65 elements that were then known in a grid or table so that each element had:

• 1. A higher atomic weight than the one on its left.

• 2. Similar chemical properties to other elements in the same column.

What about the other elements?

• In his table he noted gaps - spaces where elements should be but none had yet been discovered.

• Mendeleev could be said to have discovered germanium (which he called eka-silicon because he observed a gap between silicon and tin), gallium (eka-aluminum) and scandium (eka-boron) on paper, for he predicted their existence and their properties before their actual discoveries.

Discovering Elements

• The gaps in Mendeleev’s table helped him predict the existence of elements that had not been discovered yet.

Discovering Elements

• Elements with different numbers of protons are different elements.

• The gaps in atomic number on his table showed where new elements existed.

Adding to Today’s Table

• When you look at the modern periodic table, do you see any skipped atomic numbers that would be undiscovered elements?

Adding to Today’s Table

• New elements today aren't discovered. • They are made in labs. • You can still use the periodic table to predict

the properties of these new elements.

Properties and Trends

• The periodic table helps predict some properties of the elements compared to each other.

Properties and Trends

• Atom size, energy required to remove an electron, the ability to form a chemical bond, and other properties form patterns across and down the periodic table.

Today's Table

• The most important difference between Mendeleev's table and today's table is the modern table is organized by increasing atomic number, not increasing atomic weight.

Why was the table changed?

• Henry Moseley worked with Rutherford on the model of the atom.

• In 1914, Henry Moseley learned you could experimentally determine the atomic numbers of elements.

• Before that, atomic numbers were just the order of elements based on increasing atomic weight.

• Once atomic numbers had significance, the periodic table was reorganized.

Mendeleev’s Mistake

• They found that the atomic number (electric charge) is most fundamental to the chemical properties of any element.

• Mendeleev had believed chemical properties were determined by atomic weight.

• Moseley correctly predicted the existence of new elements based on atomic numbers.

Today’s Arrangement

• Today the chemical elements are still arranged in order of increasing atomic number (Z) as you go from left to right across the table.

• We also know now that an element's chemistry is determined by the way its electrons are arranged – by its electron configuration.