Section 2.0 An understanding of the nature of matter has developed through observations over time.

Goals for this Section: 1.Describe ancient theories of matter and chemical change 2.Distinguish between observation and theory 3.Describe the different models of the structure of the atom 4.Identify patterns that are used to group elements 5.Describe the arrangement of atoms within the periodic table

6.Use the periodic table to describe the properties of individual atoms 7.Explain relationships within periods and groups within the periodic table

2.1 – Evolving Theories of Matter We could consider stone- age people to be the very first chemists They learned to use fire as a tool to produce chemical changes that made their lives easier

Early Interests in Metals & Liquids Until about 1000 B.C., chemists only investigated materials that had high value to humans Most of these materials were metals, such as gold, copper, and iron Other substances of interests were liquids such as oils and plant juices

The Idea of the Atom The first scientist to suggest an idea of an atom was Democritus, in about 400 B.C. He suggested that all matter was made up of very small, indivisible particles known as atomos However, another Greek philosopher had a more popular idea…

Earth, Wind & Fire (… and Water) Aristotle suggested that all materials contained different proportions of four elements – Earth, Wind, Fire, and Water This idea was the preferred theory about the composition of matter for about 2000 years

Alchemy Alchemy was a field of study where the “scientists” attempted to find elixirs that would extend life, or to develop a method of turning base metals into gold

Alchemy and Chemistry In fact, some of the most famous alchemists were in fact good con-men who managed to convince nobles to provide them with money in return for recipes for creating gold from base metals However, these alchemists did contribute to what we now know as legitimate chemistry

Many of the chemical reactions that were used by alchemists are now used by chemists, and much of the specialized apparatus that is used in chemistry was developed by alchemists as well

The Modern Chemists Early chemists like Robert Boyle and Antoine Lavoisier began to develop theories about matter and reactions based on observations and experiments Eventually, by the 1800s, an atomic theory began to take shape

Dalton’s Model

J.J. Thomson’s Experiment

Thomson’s Model

Rutherford Scattering

Rutherford’s Model

Bohr & Atomic Spectra

The Bohr Model

Modern Models

2.2 – Organizing the Elements Rather than using names for chemicals early scientist used symbols for the elements that they knew

Scientists tried to organize elements by using their properties, however, this was difficult because different scientists used different properties John Dalton first attempted to categorize and order the elements in the early 1800’s. He developed a new system of symbols

Jöns Jacob Berzelius In 1814 Berzelius suggested using letters rather than symbols the first letter is capitalized if two elements had the same first letter (like Hydrogen and Helium) then the second letter would be used this system is still in use today

The Evolution of Symbols

Ordering the Elements order for the elements was soon discovered by using the atomic mass as a means to list them scientists could establish the average mass of an atom by comparing it to the mass of a carbon atom in 1864 John Newlands recognized that there was a pattern of repeating properties within the elements when they were listed in increasing mass

Dimitri Mendeleev Dmitri Mendeleev arranged elements by their chemical properties When he did this, a clear pattern emerged There were holes in his pattern created by undiscovered elements These holes were filled over time – showing that Mendeleev’s table was accurate

2.3 – The Periodic Table

Divisions Within the Periodic Table The table can be divided into horizontal rows (periods) or vertical columns (groups or families) Each box within a periodic table contains important information

Useful Information on Each Elements O oxygen

Atomic Mass and Isotopes The atomic mass of an element provides you with the total mass of the protons and neutrons in an atom However, you will notice that not all atoms have atomic masses that are whole numbers Because you cannot have fractions of protons and neutrons, what is happening here?

It turns out that the atomic mass is the average weighted mass of different isotopes of an atom For example, there are three different isotopes of hydrogen:

Protons + Neutrons = Atomic Mass Fun! We can determine the number of protons and neutrons in the nucleus of an atom by using its atomic number and its atomic mass # of Protons (Atomic Number) + # of Neutrons _______________ Atomic Mass

Example: Number of Protons & Neutrons in Uranium Uranium has an atomic mass of How many protons and neutrons are present in this element?

Metals & Nonmetals There are two major groups of elements in the periodic table – the metals and the nonmetals

Metals Properties:

Nonmetals Properties:

Groups There are 18 groups on the modern periodic table The elements that make up a particular group typically have similar reactivity and often engage in the same reactions and form similar products

Periods Periods are repeating patterns of elements created by the separation of elements into their groups The atomic mass of elements increases as you move from left to right along a period

General Reactivity of Elements

Group 1 – Alkali Metals These are the most reactive metals They will react when exposed to air or water Typically will not be found in their elemental state in nature

Group 2 – Alkaline Earths Reactive metals, although their reactivity is much lower than the alkali metals React readily with air and water Many are combustible

Group 17 – The Halogens These are the most reactive nonmetals They are all diatomic, meaning that in their natural state, two atoms of the element are bonded together

Group 18 – The Noble Gases These gases are all very stable – in fact, until 1962 it was believed that they would not react at all