What Are Elements? come on meet the elements Topic 3

I Can… statements  I can distinguish between observation and theory, and provide examples of how models and theoretical ideas are used in explaining observations  Example: Describe how observations of electrical properties of materials led to ideas about electrons and protons; describe how observed differences in the densities of materials are explained, in part, using ideas about the mass of individual atoms)

History of Elements 1. Greek philosophers observed that rock could be broken down into smaller and smaller pieces until it became powder. 1. They then wondered how many times you could continue to break the particles of powder down until they couldn’t be broken any further

History of Elements 2. Democritus described the smallest particle as atomos, meaning “indivisible.” 1. He believed that each type of material was made up of a different atomos. These different particles gave materials their own unique set of properties. These could then be added to make new materials with their own properties.

History of Elements 3. Aristotle then stated that everything was made of earth, fire, air and water. Since he was well respected, his description of matter was preferred for 2000 years.

From Alchemy…  For the next 2000 years, alchemists, people who were part magician, part scientist, carried out experiments.  The study of alchemy is known as a pseudo-science. It is a not a real science because it contains magic.  They believed that they should be able to turn any metal into gold and they were not interested in understanding the nature of matter.  They did, however, perform some of the first chemistry experiments.

… to Chemistry  From the 1500s onwards, scientific experiments were carried out and Sir Francis Bacon proposed that science should be based on experimental evidence instead of thoughts and ideas.  Robert Boyle researched and realized that elements formed compounds. He was convinced that matter was made of smaller particles (just like Democritus had proposed) and that that the purpose of chemistry was to determine the types of particles making up each substance.  The work of these two men led to continued research on the topic.

 In the 17 th and 18 th centuries, scientists continued to search for elements.  Antoine Lavoisier is known as the “father of modern chemistry.”  He heated, burned, mixed and cooled matter and found 23 pure elements.  Unfortunately, he was then beheaded.

Antoine Lavoisier  He was very careful with his experiments and kept very organized observations and data.  Since he measured the masses of the matter he worked with, he developed the… law of conservation of mass.  Law of Conservation of Mass  In a chemical change, the total mass of the new substances is always the same as the total mass of the original substances.

Antoine Lavoisier  His accurate measuring also led others to determine the Law of Definite Composition  Law of Definite Composition  Compounds are pure substances that contain two or more elements combined together in fixed (or definite) proportions  Examples  Water (H 2 O) always contains 89% Hydrogen and 11% Oxygen.  Hydrogen Peroxide (H 2 O 2 ) always contains 6% Hydrogen and 94% Oxygen.

Dalton’s Atomic Theory  Earlier, we mentioned the Particle Model of Matter – but that was revised because it didn’t explain everything.  John Dalton created a new way to think about substances.  Dalton’s Atomic Theory  All matter is made up of small particles called atoms  Atoms cannot be created, destroyed or divided into smaller particles  All atoms of the same element are identical in mass and size  Atoms of one element are different in mass and size than another element.

 An element is a pure substance made up of one type of particle, or atom. Each element has its own distinct properties and cannot be broken down into simpler substances by means of chemical change.  A compound is a pure substance that is made up of two or more elements chemically combined together. Compounds can be broken down into elements again by chemical means.

Laws, Theories, Models and Observations  Law – an action or condition that has been observed so consistently that scientists are convinced it will always happen  Theory – an explanation of an event that has been supported by consistent, repeated experimental results and has therefore been accepted by a majority of scientists  Model – a mental image, used as a building block that helps to explain an event.  Observation – the use of the senses to gather information.

Laws, Theories, Models and Observations Time and more experiments ObservationsHypothesisExperiments TheoryRevised Theory Repeated as many Revised times as necessary Hypothesis

History of Atomic Models Scientist(s)Model J.J. ThomsonPlum Pudding Model Hantaro Nagaota and Ernest RutherfordPlanetary Model Niels BohrRevised Planetary Model VariousCurrent Model Refer to page 113 in your textbook

Bohr Diagram  We continue to look at Niels Bohr’s diagram of the atom  Electrons in the outer shell are called valence electrons  An atom is the most stable when the outer shell is full  Atoms will give or take electrons to make their outer shell full. Inside nucleus: Protons (+1) Neutrons (N) Electrons (-1) Nucleus Electron shells 1 st shell: 2 e - 2 nd shell: 8 e - 3 rd shell: 8 e - 4 th shell: 18 e -

Example  Once an atom gives or takes electrons, it becomes an ion.  The charge on the ion depends on how many electrons are transferred.  This nitrogen atom has 5 electrons in its outer shell – it can give 5 or take 3 to make its outer shell full.  What is easier? Take 3 electrons.  If it takes 3 negative electrons, the charge becomes -3. We write this as N -3.

Practice  Element: Oxygen  Components: 8 protons, 7 neutrons, 8 electrons  Element symbol: O  Bohr Diagram:  Electron transfer: Take 2 e -  Ion symbol: O -2

Practice  Element:  Components:  Element symbol:  Bohr Diagram:  Electron transfer:  Ion symbol: