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ChemistryDevelopment of the Atomic Model
What is an atom?
• An atom is the smallest particle of any element that still retains the characteristics of that element.
• Atoms consist of even smaller particles also known as subatomic particles – protons, neutrons and electrons.
The Atom and BaseballAtom = Skydome, Toronto
Nucleus = Baseball
Protons = Marbles inside baseball
Electrons = mosquitoes buzzing around the baseball
Ancient Greek Philosophers
• ~500 BCE▫Wondered why matter behaves as it does
▫They manipulated ideas in their minds but did almost no real experimentation.
Early Models:Ancient Greek PhilosophersEmpedocles: • 450 BCE
• Believed everything in the world was made up of 4 substance – earth, fire, air, and water
• Known as the “four element model”
Democritus• 400 BCE• Believed that matter was made up of
tiny particles that cannot be broken down further.
• If you were to rip something into smaller and smaller pieces, eventually you would not be able to rip apart any more pieces.
• He called these particles “atomos” which mean indivisible.
• Aristotle
• 350 BCE
• After the death of Democritus, Aristotle and Socrates rejected the atomic model and adopted Empedocles’ four element model.
• This model influenced and dominated scientific thinking for almost 2000 years.
Early Chemists
• Scientists learned to take matter apart by ordinary chemical means until it would not break down any further – able to determine if a substance was an element or compound.
• Through the 17th and 18th century scientists worked with matter by heating, burning, mixing and cooling.
Alchemists
500-1600 CE • They were the first people to perform hands on
experimentation.
• They were part philosopher, mystic, magician, and chemist.
• They had 3 main beliefs.
Alchemists1. That some elements could be changed into
others, such as turning lead into gold (interestingly they discovered a lot of other elements in the process)
2. They could find a substance that would give them eternal life
3. They could produce a universal solvent that would dissolve all substances
Modern Chemists
• 1600 – Present Day
• Use the scientific method to investigate the physical world
• Focused on identifying pure substances and what they are made of
Sir Francis Bacon
•1600s
•One of the first scientists to develop new knowledge as a result of experimentation
•Often considered the father of the modern scientific method
Robert Boyle• 1650
• Went back to the “four elements” model and thought it could be improved upon
• He helped lay the foundation for the concepts of elements vs. compounds
• Recognized that elements could be combined to make compounds
• Believed that air was not an element but a mixture – he was correct!
Antoine de Lavoisier•1700s
• Defined the term element as a pure substance that cannot be chemically broken down into simpler substances
• He discovered and identified 23 elements
• His work was based on careful measurements & observations
Henry Cavendish• Late 1700s
• Experimented by mixing metal with acid, which produced a flammable gas (hydrogen).
• He discovered that his gas would burn in oxygen and produce water.
• Until that time, water was thought to be an element
Development of the Atomic ModelJohn Dalton (1766 - 1844)• Credited with developing a theory
that was a new way of explaining matter.
• He studied gases that make up the Earth’s atmosphere.
• Based on his studies, he suggested that:▫ matter is made of small, hard spheres that are
different for different elements ▫ the smallest particle of an element is called an
atom • This is the basis for Dalton’s Atomic Theory.
Dalton’s Model of the Atom“Billiard Ball Model”
• Atom is a solid, indivisible, indestructible sphere
• Atom contains no subatomic particles.
Dalton’s Atomic Theory1. All matter is made of small particles called
atoms.2. Atoms cannot be created, destroyed, or
divided into smaller particles.3. All atoms of the same element are identical in
mass and size, but they are different in mass and size from the atoms of other elements.
4. Compounds are created when atoms of different elements link together in definite proportions.
Dalton’s Atomic Theory
• Dalton’s model uses the idea that elements are different because their ‘particles’ (atoms) are different.
• This led to a more accurate definition of the word element.
• Element: ▫ 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 a chemical change.
J.J. Thomson (1856 – 1940)
• Conducted experiments which identified first negative charges and then later positive charges.
• This disproved Dalton’s model
• Considered the “Plum Pudding” or “Raisin Bun” Model
Thomson proposed:• Electrons have a negative charge & a small
mass
• An atom is a sphere of positive electricity
• Negative electrons are embedded in the positive sphere, resulting in a neutral charge
Ernest Rutherford (1871 – 1937)
• Ernest Rutherford (a student of Thomson's) was responsible for a remarkable series of discoveries in the fields of radioactivity and nuclear physics.
• He performed many experiments and discovered that radioactivity included 3 types of radiation which he named: alpha particles, beta particles and gamma rays.
• After experimenting with charged particles, he found that some particles were deflected in directions not originally predicted.
• He suggested that the deflection of the charged particles was because the atom contained a tiny dense center called a nucleus and electrons moved around the nucleus.
• Most importantly – he essentially ‘predicted’ the nuclear structure of the atom.
• Nucleus is surrounded mostly by empty space.
Rutherford’s theory suggests that an atom has a small, dense, positively charged nucleus with electrons moving around the nucleus.
Niels Bohr (1885 – 1962)
• Niels Bohr, a young Danish physicist, and a student of Rutherford’s, believed Rutherford's model needed improvement.
• The nuclear atom certainly explained the results of the gold foil experiment, but it did not agree with what was already known about electricity.
• Bohr studied gaseous samples of atoms, which were made to glow by passing an electric current through them.
• It was Bohr who explained why electrons do not spiral into the nucleus.
• Based on his observations, Bohr proposed that electrons surround the nucleus in specific “energy levels” or “shells.”
• He inferred a model that compared the atom to the solar system.
• He compared the nucleus to the sun and the electrons to the planets. Considered the Planetary Model.
• The sun emits an enormous gravitational pull on the planets but they do not spiral inward and crash. Why?▫ Because they revolve at just the right speed to
remain in their orbits.
• Similarly, the atom's positive nucleus exerts a strong force of attraction on the negative electrons.
• The electrons do not spiral in and crash because they are moving rapidly in fixed regions around the nucleus. Electrons move around the nucleus in nearly circular orbits, at a fixed distance from the nucleus.
• These regions are 3 dimensional and sphere like → called electron shells.
• Each electron in an orbit has a definite amount of energy. The further the electron is from the nucleus, the greater its energy.
• Electrons cannot exist between the orbits, but can move up or down if excited by heat, light or electrical energy.
• Electrons are more stable when at lower energy levels (closer to the nucleus).
• The model of the atom with a central nucleus and electron shells is called the Bohr Rutherford Model.
The Quantum Mechanical Model
• This model is based on mathematics. Although it is more difficult to understand than the Bohr model, it can be used to explain observations made on complex atoms.
▫ Resulted from several discoveries in the field of physics by various scientists. It is the model we accept today. It is also known as the wave model.
• The quantum mechanical model is based on quantum theory, which says matter also has properties associated with waves. According to quantum theory, it’s impossible to know the exact position and momentum of an electron at the same time.
The quantum mechanical model of the atom uses complex shapes of orbitals (sometimes called electron clouds), volumes of space in which there is likely to be an electron. So, this model is based on probability rather than certainty.
• The main points of this model:
▫ Electrons don’t move around the nucleus in a definite path
▫ It is impossible to determine their exact location.
▫ The probable location of the electron is based on its energy.
▫ The Quantum Model shows how electrons move randomly in electron clouds called an orbital.
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