Models of the AtomModels of the AtomParticle model of matter: Particle model of matter: All matter is composed of tiny All matter is composed of tiny

particles called atoms. The particles called atoms. The combination of the types of combination of the types of particles in a substance particles in a substance determines what kind of matter it determines what kind of matter it is . is .

Models of the Atom Through Time:Models of the Atom Through Time:

500 BC: Democritus 500 BC: Democritus Democritus inferred that all matter is Democritus inferred that all matter is

composed of small, indestructible particles composed of small, indestructible particles he named atomos. He taught that there he named atomos. He taught that there was an infinite number of them, and that was an infinite number of them, and that atoms differed in their size, shape, and atoms differed in their size, shape, and weight. He also thought that different weight. He also thought that different substances were made up of different substances were made up of different kinds of atoms. kinds of atoms.

400 BC: Aristotle400 BC: AristotleAristotle didnAristotle didn’’t believe in t believe in

atoms. He thought all atoms. He thought all matter was composed matter was composed of a combination of of a combination of four elements- air, four elements- air, fire, water and earth. fire, water and earth.

His model was, of His model was, of course, accepted for course, accepted for the next 2000 years. the next 2000 years.

1803: John Dalton1803: John DaltonAfter experimenting with After experimenting with

gases, especially gases, especially hydrogen, methane hydrogen, methane and oxygen, Dalton and oxygen, Dalton rediscovered the rediscovered the particle theory of particle theory of matter. matter.

Dalton was able to infer 4 things about matter and Dalton was able to infer 4 things about matter and atoms: atoms:

1.1. Atoms of one element have masses that are Atoms of one element have masses that are different from atoms of other elementsdifferent from atoms of other elements

2.2. Atoms of the same element are identicalAtoms of the same element are identical3.3. An atom of one element cannot be changed An atom of one element cannot be changed

into an atom of another elementinto an atom of another element4.4. Atoms of different elements can be combined Atoms of different elements can be combined

and /or rearranged to form new substances. and /or rearranged to form new substances.

Like Democritus, Dalton thought an atom was a Like Democritus, Dalton thought an atom was a tiny, irreducible sphere that could not be tiny, irreducible sphere that could not be destroyed. destroyed.

1890: JJ Thomson 1890: JJ Thomson Excited by the discovery of X-rays a couple Excited by the discovery of X-rays a couple

of years earlier, Thomson played around of years earlier, Thomson played around with radioactive waves and discovered with radioactive waves and discovered that atoms have electrical charges. Not that atoms have electrical charges. Not only that, he discovered that those only that, he discovered that those charges come from small particles which charges come from small particles which make up part of the atom. He called the make up part of the atom. He called the particles electrons. particles electrons.

Thomson thought that Thomson thought that atoms consisted of atoms consisted of positively-charged main positively-charged main bodies, and attached to bodies, and attached to these were the these were the negatively-charged negatively-charged electrons (plum pudding electrons (plum pudding model). He was wrong model). He was wrong about the details, but his about the details, but his discovery that atoms can discovery that atoms can be broken down into even be broken down into even smaller parts earned him smaller parts earned him science immortality. science immortality.

+

---

---

---

++

+

1909: Earnest Rutherford: 1909: Earnest Rutherford: A student of Thompson, A student of Thompson,

Rutherford designed a Rutherford designed a very famous experiment very famous experiment that proved that that proved that the atomthe atom is mostly empty spaceis mostly empty space: : he bombarded a very thin he bombarded a very thin piece of gold leaf with piece of gold leaf with alpha particles. Although alpha particles. Although some alpha particles were some alpha particles were reflected, most passed reflected, most passed through, proving that a through, proving that a dense nucleus was at the dense nucleus was at the center of the atom, but center of the atom, but not much else. not much else.

RutherfordRutherford’’s model of the Atom s model of the Atom

1912: Niels Bohr1912: Niels BohrBohr improved RutherfordBohr improved Rutherford’’s s

model by noticing that model by noticing that energy levels in atoms energy levels in atoms went up and down by went up and down by specific, specific, ““pre-setpre-set”” amounts. He suggested amounts. He suggested that electrons move that electrons move around the nucleus of an around the nucleus of an atom like planets around atom like planets around the sun, and that they the sun, and that they move frommove from orbit to orbitorbit to orbit as they gain and lose as they gain and lose energy.energy.

This model of the atom is This model of the atom is still the most famous of all still the most famous of all models, and though it is models, and though it is not the most accurate, it not the most accurate, it predicts chemical predicts chemical reactions very well. (It is reactions very well. (It is important to remember, important to remember, though, that atoms though, that atoms probably do not really probably do not really look like this.) look like this.)

1920-present: Particle-Wave models1920-present: Particle-Wave modelsSince Bohr, most scientists have tried to describe Since Bohr, most scientists have tried to describe

atoms using mathematical models instead of atoms using mathematical models instead of physical ones. Generally, in these modern models, physical ones. Generally, in these modern models, they keep the nucleus where Rutherford and Bohr they keep the nucleus where Rutherford and Bohr left it, and try to describe the location and left it, and try to describe the location and movement of electrons, which can best be movement of electrons, which can best be described as a described as a ““cloudcloud”” which surrounds the nucleus. which surrounds the nucleus. Even more recent theories suggest that matter, at Even more recent theories suggest that matter, at its most basic level, isnits most basic level, isn’’t even made of particles at t even made of particles at all, but consists of really tiny fields of vibrating all, but consists of really tiny fields of vibrating energy. energy.

Wave Model of Atoms: Wave Model of Atoms:

According to modern physicists, the location According to modern physicists, the location and/or motion of electrons cannot be known for and/or motion of electrons cannot be known for sure. The best we can do is to talk about the sure. The best we can do is to talk about the probability of some event occurring or some probability of some event occurring or some particle existing at a given location. particle existing at a given location.

Structure of a Bohr Atom: Structure of a Bohr Atom: Electron- negative charge (-)

Neutron-no charge

Proton- positive charge (+)