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S3 “Atomic Structure”. Defining the Atom OBJECTIVES: OBJECTIVES: Describe Democritus’s ideas about atoms. Describe Democritus’s ideas about atoms.

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Presentation on theme: "S3 “Atomic Structure”. Defining the Atom OBJECTIVES: OBJECTIVES: Describe Democritus’s ideas about atoms. Describe Democritus’s ideas about atoms."— Presentation transcript:

1 S3 “Atomic Structure”

2 Defining the Atom OBJECTIVES: OBJECTIVES: Describe Democritus’s ideas about atoms. Describe Democritus’s ideas about atoms.

3 Defining the Atom OBJECTIVES: OBJECTIVES: Explain Dalton’s atomic theory. Explain Dalton’s atomic theory.

4 Defining the Atom The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) The Greek philosopher Democritus (460 B.C. – 370 B.C.) was among the first to suggest the existence of atoms (from the Greek word “atomos”) He believed that atoms were indivisible and indestructible He believed that atoms were indivisible and indestructible His ideas did agree with later scientific theory,and was not based on the scientific method – but just philosophy His ideas did agree with later scientific theory,and was not based on the scientific method – but just philosophy

5 Dalton’s Atomic Theory (experiment based!) 3)Atoms of different elements combine in simple whole-number ratios to form chemical compounds 1)All elements are composed of tiny indivisible particles called atoms 2)Atoms of the same element are identical. Atoms of any one element are different from those of any other element. John Dalton (1766 – 1844)

6 Sizing up the Atom  Elements are able to be subdivided into smaller and smaller particles – these are the atoms, and they still have properties of that element  If you could line up 100,000,000 copper atoms in a single file, they would be approximately 1 cm long  Despite their small size, individual atoms are observable with instruments such as scanning tunneling (electron) microscopes

7 Structure of the Nuclear Atom OBJECTIVES: OBJECTIVES: Identify three types of subatomic particles. Identify three types of subatomic particles.

8 Structure of the Nuclear Atom OBJECTIVES: OBJECTIVES: Describe the structure of atoms, according to the Rutherford atomic model. Describe the structure of atoms, according to the Rutherford atomic model.

9 Structure of the Nuclear Atom One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles: One change to Dalton’s atomic theory is that atoms are divisible into subatomic particles: Electrons, protons, and neutrons are examples of these fundamental particles Electrons, protons, and neutrons are examples of these fundamental particles There are many other types of particles, but we will study these three There are many other types of particles, but we will study these three

10 Discovery of the Electron In 1897, J.J. Thomson used a cathode ray tube to deduce the presence of a negatively charged particle: the electron

11 Modern Cathode Ray Tubes  Cathode ray tubes pass electricity through a gas that is contained at a very low pressure. TelevisionComputer Monitor

12 Mass of the Electron 1916 – Robert Millikan determines the mass of the electron: 1/1840 the mass of a hydrogen atom; has one unit of negative charge The oil drop apparatus Mass of the electron is 9.11 x g

13 Conclusions from the Study of the Electron: a)Cathode rays have identical properties regardless of the element used to produce them. All elements must contain identically charged electrons. b)Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons c)Electrons have so little mass that atoms must contain other particles that account for most of the mass

14 Conclusions from the Study of the Electron:  Eugen Goldstein in 1886 observed what is now called the “proton” - particles with a positive charge, and a relative mass of 1 (or 1840 times that of an electron)  1932 – James Chadwick confirmed the existence of the “neutron” – a particle with no charge, but a mass nearly equal to a proton

15 Subatomic Particles ParticleCharge Mass (g) / Relative mass Location Electron (e - ) (e - ) 9.11 x x /1840(negligible) Electron cloud Proton (p + ) x x Nucleus Neutron (n) (n) x x Nucleus

16 Thomson’s Atomic Model Thomson believed that the electrons were like plums embedded in a positively charged “pudding,” thus it was called the “plum pudding” model. J. J. Thomson

17 the “plum pudding” model of JJ Thomson

18 Ernest Rutherford’s Gold Foil Experiment Rutherford was a student of Thomson and decided to see if there was more evidence for the 'plum pudding /blueberry muffin' theory. His idea: to bombard the atom with dense alpha particles alpha particle = He atom with no electrons - this was like shooting bullets at a blueberry muffin

19 Ernest Rutherford’s Gold Foil Experiment this was like shooting bullets at a blueberry muffin…. So what did Rutherford expect?

20 ?v=Q8RuO2ekNGw Surprising result of Rutherford’s Gold Foil Experiment

21 Rutherford’s Findings  Most of the particles passed right through  A few particles were deflected VERY FEW were greatly deflected  VERY FEW were greatly deflected “Like gun shells bouncing off of tissue paper!”

22 FindingsDeductions Most of the particles passed right through an atom was much more than just empty space and scattered electrons A few particles were deflected, very few were greatly deflected Since alpha particles are relatively heavy, positively charged particles, the fact that the occasional particle would be deflected by either a small or large extent, an atom must have a positively charged center that contains most of its mass (which Rutherford termed as the nucleus). Rutherford’s Findings and deductions

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24 The top structure shows how the alpha particles would have passed through the gold foil atoms if the plum pudding model was correct in its assumptions. The bottom structure shows what Rutherford and his collogues observed and is the true depiction of an atom's structure. An illustration that compares the plum pudding model of an atom to what Rutherford observed in his experiment.

25 a) The nucleus is small b) The nucleus is dense c) The nucleus is positively charged The nucleus as described by Rutherford:


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