History of the Atom

Compare and contrast the major models of the atom. Interpret the periodic table to describe an element’s atomic makeup.

John Dalton

John Dalton The Atom 1808 Studied chemical reactions and mass ratios Concluded that… Matter is composed of extremely small particles Atoms are indivisible and indestructible Atoms of a given element are identical in size, mass and properties Atoms of a specific element are different from those of another element

Dalton – Model of the Atom Dalton’s Model of the Atom is called the Solid Sphere Model

Dalton – Model of the Atom Now, what does this look like? Draw Dalton’s atomic model for hydrogen and oxygen Hydrogen Oxygen

Where did Dalton go wrong? Are atoms indivisible? No – atoms are made of smaller parts – protons, neutrons, & electrons Do atoms of the same element always have identical mass? No – isotopes same # p different #n  different mass Example: Ne – 20 & Ne- 22 Do atoms of the same element always have identical properties? No – allotropes Same element but different properties Example: diamond, coal, graphite

JJ Thomson – The Electron 1897

JJ Thomson – Cathode Ray Tube Used a Cathode-Ray Tube What is a vacuum? An enclosed space from which matter, esp. air, has been partially removed so that the matter or gas remaining in the space exerts less pressure than the atmosphere. Cathode-Ray Tube Tube with a vacuum inside Connect to anode & cathode power supply – electricity running through the tube

Cathode Ray Tube

Discovering Electrons Cathode ray deflected away from the negative pole of the magnetic field Since the rays were deflected away from a negatively charged object, the ray must also be NEGATIVE FOUND THE ELECTRONS – the negatively charged subatomic particle Studied charge-to-mass ratios The mass of the electrons is 2000 times smaller than the mass of a H atom

Plum Pudding Model What would Thomson’s model look like? Draw Thomson’s model of the atom for hydrogen and oxygen.. Hydrogen - 1 e- Oxygen – 8e-

Plum Pudding Model What element is represented in the plum pudding model shown? What would the overall charge of the particle be?

What is next? After Thomson, scientists inferred: Atoms must contain a positive charge. WHY? The overall charge of the atom is not negative bc net charge = 0 Atoms must contain other particles (regardless of charge). Electrons do not account for the entire mass of the atom

Ernest Rutherford positive proton dense positive nucleus Atom is mostly empty space 1911

Gold Foil Experiment Shot a beam of positively charged alpha particles at a piece of gold foil Expected the positive particles to pass through with little deflection

What Rutherford expected to see...

What Rutherford ACTUALLY observed

Gold Foil Experiment

What actually happened? Most particles passed through with no deflection Most of the atom is EMPTY SPACE Some particles were slightly deflected The atom contains negative electrons 1/8000 particles were deflected back towards the source Some positive alpha particles came near positive particles in the atom  densely packed protons

Rutherford’s Conclusion the atom is mostly empty space with a dense positively charged nucleus Still a problem though…Rutherford’s conclusion doesn’t account for the entire mass of the atom.

James Chadwick Discovered the neutron helped explain & account for the mass of the atom 1932

space surrounding nucleus Subatomic Particles Particle Symbol location in atom charge relative mass actual mass (g) electron e- space surrounding nucleus 1- 1/1840 9.11x10-28 proton p nucleus 1+ 1 1.673x10-24 neutron n 1.675x10-24

Nuclear Model of the Atom For the sake of simplicity when referencing the “Rutherford Model” we will include James Chadwick’s contribution as well. Steps in drawing Rutherford’s model: Determine the number of protons, neutrons and electrons in the atom Draw the nucleus with protons & neutrons Draw the electrons randomly around the outside of the nucleus

Nuclear Model Hydrogen Oxygen p+=1 n0=0 e-=1 p+=8 n0=8 e-=8 We are now going to draw the Rutherford/Chadwick Models for the elements in the table Do they give you any reason to think they are changed? NO – assume they are neutral  p=e Hydrogen Oxygen p+=1 n0=0 e-=1 p+=8 n0=8 e-=8 p=1 n=0 p=8 n=8

Summary Dalton Thomson Rutherford Started studying the atom & chemical reactions Mostly wrong Thomson Electrons Cathode Ray Tube Plum Pudding Model Rutherford Protons Gold Foil Experiment Model combined with Chadwick (neutrons)