1. Introduction to Atoms
Chapter 4

3. Democritus Greek Philosopher “uncutable particle” Named the atom from
the Greek word Atomos
Atoms are small, hard, different shapes and sizes, always moving and join together

4. Aristotle
Greek philosopher
Disagreed with Democritus
Strong influence

5. Dalton British Chemist 1766
Elements combine in specific proportions because they are made of individual atoms
He observed that no matter how large or small the sample, the ratio of the masses of the elements in the compound is always the same.
Compounds have a fixed position

6. Dalton’s Theory
Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

7. Main Points of Dalton’s Theory
All elements are composed of atoms
All atoms of the same element have the same mass, and atoms of different elements have a different masses.
Compounds contain atoms of more than one element.
In a particular compound, atoms of different elements always combine in the same way

8. What did Dalton notice that all compounds have in common?
Dalton noticed that the ratio of masses of elements in a compound is always the same!

9. Objects with like charges repel, or push apart.
Objects with opposite charges attract, or pull together.

10. Thomson’s Experiments
He used a sealed tube of gas in his experiments.
When the current was on, the disks became charged and glowing beam appeared in the tube.
The beam bent toward a positively charged plate placed outside the tube.
He concluded that the particles in the beam had a negative charge because they were attracted to the positive plate.

11. Thomson’s experiments provided the first evidence that atoms are made of even smaller particles

12. Thomson’s Model
Thomson found a negative charge, but knew that atoms were neutral.
Atoms must contain a positive charge as well.
The negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter.
Plum Pudding Model

13. Rutherford’s Atomic Theory (1871-1937)
He hypothesized that the mass and charge at any location in the gold would be too small to change the path of an alpha particle (positive charge).

14. When the gold foil experiment was done more particles were deflected and bounced straight back, as though they had struck an object.
The alpha particles must have come close to another charged particle

16. Rutherford concluded that the positive charge of an atom is not evenly spread through the atom.
It is concentrated in a very small, central area called the nucleus.

17. According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus.

18. Bohr
Danish scientist
Discovered electrons travel around nucleus in definite paths
Located in levels around nucleus
Electrons can jump from one level to another

19. Schrodinger and Heisenberg
Modern theory
Electron clouds surround nucleus
Electrons do not travel in definite paths
Movement cannot be predicted

20. Chapter 4 Section 2
Atom Structure

21. Nucleus
A dense, positively charged mass located at the center of the atom
New Model: All of an atom’s positive charge is concentrated in its nucleus.

22. Properties of subatomic particles
1. Protons
2. Electrons
3. Neutrons

23. Protons
Positively charged subatomic particle that is found in the nucleus of an atom
Charge of 1+

24. Electron
A negatively charged subatomic particle that is found in the space outside the nucleus
Charge of 1-

25. Neutrons
Neutral subatomic particle that is found in the nucleus of an atom
Mass almost exactly equal to that of a proton

26. Subatomic particles
Distinguished by their mass, charge and location in the atom

27. Atomic Number Atoms of any element have the same number of protons
Atomic number = number of protons in an atom of that element
Ex: Hydrogen has 1 proton, atomic number is 1

28. Mass Number Sum of protons and neutrons in the nucleus of an atom
Number of neutrons = Mass number – Atomic Number

29. Isotopes
Every atom of a given element has the same number of protons and electrons, but every element does not have the same number of neutrons.
Isotopes – have same atomic number, but different mass numbers because they have different number of neutrons

30. Isotopes
Atoms that have the same number of protons but have a different number of neutrons
Atoms that are isotopes of each other are always the same element because the number of protons in each atom is the same

31. Bohr’s Model of the Atom
Focused on Electrons that move with constant speed in fixed orbits around the nucleus
Electrons can change energy levels when the atom gains or loses energy

32. Electron Cloud Model
Visual model used to find the locations for electrons

33. Energy levels, orbitals and Electrons
# of Orbitals
Maximum # of Electrons 1 2 4 8 3 9 18 16 32

34. Forces that Work in Atoms
* Gravity
* Electromagnetic Force
* Strong Force
* Weak Force