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
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
Dalton’s Theory
• Dalton proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.
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
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!
Objects with like charges repel, or push apart.
• Objects with opposite charges attract, or pull together.
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.
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
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).
• 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
• 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.
Bohr
• Danish scientist
• Discovered electrons travel around nucleus in definite paths
• Located in levels around nucleus
• Electrons can jump from one level to another
Schrodinger and Heisenberg
• Modern theory
• Electron clouds surround nucleus
• Electrons do not travel in definite paths
• Movement cannot be predicted
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.
Protons
• Positively charged subatomic particle that is found in the nucleus of an atom
• Charge of 1+
Electron
• A negatively charged subatomic particle that is found in the space outside the nucleus
• Charge of 1-
Neutrons
• Neutral subatomic particle that is found in the nucleus of an atom
• Mass almost exactly equal to that of a proton
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
Mass Number
• Sum of protons and neutrons in the nucleus of an atom
• Number of neutrons = Mass number – Atomic Number
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
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
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
Energy levels, orbitals and Electrons
Energy Level # of Orbitals Maximum # of Electrons
1 1 2
2 4 8
3 9 18
4 16 32