Chapter 4, section 1 – Introduction to Atoms
Atom
1. The smallest part of an element.
2. Atomic theory grew as a series of models that developed from
experimental evidence.
Dalton’s Atomic Theory
1. Still accepted today with minor changes.
2. All elements are composed of atoms that cannot be divided.
3. All atoms of the same element are exactly alike and have the same
masses.
4. An atom of one element cannot be changed into an atom of another
element. Atoms cannot be created or destroyed, only rearranged.
5. Every compound is composed of atoms of different elements,
combined in a specific ratio.
J.J. Thomson
1. An atom consists of negatively charges scattered throughout a ball of
positive charges.
2. Electrons – negatively charged particles.
Rutherford and the nucleus
1. Nucleus : tiny region in the center of the atom.
2. positively charged particles in the nucleus of an atom called protons.
Responsible for most of an atom’s mass.
Bohr’s model
1. Electrons have specific amounts of energy which require them to
stay in orbits or paths.
Cloud of electrons
1. Electrons can be anywhere in a cloudlike region around the nucleus.
2. energy level – a region of an atom where electrons with the same
energy are likely to be found.
The modern atomic model
1. James Chadwick discovered another part of an atom with a neutral
charge and almost the same mass as a proton.
2. An atom consists of nucleus that contains protons and neutrons,
surrounded by a cloudlike region of moving electrons.
Particle charges
1. Protons (+) positive charge.
2. electrons (-) negative charge.
3. neutrons (0) neutral charge.
4. atoms are neutral – number of protons equals the number of
electrons. Number of neutrons does not have to equal the number
electrons or protons because neutrons are neutral.
Particle masses
1. Protons and neutrons have masses that are equal and are measured
in Atomic Mass Units (amu).
2. Electrons have masses that are much less.
Size of atoms
1. Too small to see.
2. most of the space is taken up by the area required for the electron
movement.
Atomic number
1. Identifies an element.
2. Unique number of protons in the nucleus of an atom of an element.
3. will never be the same in another element.
Isotopes
1. Atoms of an element will always have the same number of protons,
but the number of neutrons can vary.
Example: Carbon 12, 6p, 6e, 6n
Carbon 13, 6p, 6e, 7n
Carbon 14, 6p,6e, 8n
Mass number
1. Sum of the number of protons and neutrons in the nucleus of an
atom.
2. Also called the atomic mass.
3. Can be used to identify an isotope. Example: Carbon 12 – 12 is its
mass number.