2.02 Properties of the Atom
atomic number
The number of protons in the nucleus of an atom. (determines what element is it)
atomic mass unit (u)
a unit of mass, abbreviated u, that is exactly 1/12 the mass of a carbon-12 atom. 1 u equals 1.66 x 10^-24 grams. (a carbon atom containing 6 protons and 6 neutrons in its nucleus)
element
any material made up of only one type of atom
subatomic particles
any particles of matter smaller than an atom, including protons, neutrons, and electrons.
number of protons = atomic number number of electrons = atomic number (if atom is neutral) number of neutrons = mass number - atomic number
examples: •if a neutral hydrogen atom has an atomic number of 1 and a mass number of 1 the atom has... 1 proton, 1 electron, 0 neutrons. •if a neutral atom of chlorine has an atomic number of 17 and a mass number of 35 the atom has... 17 protons, 17 electrons, and 18 neutrons. •if a neutral fluorine atom has 9 protons and 10 neutrons, the atoms atomic number is 9, and it's mass number is 19.
electron
found in the electron cloud surrounding the nucleus. -1 charge (negative). 9.109 x 10^-28 grams. 0 u (the mass of the electron is so much smaller than the masses of protons and neutrons that it is ignored when determining the relative mass of atoms)
neutron
found in the nucleus with the protons. no charge (neutral). 1.675 x 10^-24 grams. 1 u (a neutron has the same relative mass as a proton, and each one has about 2,000 times the mass of an electron. that means that almost all of the mass of an atom is located in the tiny nucleus)
proton
found in the small nucleus at the center of the atom. +1 charge (positive charge). 1.673 x 10^-24 grams. 1 u (which equals 1.66 x 10^-24 grams)
why are atoms invisible?
individual atoms are so small that waves of visible light pass by them without being reflected at all. this is why they cannot be seen directly, even with the strongest microscopes.
counting subatomic particles
the number of protons, neutrons, and electrons within an atom affects its properties. you can determine the number of each of these subatomic particles when given the atomic number and mass number for a given atom.
atomic mass
the relative atomic mass of an atom or subatomic particle, measured in atomic mass units (u). (the mass of a proton and the mass of a neutron are both about 1 u. this means that an atom's mass number is approximately equal to the atom's total mass.)
mass number
the total number of protons and neutrons in the nucleus of a given atom. (mass number = number of protons + number of neurons)
there are just over 100 distinct elements, and 90 are found in nature
these elements are found on the periodic table
mass number vs. atomic mass
•the mass number does not have any units. an atom's mass number is always a whole number because it is a counted value. it is a number determined by counting the protons and neutrons in an atom's nucleus. for example, an oxygen atom that has 8 protons and 8 neutrons has a MASS NUMBER of 16. •the atomic mass is not usually a whole number because it is an actual measurement of the total mass of the atom. because it is a measurement, it is given the unit u (atomic mass units). for example, scientists can measure the mass of an oxygen atom and determine that it has an ATOMIC MASS of 15.99 u.
