What is an atom? Atom: the smallest unit of matter that retains the identity of the substance
Atomic Structure Atoms have 2 regions 1) Nucleus: the center of the atom that contains most of the mass of the atom. 2) Electron cloud: surrounds the nucleus & takes up most of the space of the atom. Nucleus Electron Cloud
What’s in the Nucleus? In the nucleus we find: Protons: positively charged subatomic particles Mass of 1 amu Neutrons: neutrally charged subatomic particles Mass of 1 amu
What’s in the Electron Cloud? In the electron cloud we find: Electrons: the subatomic particle with a negative charge and relatively no mass Mass of ~ 1/1836 amu
Subatomic Particles ParticleChargeMass (g)Location Electron (e - ) 9.11 x 10 -28 Electron cloud Proton (p + ) +1 1.67 x 10 -24 Nucleus Neutron (n o )0 1.67 x 10 -24 Nucleus
How do we know the number of protons in an atom? Atomic number (#)= # of protons in an atom Ex: Hydrogen’s atomic # is 1 hydrogen has 1 proton Ex: Carbon’s atomic # is 6 carbon has 6 protons **The number of protons identifies the atom-it’s an atom’s fingerprint.
Mass #: the # of protons plus neutrons in the nucleus # of neutrons = mass # - atomic # Example Li has a mass # of 7 and an atomic # of 3 Protons = 3 (same as atomic #) Neutrons= 7-3 = 4 (mass # - atomic #) How do we know the number of neutrons in an atom?
Mass # vs. Atomic Mass Mass # ? = The Atomic mass on the periodic table rounded either up or down
How do we find the number of electrons in an atom? Most atoms are neutral (have no overall charge) Because the only charged subatomic particles are the protons and electrons… they must balance each other out in an electrically neutral atom. Therefore.. # Electrons = # Protons * * (in a neutral atom..)
Examples He has a mass # of 4 and an atomic # of 2 p+ = 2n o = 2e - = 2 Cl has a mass # of 35 and an atomic # of 17 p + = 17,n o = 18, e - = 17
How exactly are the particles arranged? Bohr Model of the atom: electron configurations All of the protons and the neutrons The 1 st ring can hold up to 2 e - The 2 nd ring can hold up to 8 e - The 3 rd ring can hold up to 8 e-
What does carbon look like? Mass # = 12 atomic # = 6 p + = 6 n o = 6e - = 6 6 p and 6 n live in the nucleus
Isotopes Dalton’s 1 st postulate was wrong. Atoms of the same element can be different (they can have different # of neutrons) Thus, different mass numbers. These are called isotopes.
Isotopes are atoms of the same element having different masses, due to varying numbers of neutrons.
Naming Isotopes We name the isotope based on its mass number carbon-12 carbon-14 uranium-235
Isotopes Elements occur in nature as mixtures of isotopes.
Atomic Mass How heavy is an atom of oxygen? It depends.. b/c there are different oxygen isotopes. We are more concerned with the average atomic mass. This is determined based on the abundance of each isotope We don’t use grams for this mass because the numbers would be too small.
Measuring Atomic Mass Instead we use the Atomic Mass Unit (amu) defined as one-twelfth the mass of a carbon-12 atom. Each isotope has its own atomic mass, thus we determine the average from percent abundance.
Atomic Mass IsotopeSymbolComposition of the nucleus % in nature Carbon-12 12 C6 protons 6 neutrons 98.89% Carbon-13 13 C6 protons 7 neutrons 1.11% Carbon-14 14 C6 protons 8 neutrons <0.01% Atomic mass is the average of all the naturally occurring isotopes of that element. Carbon = 12.011