Atom – basic unit of matter 3 subatomic particles: protons, electrons, neutrons Atom is mostly empty space Atoms have small dense positive core (nucleus) and negative electron cloud surrounding the nucleus Elements are composed of atoms with the same atomic number Atoms of the same element are similar Atoms of different elements are different Nucleus - ________________________________________ Contains ___________ (+) and neutrons (no charge) Overall charge of the nucleus is _____________ due to the protons Compared to the entire atom, the nucleus is very small but very dense Most of the atom’s mass is due to the mass of the nucleus

Protons – positively charged subatomic particles found in the nucleus of the atom Has a mass of 1 atomic mass unit (amu) and a +1 charge Is about 1836 times more massive than an electron Located inside the nucleus Number of protons is the same as the atomic number of the element All atoms of the same element must have the same number of protons The number of protons in the nucleus is also the nuclear charge of the element

Electrons – negatively charged subatomic particles found in orbitals outside the nucleus of the atom Have insignificant mass and a -1 charge Have a mass that is 1/1836 th that of a proton or neutron Found in orbitals outside the nucleus Arrangements in an atom determine the chemical properties of the elements Number of electrons is always equal to the number of protons in a neutral atom Neutrons – neutral subatomic particles located inside the nucleus of an atom Have a mass of 1 amu and a zero charge Have the same mass as a proton Located in the nucleus along with protons Atoms of the same element can differ in the number of neutrons

Subatomic Particle SymbolMassChargeLocation Protons 1 +1 p1+1Nucleus Neutrons 10n10n10Nucleus Electrons 0 -1 e0Orbital outside nucleus

Atomic number of an element identifies each element. Equal to the number of _________________ All atoms of the same element have the same atomic number and the same number of protons Found on the periodic table Elements on the periodic table are arranged in order of increasing atomic number Nucleons - __________________________________________ Account for the total mass of an atom Total number of nucleons is equal to the sum of __________ plus ________________

Mass number identifies different isotopes of the same element. Atoms of the same element differ by their mass numbers Mass number is equal to the number of _____________ plus ________________ Mass number shows the total number of nucleons

Relationships in Neutral Atoms Number of protons = atomic number of the element = number of electrons = number of nuclear charge = number of nucleons – neutrons = mass number – number of neutrons Number of electrons = atomic number of the element = number of protons = number of nuclear charge = mass number – number of neutrons Number of neutrons = mass number – number of protons = mass number – atomic number = mass number – number of electrons = nucleons – number of protons

Relationships cont’d Atomic number = number of protons = number of electrons = number of nuclear charge = mass number – number of neutrons Mass number = number of protons + number of neutrons = number of electrons + number of neutrons = nuclear charge + number of neutrons = number of nucleons

Relationships cont’d Number of nucleons = mass number = number of protons + number of neutrons = number of electrons + number of neutrons = nuclear charge + number of neutrons Nuclear charge = number of protons = atomic number = mass number – number of neutrons

Isotopes - __________________________________________ ___________________________________________________ Isotopes of the same element must have different __________________________________ same _____________________________________ Element – mass numberlithium – 7lithium – 8 Symbol – mass numberLi-7Li-8 Isotope notation 7 3 Li 8 3 Li Nucleus symbol notation 4n 3p 5n 3p

To determine which two symbols are isotopes of the same element LOOK for two symbols that have different mass number, same chemical symbol, same atomic number To determine which two nucleus notations are isotopes of the same element LOOK for nucleus symbols that contain same number of protons, different number of neutrons 34 number of protons + neutrons number of nucleons mass number P 16 atomic number number of protons nuclear charge number of electrons

Atomic mass unit – unit for measuring mass of atoms based on carbon-12 1 amu = 1/12 th of C-12 Hydrogen – 1 has a mass that is 1/12 th of the mass of C-12 Lithium – 6 has a mass that is 6/12 th or half the mass of C-12 Magnesium – 24 has a mass that is 24/12 th or 2 times the mass of C-12 Atomic mass of an element is the average mass of all the naturally occurring stable isotopes of an element

Atomic mass of an element is the average mass of all the naturally occurring stable isotopes of an element. It is based on the masses of the stable isotopes and their percent in a sample. A natural sample of an element consists of a mix of two or more isotopes. Usually there is a lot of one isotope and very little of others. Approximately 75% of chlorine atoms are Cl-35 and 25% of chlorine atoms are Cl-37. The average atomic mass of chlorine is Cl ● 35 = Cl ● 37 = 9.25 Avg 35.5 amu

Practice 1. A sample of unknown element X contains the following isotopes: 80% of X-64, 15% of X-65, 5% of X-66. What is the average atomic mass of element X?

Isotopes of Hydrogen ProtiumDeuteriumTritium Name – mass Isotope symbol Hydrogen – H Hydrogen – H Hydrogen – H Mass number123 Protons111 Neutrons012 Nucleus diagram 1p 0n 1p 1n 1p 2n H-1 most abundant isotope of hydrogen H-2found in heavy water which is used in nuclear power plants to cool reactors H-3used as reactant in nuclear fusion