Unit 5 The Structure of Matter
Chapter 16 The Atom
16B – The Orderly Atom
• Objectives:
– Describe the three major subatomic particles and
their locations in an atom
– Define atomic number and explain its significance
– Discuss the significance of isotopes and how a mass
number identifies them
– Use isotopic notation to identify isotope of an
element and to determine the number of neutrons
in an atom’s nucleus
– Define atomic mass and explain its significance
– Compare and contrast the mass number and the
atomic mass of an element
– State and use the general rules that establish the
electron configuration of an atom.
• Assignments: Outline, Worksheet, and Section
Review page 396
Subatomic Particles
• The atom is made up of three principle particles
• Protons
– Located in the atom’s nucleus
– Has a mass of 1.6726 x 10-27 kg
– Has a single positive fundamental electrical charge
• Neutrons
–
–
–
–
Located in the nucleus
Have about the same mass as the proton
Discovered last because they have no electrical charge
Neutrons and protons located in the nucleus are called
nucleons
– Neutrons outside the nucleus will break apart into a proton
and electron
• Electrons
–
–
–
–
–
Smallest of the main subatomic particles
Mass of 9.11 x 10-31 kg
Occupy a spherical region surrounding the nucleus
Has a single negative fundamental electrical charge
Responsible for all the chemical properties of an atom
Elementary Particles
• Read Facet of Science
– Page 390
Atomic Number
• The number of protons in the nucleus
determines the kind of atom
• This number is known as the atomic
number (Z)
• It also tells you the number of electrons
surrounding the nucleus in a neutral
atom
• The atomic number is an element’s ID
number and is unique to each element
• This number is found on the periodic
table
Mass Number and Isotopes
• Atoms of the same element always have the same
number of protons
• Atoms of an element can have different numbers of
neutrons and are called isotopes
• In order to tell isotopes apart they are assigned a
special number called the mass number
– The sum of the protons and neutrons in the nucleus
• Isotopes are identified by the chemical name
followed by the mass number
– Some can have special names
• Hydrogen-1, protium
• Hydrogen-2, deuterium
• Hydrogen-3, tritium
• An isotope can also be identified by isotopic
notation
Mass number
Atomic number
X
Atomic Mass
• Determining the atomic mass is not as easy as
the mass number
• Nucleons are held together by nuclear binding
energy
• Elements in their natural state do not exist as a
single isotope, but as a combination
• The atomic mass is determined by finding the
average mass of its atoms in nature, taking into
account the percentage occurrence of each of its
isotopes and expressing that mass in atomic
mass units
• Expressed as decimals, not whole numbers like
the mass number
• Continually updated when new isotopes are
discovered
Electron Arrangement in Atoms
• Electrons are arranged and found in energy
levels
– a region where an electron with a certain amount
of energy is most likely to be found
• Energy levels do not have well defined
boundaries and sometimes overlap each other
• In general, higher energy levels are farther
from the nucleus
• Electrons with the lowest energy occupy the
first energy levels, and increase levels with
increased energies
• The ground state of an atom is the lowest and
most stable energy state an atom can have
Electron Order Rules
• Each energy level has a limit on the number
of electrons it can hold
– 2n2
• The maximum number of electrons an atom
can have in its outermost energy level is eight
– Called the octet rule
• Some elements do not strictly follow these
rules
• Since it is nearly impossible to draw both the
energy levels and all the electrons in an atom
as 3D orbital, Bohr’s planetary model will be
used for the sake of simplicity and clarity (not
completely accurate)
Electron Structure Template
Electron Configuration
• The unique arrangement of the energy
levels and the positions that electrons
can take within those energy levels for a
given number of electrons in an atom
• Let’s do Titanium, atomic number 22
• Valence electrons
– The electrons in the outermost energy level
• How these electrons are gained, lost, or
shared determines the element’s
chemical properties
• These electrons are colored gold in the
Bohr models in this section