5.2 The Structure of the Atom
An Atom
The smallest particle of an element that retains the identity of the element
Consists of electrons orbiting around the nucleus
Electron: a negatively charged particle within the atom
Nucleus: the positively charged centre of an atom
Nucleus
Made up of protons and neutrons
Proton: a positively charged particle that is
part of every atomic nucleus
Neutron: an uncharged (neutral) particle that
is part of almost every atomic nucleus
Structure of An Atom
NameRelative
Mass
Electric
Charge
Location in
the Atom
proton 1836 + nucleus
neutron 1837 0 nucleus
electron 1 -
energy
levels
surrounding
the nucleus
Structure of an Atom
Electron
Proton
Neutron
Nucleus
Atomic numbers, Mass numbers
Elements are often symbolized with their mass
number and atomic number
Oxygen: O16
8
These values are given on the periodic table.
For now, round the mass # to a whole number.
These numbers tell you a lot about atoms.
# of protons = # of electrons = atomic number
# of neutrons = mass number – atomic number
Atomic numbers, Mass numbers
# of protons = atomic number
# of electrons = atomic number
# of neutrons = mass number – atomic number
atomic number = # of protons
= # of electrons (neutral atom)
mass number = # of protons + # of neutrons
SO…
(combining capacity)
3545358035Br
1822184018Ar
2020204020Ca
e–n0p+MassAtomic
Calculate # of protons (p+), neutrons (n0), and
electrons (e–) for Ca, Ar, and Br.
Bohr - Rutherford diagrams1. Find : # p+ , # e- and # n0
2. Draw nucleus in circle: # p+ and # n0
3. Draw electrons around in shells: 2, 8, 8
Bohr - Rutherford diagrams
2 p+
2 n0
He
3 p+
4 n0
Li
Draw He and Li:
p+ =
e- =
n0 =
2
2
2
p+ =
e- =
n0 =
3
3
4
4 p+
5 n°
Bep+ = 4
e- = 4
n0 = 5
5 p+
6 n°
Bp+ = 5
e- = 5
n0 = 6
13 p+
14 n°
Alp+ = 13
e- = 13
n0 = 14
Draw Be, B, and Al:
Isotopes Atoms of the same element that have different
numbers of neutrons are called isotopes.
Isotopes can be written with the name followed
by the mass number: lithium-6 Due to isotopes, atomic masses are not round
numbers. Li (atomic mass = 6.9) is made up of both 6Li
and 7Li.
Sometimes an isotope is written without its atomic number - e.g. 35S (or S-35). Why?
A- The atomic # of an element does not change. Although the number of neutrons can vary,
atoms have definite numbers of protons.
IsotopesDraw B-R diagrams for the two Li isotopes.
3 p+
3 n0
Lithium-6
3 p+
4 n0
Lithium-7
Learning Check
Page 192 #1-4
Homework
Section 5.2 questions
– See “Unit At A Glance”