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”