Atomic Structure
Basic Structure & Bohr’s Model
Lesson Objectives
At the end of the lesson, you should be able to:
1. State the relative charges and appropriate relative masses of a proton, a
neutron and an electron.
2. Describe with the aid of diagrams the structure of an atoms as containing
protons and neutrons in the nucleus and electrons arranged in shells
3. define proton (atomic) number and nucleon (mass) number.
4. deduce the number of protons, neutrons and electrons in atoms.
5.
State the electronic configuration of atoms of various elements.
6.
Define the term isotopes
7.
Calculate the relative mass of elements using mass data.
8.
Give examples of applications of isotopes.
References
Glencoe Science
1. Concepts and Application: Chapter 2, 3, 7
2. Matter and Change: Chapter 4
Bohr’s Model
Protons
Neutrons
Electrons
An atom is the smallest unit of matter that retains all the chemical properties of
an element.
Recall
What is formed when atoms chemically combine?
H
O
H H
H
Molecules
Recall
What is formed when the same kinds of atoms chemically combine?
H H
P
Hydrogen
P
P
P
Element
Phosphorus
What is formed when different kinds of atoms chemically combine?
H
O
Water
H
H
H
N
Compound
H
Ammonia
Basic Structure of an Atom
An atom is made up of 3 basic particles:
• the proton (p), the neutron (n) & the electron (e-)
• These particles are called the subatomic particles.
• An atom consists of a nucleus (made up of protons and neutrons)
surrounded by electrons arranged in electron shells.
• The 3 particles have different masses and charges.
Masses of subatomic particles
• All the subatomic particles are very light.
1
• Their masses are compared to the mass of
of a carbon-12
12
atom.
1
• Proton has the same mass as
of a carbon-12 atom
12
à It has a mass of one unit (amu)
Masses of subatomic particles
NEUTRON
PROTON
A neutron and proton have the same mass
Therefore, neutron has a mass of one unit (amu).
Masses of subatomic particles
ELECTRON
PROTON
The mass of an electron is 0.00055 of the mass of a proton.
Mass of electron is almost negligible.
Mass of Atom
Question: What does the mass of an atom depend on?
• Protons and neutrons.
• The mass of an electron is negligible.
Charges of subatomic particles
• The proton is positively charged (+1)
• The electron is negatively charged (-1)
• no. e- = no. p+
• The neutron has no charge (0), it is neutral
What is the charge of an atom?
Electrons
Nucleus
(Protons +
Neutrons)
No. of protons = No. of electrons
SUMMARY
Sub-atomic
particles
Relative
Mass
Relative
Charge
Proton
1
+1
Neutron
1
0
Electron
1/1836
(~0.0005)
-1
Symbol Form
Structure of an atom can be written in symbol
a : mass no./ nucleon no.
b
Example
X : symbol
39
b/z : atomic no./proton no.
19
K
Example
Mass number
= 12
Proton number
=6
Proton Number (Atomic No.)
• Proton number = number of protons in an atom
• Proton number also tells us the number of electrons in the
atom.
à An atom is neutral, ∴ no. of protons = no. of electrons
Nucleon Number (Mass number)
Nucleon number = no. protons + no. neutrons (in the nucleus)
Nucleon number gives the relative atomic mass of an atom.
Mass of electron is negligible, hence almost all the mass
is in the nucleus
Example:
Determine the number of protons, neutrons and electrons in
Potassium
From the periodic table, 39.10
19 K
Mass number is not a whole
number because it is the
average mass of all the natural
isotopes for that element.
No. protons = 19
No. electrons = 19
No. Neutrons = Nucleon No.– Proton No.
= 39 – 19
= 20
Check point
• Try Assignment 4
Part A Q 3
Electronic Structure (Bohr’s model)
• Electrons orbit around the
nucleus of the atom.
• Electrons are held in place
by electrostatic force.
à It is a force between the
positive charge on the
nucleus and the negative
charge on the electrons.
Electrons
nucleus
Electrostatic force
of attraction
Electronic Structure (Bohr’s model)
• The way the electrons are arranged is called its electronic
configuration.
• Each energy level is capable of holding up to a certain
number of electrons
• The energy levels are numbered from the nucleus
outwards
Electronic Structure (Bohr’s model)
1st energy level: 2 e-
2nd energy level: 8 e-
3rd energy level: 18 eNucleus
Proton no. ≤ 20
Electronic Structure (Bohr’s model)
• Electrons will fill up lowest energy level first before
moving on to higher energy levels.
• Each energy level can hold up to 2n2 electrons.
n = quantum number (energy level no.)
Example: Draw a model to represent the electronic
configuration of Na atom.
Sodium has a proton number of 11.
Therefore it has 11 electrons
Electronic Configuration
2, 8, 1
Na
1st energy level
2nd energy level
3rd energy level
Outermost Electrons (Valence electrons)
• Outermost (valence) electrons
à furthest away from nucleus.
• These electrons form chemical bonds with other atoms.
àelectrons with the highest energy in the atom and have the weakest
attraction to the nucleus.
àinner-level electrons do not take part in chemical reactions as they have
a stronger attraction to the nucleus.
Example:
State the electronic configuration for Aluminium using
Bohr’s model
Electron arrangement : 2, 8, 3
There are 3 outermost (valence) electrons.
These 3e- will later take part in chemical reactions (form
chemical bonds).
Check point
• Try Assignment 4
• Part A Q 4