The structure of �the atom
3A
Content descriptor
All matter is made of atoms which are composed of protons, neutrons and electrons; natural radioactivity arises from the decay of nuclei in atoms.
Elaborations
Learning intention
I will be able to describe the structure of the atom.
Success measure
I can use protons, neutrons, and electrons to model the structure of the atom.
Atoms make up everything. What makes an atom?
What do we know?
The development of the atomic model
Dalton (1803) stated that atoms are indivisible and indestructible.
Thomson (1904) produced the ‘plum pudding’ model – showing that the atom is comprised of electrons spread throughout a positive sphere.
Rutherford (1911) proposed the nuclear model – the atom is mostly empty space with a positive centre called the nucleus.
Bohr (1913) proposed the planetary model – modelling that electrons orbit the nucleus in different energy levels.
Schrödinger (1926) stated that electrons move in waves – orbiting in a cloud around the nucleus.
THE STRUCTURE OF THE ATOM
atoms the smallest unit of matter
indivisible cannot �be divided into smaller parts
nucleus the core of an atom, made up of protons and neutrons
orbit move in a circle
KEY TERMS
Current knowledge of the atom comes from the work of many scientists, including:
Theory
Atoms are made up of subatomic particles
THE STRUCTURE OF THE ATOM
The research of previous scientists has led to our current model of the atom. This model shows the atom is made up of three subatomic particles. These are outlined below.
electron negatively charged particle orbiting the nucleus
proton positively charged particle located inside the nucleus
neutron particle of no charge located inside the nucleus
KEY TERMS
Subatomic particle | Charge | Relative mass | Location |
electron | negative (–1) | 1/1840 | orbiting the nucleus in levels |
proton | positive (+1) | 1 | inside the nucleus |
neutron | neutral (0) | 1 | inside the nucleus |
Bohr-Rutherford model of the atom
Theory
Proton number determines the element
THE STRUCTURE OF THE ATOM
Atomic number = number of protons
Mass number = number of protons + number of neutrons
Atoms are the smallest units of an element that retain the element's properties. Each element is defined by the number of protons in the nucleus of the atom. For example, oxygen has 8 protons, and so it is number 8 on the periodic table. Elements can be represented in two key ways:
Symbolic representation
Periodic table representation
In an atom with overall zero charge the number of protons equals the number of electrons.
element atoms �with the same characteristics and number of protons
atomic number the number of protons in an element
mass number a sum of the number of neutrons and protons in an element
KEY TERMS
DON’T GET TRICKED
The number of protons and neutrons are not always the exact same.
Theory
The periodic table
THE STRUCTURE OF THE ATOM
Theory
Electrons orbit the nucleus in shells
THE STRUCTURE OF THE ATOM
Electrons can be found in different orbits around the nucleus (also known as shells or energy levels). Each orbit is often represented as a circle.
The arrangement of electrons in an atom is known as its electron configuration. Electrons usually fill the closest shell to the nucleus first before occupying the next shell. Up to element 20, each shell of an atom has a maximum capacity of electrons it can hold. The maximum number of electrons for each shell is:
energy levels distances from the nucleus where electrons can be found
electron configuration �the arrangement of electrons in each level around a nucleus
KEY TERMS
DID YOU KNOW?
Atoms with full outer shells of electrons are far more stable than those without, which are more reactive.
Theory
Electrons can be lost or gained
THE STRUCTURE OF THE ATOM
An atom is more stable when its outer shell has a full octet of electrons. Electrons can be lost or gained (from another atom) in order to achieve this.
When an atom gains electrons it becomes an ion with a negative overall charge �(an anion). When an atom loses electrons it becomes an ion with a positive overall charge (a cation).
octet atoms tend to prefer 8 electrons in the outer shell
ion an atom that has a positive or negative charge
anion negatively charged ion
cation positively charged ion
KEY TERMS
MEMORY DEVICE
Cations are ‘pawsitive’.
DON’T GET TRICKED
Helium is a special case which prefers 2 electrons in its outer shell.
Theory
Sum it up
THE STRUCTURE OF THE ATOM
Theory
Skill sharpener: Processing and analysing
Representing the atom
Include in your model
Extension
Analyse the model you have developed. Come up with a list of advantages and disadvantages of the model when considering all the information and models known about the atom.
THE STRUCTURE OF THE ATOM
Achievement standards |
Year 8: Select and construct appropriate representations to organise and process data and information. |
Year 9: Select and construct appropriate representations to organise, process and summarise data and information. |
Year 10: Select and construct effective representations to organise, process and summarise data and information. |
Activity
Example answer
THE STRUCTURE OF THE ATOM
Representing a Carbon atom
Advantages | Disadvantages |
|
|
Extension
Activity
THE STRUCTURE OF THE ATOM
Question 1
Which description best fits the current model of the atom?
A | | negative electrons contained within a positive sphere |
| ||
| ||
B | | electrons and neutrons contained within a nucleus, protons surrounding the nucleus |
| | |
| ||
C | | negative protons and positive neutrons inside a nucleus, neutral electrons orbiting in shells |
| | |
| ||
D | | positive protons and neutral neutrons inside a nucleus, negative electrons orbiting in shells |
| | |
Check for understanding
THE STRUCTURE OF THE ATOM
Question 1 – answer
Which description best fits the current model of the atom?
Protons are positive, neutrons are neutral, and electrons are negative.
A | | negative electrons contained within a positive sphere |
| ||
| ||
B | | electrons and neutrons contained within a nucleus, protons surrounding the nucleus |
| | |
| ||
C | | negative protons and positive neutrons inside a nucleus, neutral electrons orbiting in shells |
| | |
| ||
D | | positive protons and neutral neutrons inside a nucleus, negative electrons orbiting in shells |
| | |
Check for understanding
THE STRUCTURE OF THE ATOM
Question 2
A | | Mass number: 5, Atomic number: 10 |
| ||
B | | Mass number: 10, Atomic number: 5 |
| ||
C | | Mass number: 15, Atomic number: 5 |
| ||
D | | Mass number: 5, Atomic number: 15 |
Determine the mass number and atomic number of an atom with 5 protons and 5 neutrons.
X
?
5
Check for understanding
THE STRUCTURE OF THE ATOM
The mass number is the sum of the number of protons and neutrons.
Question 2 – answer
A | | Mass number: 5, Atomic number: 10 |
| ||
B | | Mass number: 10, Atomic number: 5 |
| ||
C | | Mass number: 15, Atomic number: 5 |
| ||
D | | Mass number: 5, Atomic number: 15 |
Determine the mass number and atomic number of an atom with 5 protons and 5 neutrons.
X
?
5
Check for understanding
Atoms make up everything. What makes an atom?
What do we know?
Atoms make up everything. What makes an atom?
Subatomic particles make up an atom: protons, neutrons, and electrons. The number of each determines the properties of an atom.
What do we know?
I can use protons, neutrons, and electrons to model the structure of the atom.
THE STRUCTURE OF THE ATOM
Success measure
Not yet
Partly
Yes