Atomic
Structure

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

Gold and iron are both metals, yet gold is shiny and unreactive while iron rusts. What makes their atoms different?

2

Which particle in an atom has almost no mass, and where is it found?

3

Carbon atoms in fossil fuels have 6 neutrons, but some carbon atoms in living things have 8. How can both be carbon?

4

A calcium atom has 20 electrons. Why does a calcium ion have only 18?

5

Why are helium and argon unreactive?

6

How are the 19 electrons of a potassium atom arranged?

Starter

PredictGold is shiny yellow and not very reactive. Iron looks silvery and rusts. Both are metals.

What could make their atoms behave so differently?

NoteAtoms of different elements contain different numbers of sub-atomic particles. This topic shows what those particles are and how they are arranged.

1

Sub-atomic Particles

1.1What an atom is made of

An atom is the smallest particle of an element that still has the chemical characteristics of that element.

Atoms are made of three sub-atomic particles: protons, neutrons and electrons.
Protons and neutrons are together called nucleons. They are in the nucleus at the centre of the atom.
Electrons move very quickly in electron shells around the nucleus.
Atoms are electrically neutral, so the number of protons = the number of electrons.

1

Sub-atomic Particles

1.1Charge, mass and location

Particle	Relative charge	Relative mass	Location
pproton	+1	1	nucleus
nneutron	0	1	nucleus
eelectron	−1	1/1836	electron shells

AnalogyPut a proton and an electron on a balance and it is like a bowling ball against a teaspoon of sugar. Almost all the mass of an atom is in its nucleus.

1

Sub-atomic Particles

126C

12 nucleon (mass) number 6 proton (atomic) number read as carbon-12

1.2Proton number and nucleon number

DefinitionProton number: the number of protons in the nucleus of an atom. It tells us which element the atom is from.

DefinitionNucleon number: the total number of protons and neutrons in the nucleus of an atom.

number of neutrons = nucleon number − proton number

AnalogyThe proton number is an element's identity card number. Every carbon atom carries 6, and no other element has that number.

1

Sub-atomic Particles

Worked exampleTungsten is used to make drill bits. Tungsten-184 is written ¹⁸⁴₇₄W. Deduce the numbers of protons, neutrons and electrons in one atom of tungsten-184.

Step 1Protons = proton number = 74

Step 2Neutrons = nucleon number − proton number = 184 − 74 = 110

Step 3The atom is neutral, so electrons = protons = 74

1

Sub-atomic Particles

1.3Sub-atomic particles in ions

An ion is a particle formed when an atom or group of atoms gains or loses electrons. The numbers of protons and neutrons stay the same.

Cation: the atom loses electrons, so it has fewer electrons than protons, e.g. Ca²⁺.
Anion: the atom gains electrons, so it has more electrons than protons, e.g. O²⁻.

1

Sub-atomic Particles

Particle	Protons	Neutrons	Electrons
Ca	20	20	20
Ca²⁺	20	20	18
O	8	8	8
O²⁻	8	8	10

Ca²⁺ has a charge of +2, so it has 2 more protons than electrons.
Electrons = 20 − 2 = 18.
O²⁻ has a charge of −2, so it has 2 more electrons than protons.
Electrons = 8 + 2 = 10.

Exam tipElectrons in an ion = protons − charge. Protons and neutrons never change when an atom becomes an ion.

1

Sub-atomic Particles

Your turnComplete the table. The first row is done for you.

Particle	Protons	Neutrons	Electrons
¹⁸⁴W	74	110	74
uranium-235 atom (proton number 92)	92	143	92
sodium ion with a charge of +1 (11 protons, 12 neutrons in the atom)	11	12	10
³²₁₆S²⁻	16	16	18

2

Isotopes

hydrogen-11 proton, 0 neutrons

hydrogen-21 proton, 1 neutron

hydrogen-31 proton, 2 neutrons

2.1Isotopes of an element

DefinitionIsotopes are atoms of the same element with the same proton number but different nucleon numbers. They have different numbers of neutrons.

NoteIsotopes are named with their nucleon number, e.g. chlorine-35 and chlorine-37.

AnalogyIsotopes are like identical twins carrying different backpacks. They look and behave the same, but one is heavier.

2

Isotopes

2.1Three isotopes of hydrogen

	hydrogen-1	hydrogen-2	hydrogen-3
Notation	¹₁H	²₁H	³₁H
Protons	1	1	1
Neutrons	0	1	2
Electrons	1	1	1
Nucleon number	1	2	3

Chemical propertiesIsotopes have the same number of electrons, so they react in the same way.

Physical propertiesIsotopes have different masses, so they may have different densities, melting points and boiling points.

2

Isotopes

Quick checkWhich statement about atoms of chlorine-35 and chlorine-37 is correct?

AThey have the same number of neutrons.

BThey have the same nucleon number.

CThey have the same number of electrons.

DThey have the same mass.

NoteC: both atoms have 17 protons, so both have 17 electrons and react in the same way. They differ in neutrons (18 and 20), nucleon number (35 and 37) and mass.

3

Electrons in Shells

3.1Electron shells

Electrons in the shell closest to the nucleus have the lowest energy. Electrons fill the lowest energy shells first.
The first shell holds up to 2 electrons. The second and third shells usually hold up to 8 electrons each.
The outermost shell is the valence shell, and its electrons are valence electrons.

AnalogyElectrons fill shells like guests checking into a hotel whose lifts are broken. Everyone takes the lowest floor with a free room, and the ground floor has only 2 rooms.

3

Electrons in Shells

3.1Full valence shells

A full valence shell is stable, so atoms of helium and argon are unreactive.
An atom with a valence shell that is not full is reactive.

3

Electrons in Shells

3.2Electronic configuration

The electronic configuration of an atom shows the number of electrons in each shell, starting from the innermost shell, e.g. sodium is 2.8.1.

The period number is the number of electron shells.
The group number gives the number of valence electrons: Groups 1 and 2 have 1 and 2, and Groups 13 to 18 have 3 to 8. Helium has 2.

3

Electrons in Shells

3.3Shell diagrams

Li2.1

Be2.2

Ne2.8

Na2.8.1

3

Electrons in Shells

3.4Configurations of the first 20 elements

Period	Group 1	2	13	14	15	16	17	18
1	H1							He2
2	Li2.1	Be2.2	B2.3	C2.4	N2.5	O2.6	F2.7	Ne2.8
3	Na2.8.1	Mg2.8.2	Al2.8.3	Si2.8.4	P2.8.5	S2.8.6	Cl2.8.7	Ar2.8.8
4	K2.8.8.1	Ca2.8.8.2

NoteElectronic configurations are needed only up to calcium (proton number 20).

3

Electrons in Shells

Worked exampleHow are the electrons arranged in an atom of potassium (proton number 19)?

Step 1Potassium has 19 protons, so it has 19 electrons.

Step 2Fill the first shell with 2, then the second with 8: 2.8. That is 10 electrons.

Step 3The remaining 9 electrons: the third shell takes 8, so 1 is left for the fourth shell.

Step 4Electronic configuration = 2.8.8.1

Exam tipIsotopes of an element have the same number of electrons, so they have the same electronic configuration.

3

Electrons in Shells

Your turnBoron-11 is the most common isotope of boron. Boron has proton number 5.

(a) How many protons, neutrons and electrons does an atom of boron-11 have?

(a) 5 protons, 6 neutrons (11 − 5) and 5 electrons[1]

(b) What is its electronic configuration?

(b) 2.3[1]

3

Electrons in Shells

Quick checkCalcium has proton number 20. What is its electronic configuration?

A2.4.6.8

B2.6.8.4

C2.8, 10

D2.8.8.2

NoteD: the first shell takes 2, the second takes 8, the third takes 8 and the last 2 go in the fourth shell. The third shell usually holds a maximum of 8 electrons, so C is wrong.

3

Electrons in Shells

ChallengeArsenic is a toxic element with proton number 33. Arsenic and its compounds react in the same way as phosphorus and its compounds. Phosphorus has proton number 15. Arsenic has 5 valence electrons. Suggest why arsenic and its compounds react in the same way as phosphorus and its compounds.

Phosphorus has the electronic configuration 2.8.5, so it also has 5 valence electrons.[1]

Elements with the same number of valence electrons have similar chemical properties.[1]

4

Summary

4.1Atomic structure at a glance

The nucleus contains protons and neutrons, together called nucleons.
Protons have a relative charge of +1 and neutrons 0. Both have a relative mass of 1.
Electrons have a relative charge of −1 and a relative mass of 1/1836.
The first shell holds up to 2 electrons and the second and third usually hold up to 8. Oxygen is 2.6.

4.2Finding the numbers

To find	Use
Protons	the proton number
Neutrons	nucleon number − proton number
Electrons in an atom	the same number as the protons
Electrons in an ion	protons − charge, e.g. Ca²⁺ has 20 − 2 = 18
Electron shells	the period number
Valence electrons	the group number

Exit ticket

1How many protons, neutrons and electrons are in ²³₁₁Na⁺?

11 protons, 12 neutrons and 10 electrons[1]

2Define the term isotope.

Atoms of the same element with the same proton number but different nucleon numbers.[1]

3Write the electronic configuration of sulfur (proton number 16).

2.8.6[1]

4Why is argon unreactive?

Its valence shell is full, so it is stable.[1]

AtomicStructure

By the end of this topic, you can answer

Starter

Sub-atomic Particles

1.1What an atom is made of

Sub-atomic Particles

1.1Charge, mass and location

Sub-atomic Particles

1.2Proton number and nucleon number

Sub-atomic Particles

Sub-atomic Particles

1.3Sub-atomic particles in ions

Sub-atomic Particles

Sub-atomic Particles

Isotopes

2.1Isotopes of an element

Isotopes

2.1Three isotopes of hydrogen

Isotopes

Electrons in Shells

3.1Electron shells

Electrons in Shells

3.1Full valence shells

Electrons in Shells

3.2Electronic configuration

Electrons in Shells

3.3Shell diagrams

Electrons in Shells

3.4Configurations of the first 20 elements

Electrons in Shells

Electrons in Shells

Electrons in Shells

Electrons in Shells

Summary

4.1Atomic structure at a glance

4.2Finding the numbers

Exit ticket

Atomic
Structure