Topic 1 – Atomic Structure and the Periodic Table

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Subtopic Class Notes Revision Notes Past Exam Questions

C2.1 Mendeleev

C2.2 Structure of an atom

C2.3 The modern periodic table

C2.4 Electron shells

The history of the periodic table

1798:

Lavoisier began to divide the elements into metals and non-metals.

1860: 63 elements were known. John Newlands noticed that when the elements were put in order of atomic mass, that every eighth element had similar properties. But this only worked as far as calcium

C2.1 Mendeleev

1871: Mendeleev•Mendeleev arranged the elements

in order of atomic mass

•He arranged the elements so that

elements with similar properties

were in the same vertical columns

(groups).

•He thought there must still be some

elements to discover so left gaps for

them.

•He used the gaps to predict the

properties of the undiscovered

elements. For example he predicted

the properties of Eka Aluminium

(what we now know as Gallium)

C2.1 Mendeleev

ElementsDefinition…Element: A element is made up of only one type one atom. It cannot be spilt into simpler

substances

These atoms are ALL iron – there’s nothing else in here

C2.2 Structure of an atom

Ideas about atomic structure have changed over time.

In 1803, John Dalton said atoms were solid spheres of matter that could not be split.

Dalton also suggested that each element contained identical atoms.

Atomic StructureC2.2 Structure of an atom

In 1897, whilst studying cathode rays, JJ Thomson discovered tiny particles with a negative charge.

His discovery did not fit with Dalton’s model of the atom.

Ernest Rutherford suggested that an atom is mostly empty space with its positive charge and most of its mass in a tiny central nucleus.

Electrons orbited this nucleus at a distance, like planets around the Sun.

nucleus

electrons

C2.2 Structure of an atom

nucleus

neutron proton

electron

Atomic StructureC2.2 Structure of an atom

There are two properties of subatomic particles that are especially important:

1. Mass

2. Electrical charge

The atoms of an element contain equal numbers of protons and electrons and so have no overall charge.

1

1

+1

0

almost 0 -1

C2.2 Structure of an atom

How many protons?

The atoms of any particular element always contain the same number of protons. For example:

The number of protons in an atom is known as its atomic numberor proton number.

It is the smaller of the two numbers shown in most periodic tables.

hydrogen atoms always contain 1 proton;

carbon atoms always contain 6 protons;

magnesium atoms always contain 12 protons,

C2.3 The modern periodic table

Mass number

Electrons have a mass of almost zero, which means that the mass of each atom results almost entirely from the number of protons and neutrons in the nucleus.

The sum of the protons and neutrons in an atom’s nucleus is the mass number. It is the larger of the two numbers shown in most periodic tables.

1 0 1

3

13

4

14

7

27

C2.3 The modern periodic table

The actual mass of a hydrogen atom is 1.7x10-24g

(that’s 0.0000000000000000000000017g!)

Far too small a number to easily get your head around…

So – we use Relative Atomic Mass instead (Ar)

Relative Atomic Mass

•We compare the mass of each element to carbon-12

•Carbon 12 has 6 protons and 6 neutrons so we say its Ar = 12

•Magnesium has the same mass as 2 carbon atoms so we say its Ar = 24

C2.3 The modern periodic table

Why does chlorine have an Ar of 35.5?

…because there are different isotopes of chlorine

C2.3 The modern periodic table

…have the same number of protons and electrons, but different numbers of neutrons.

E.g. hydrogen has 3 isotopes…

H1

1 H2

1 H3

1Hydrogen Deuterium Tritium

Isotopes

Isotopes always have the same atomic number, but different mass number

C2.3 The modern periodic table

Chlorine always has 17 protons, but some have 18 neutrons, others have 20 neutrons

Isotopes

The relative abundance (proportion) of Chlorine-35 is 75%

The relative abundance (proportion) of Chlorine-37 is 25%

We can use the Ar and relative abundance of the isotopes to work out the Ar of the element.

C2.3 The modern periodic table

Example:Isotopes

The relative abundance (proportion) of Chlorine-35 is 75%

The relative abundance (proportion) of Chlorine-37 is 25%

Q1: Calculate the Ar of Neon

C2.3 The modern periodic table

Q1: Calculate the Ar of Neon

Q2: Calculate the Ar of Lithium

Q3: Calculate the Ar of Iron

Q4: Calculate the Ar of Silicon

Q5: Calculate the Ar of MercuryThe table shows the natural relative abundance of the main isotopes of mercury, Hg.

Mass number 198 199 200 201 202 204

% Relative abundance 10.0 16.9 23.1 13.2 29.9 6.9

C2.3 The modern periodic table

The Modern Periodic Table

•Elements are arranged in order or increasing atomic number (number of protons)

•Horizontal rows are called periods

•Vertical rows are called groups (the period number tells you the number of electron shells)

•Elements in the same group have similar properties (because they have the same number of

outer electrons)

•Most elements are metals (in yellow on the left), non-metals are shaded in blue

C2.3 The modern periodic table

•Electrons are held in shells•The first shell can hold upto 2 electrons•The second shell can hold upto 8 electrons•The third shell can hold upto 8 electrons

2.8.1

Electronic Structure

No. of electrons in 1st shell

No. of electrons in 2nd shell

No. of electrons in 3rd hell

C2.4 Electron shells