Chemistry: Topic 1: Atomic structure and the periodic table

What are all substances made of?

Atoms

What is an atom?

The smallest part of an element that can exist

What are atoms of each element represented by?

A chemical symbol

e.g. What does O represent?

An atom of oxygen

e.g What does Na represent?

Sodium

How many different elements are there?

Around 100

What are elements shown on?

The periodic table

What are compounds formed from?

Elements by chemical reactions

Chemical reactions always involve what?

The formation of one or more new substances

What do chemical reactions often involve?

A detectable energy change

What do compounds contain?

Two or more elements chemically combined in fixed proportions

How can compounds be represented?

By formulae using the symbols of the atoms from which they were formed

How can compounds be separated into elements?

Only by chemical reactions

How can chemical reactions be represented?

By word equations or equations using symbols and formulae

What does a mixture consist of?

Two or more elements or compounds not chemically combined together

What are the chemical properties of each substance in the mixture like?

Unchanged

How can mixtures be separated?

By physical processes such as:

• Filtration

• Crystallisation

• Simple distillation

• Fractional distillation

• Chromatography

What do these physical processes not involve?

Chemical reactions - meaning no new substances are made

What might experimental evidence lead to?

A scientific model being changed or replaced

Before the discovery of the atom, what were atoms thought to be?

Tiny spheres that could not be divided (billiard ball theory)

What did the discovery of the electron lead to?

The plum pudding model of the atom

What did the plum pudding model suggest?

That the atom is a ball of positive charge with negative electrons embedded in it

What did the results from the alpha particle scattering experiment lead to?

• The conclusion that the mass of an atom was concentrated at the centre (nucleus)

• That the nucleus was charged

Describe the alpha particle scattering experiment

• A beam of alpha particles was aimed at very thin gold foil and their passage through was detected

• Some of the alpha particles emerged from the foil at different angles, and some even came straight back

• The positively charged alpha particles were being repelled and deflected by a small concentration of positive charge in the atom (nucleus)

What happened due to this?

This nuclear model replaced the plum pudding model

Who adapted this nuclear model?

Niels Bohr

How did Niels Bohr adapt the nuclear model?

By suggesting that electrons orbit the nucleus at specific distances

What agreed with his experimental observations

Bohr’s theoretical calculations

What idea did later experiments lead to?

The idea that the positive charge of any nucleus could be subdivided into a whole number of smaller particles, each particle having the same amount of positive charge

What name was given to these particles?

Proton

What did the experimental work of James Chadwick provide the evidence to show?

The existence of neutrons within the nucleus

How long after was this after the nucleus became an accepted scientific idea?

About 20 years

What is the relative electrical charge of a proton?

+1

What is the relative electrical charge of a neutron?

0

What is the relative electrical charge of an electron?

-1

In an atom, what is the number of electrons equal to?

The number of protons in the nucleus

What is the electrical charge of an atom?

0

What is the atomic number of an element?

The number of protons in an atom of an element

What do all atoms of a particular element have the same number of?

Protons

How do atoms of different elements compare to this?

They have different numbers of protons

What is the radius of an atom?

Around 0.1 nm (1×10^-10m)

What is the radius of a nucleus?

Less an 1/10,000 of that of the atom (around 1×10^-14m)

Where is almost all of the mass of an atom?

In the nucleus

What is the relative mass of a proton?

1

What is the relative mass of a neutron?

1

What is the relative mass of an electron?

very small

What is the mass number of an atom?

The sum of the protons and neutrons

What are isotopes?

Atoms of the same element that have different numbers of neutrons, but the same number of protons

What is the relative atomic mass of an element?

An average value that takes account of the abundance of the isotopes of the element

Relative atomic mass equation

(Isotope 1 mass x abundance) + (Isotope 2 mass x abundance)

abundance as a decimal

How else can relative atomic mass be written?

RAM or A_r

What is relative atomic mass compared to?

The mass of one atom of Carbon-12

What do the electrons in an atom occupy?

The lowest available energy levels (innermost available shells)

What is the electronic structure of an atom in numbers?

2, 8, 8 etc.

What does this diagram show?

• The electronic structure of sodium - 2,8,1

• Two electrons in the lowest energy level

• Eight in the second energy level

• One in the third energy level

How are the elements in the periodic table arranged?

In order of atomic (proton) number

What are elements with similar properties arranged in?

Columns/ groups

What is the table called the periodic table?

Similar properties appear at regular intervals

What are the columns of the periodic table called?

Groups

What do elements in the same group of the table have?

The same number of electrons in their outer shell (outer electrons)

This gives the, similar chemical properties

What are rows of the periodic table called?

Periods

What do the periods of the table show?

How many shells the element has

How did scientists attempt to classify the elements at first?

• Before the discovery of protons and neutrons

• Arranged elements in order of their atomic weights

What were the early periodic tables like?

• Incomplete

• Some elements were placed in inappropriate groups if the strict order of atomic weights was followed

WHo played a big part in the development of the periodic table?

Demitri Mendeleev

How did Mendeleev overcome some of the problems with the periodic table?

• By leaving gaps for elements that he thought had not been discovered

• In some places changed the order based on atomic weights

What happened with the elements with properties predicted by Mendeleev?

They were discovered and filled the gaps

What made it possible to explain why the order based on atomic weights was not always correct?

Knowledge of isotopes

What are metals?

Elements that react to form positive ions

What are non-metals?

Elements that do not react to form positive ions

What are the majority of elements?

Metals

Where are metals found on the periodic table?

To the left and towards the bottom of the table

Where are non-metals found on the periodic table?

Towards the right and top of the table

What are the elements in Group 1 of the periodic table known as?

Alkali metals

Why do the elements in Group 1 have characteristic properties?

Because of the single electron in their outer shell

What are group 1 elements like?

• soft

• low density

What are the trends as you go down the group?

• increased reactivity

• lower MPs and BPs

• higher RAMs

Why is sodium more reactive than lithium?

• sodium needs a full outer shell

• has to lose 1 electron

• the attraction between the nucleus and electron is smaller as they are further away and sodium has more shells

• the outer electron is more easily lost, so sodium is more reactive

How do group 1 elements react with non-metals?

• They don’t need much energy to lose their one outer electron to form a full outer shell, so readily form 1+ ions

• Only ever react to form ionic compounds

• These are usually white solids that dissolve in water to form colourless solutions 

How do G1 elements react with water?

• react vigorously

• produce hydrogen gas and metal hydroxides

Equation for G1 reaction with water

Element + water > element hydroxide + hydrogen

2E + 2H₂O > 2E + 2NaOH + H₂

What are metal hydroxides?

Compounds that dissolve in water to produce alkaline solutions

How does lithium react with water?

• fizzes steadily on surface

• gradually disappears

How does sodium react with water?

• fizzes rapidly, moves around surface as liquid ball

• disappears quickly

How does potassium react with water?

• Ignites with sparks and a lilac flame

• Disappears very quickly

Why does the reaction with potassium produce a flame?

The reaction releases enough energy to ignite the hydrogen in the water

What do group 1 elements react with oxygen to form?

Metal oxides

How does lithium react with oxygen?

• burns strongly

• crimson flame

• produces a white solid

What does Lithium + Oxygen form?

Lithium Oxide (Li₂O)

How does sodium react with oxygen?

• strong orange flame

• produces a white solid

What does sodium + oxygen form?

Sodium Oxide (Na₂O) and Sodium Peroxide (Na₂O₂)

How does potassium react with oxygen?

• large pieces produce a lilac flame

• smaller ones make a solid immediately

What does Potassium + Oxygen form?

A mixture of Potassium Peroxide (K₂O) and Potassium Superoxide (KO₂)

Why do G1 elements tarnish in air?

The metal reacts with oxygen in the air to form a dull metal oxide layer

How do G1 elements react with chlorine?

• React vigorously when heated in chlorine gas

• Form white metal chloride salts

Equation for reaction of G1 elements with chlorine

Element + Chlorine > Element Chloride

2E + Cl₂ > 2ECl

How does lithium react with chlorine?

• white powder is produced and settles on the sides of the container

How does sodium react with chlorine?

• burns with a bright yellow flame

• clouds of white powder produced and settle on the sides of the container

How does potassium react with chlorine?

• reaction is even more vigorous than sodium

What are the elements in Group 7 of the periodic table known as?

The halogens