C1 - Atomic Structure and the Periodic Table

What is an atom?

The smallest particle of an element that can exist

What is an element

A substance is made up of only one type of atom

What is a compound?

A substance made up of two or more elements chemically bonded together

What is a mixture?

A mixture is a substance that contains two or more elements that are not chemically bonded together

What is the difference between compounds and mixtures?

• In compounds, the elements are chemically bonded together
  

• In mixtures the substances are mixed together but not chemically bonded

How can mixtures be separated?

• Filtration, crystallisation, simple/fractional distillation, chromatography

What does filtration separate?

An insoluble solid from a liquid

What does crystallisation separate?

A soluble solid from a solvent

What did John Dalton think of atoms?

He described atoms as solid spheres that could not be divided

What did J.J. Thompson think of atoms?

The plum pudding model - showed the atom as a ball of positive charge with negatively charged electrons embedded

Alpha Scattering Experiment (Rutherford)

Alpha particles fired at thin sheet of gold foil

What observations were made?

1. Most alpha particles went straight through
   

2. Some were deflected slightly to the side
   

3. Some were deflected backwards

What did Rutherford conclude from this?

1. Most of the atom is empty space (particles went straight through)
   

2. The atom has a positively charged centre (particles were deflected)
   

3. Most of the atom’s mass is concentrated in the centre (few particles bounced back)

Compare the plum pudding model and the nuclear model

Plum pudding – All solid, no empty space; the atom is a ball of positive charge, mass is spread evenly throughout the atom, electrons are embedded in the ball of positive

Nuclear model – The atom is mostly empty space; the positive charge is in the nucleus, the mass is concentrated in the nucleus, electrons orbit the nucleus

What did Niels Bohr propose?

Electrons orbit the nucleus in shells – each shell is a fixed distance from the nucleus.

What did later experiments lead to?

The idea that the nucleus can be divided into smaller particles – protons

What did James Chadwick discover?

After 20 years, he discovered neutrons

What are the three subatomic particles in an atom?

Protons, neutrons, and electrons

What is the charge and relative mass of a proton?

1+ charge, mass 1

What is the charge and relative mass of a neutron?

0 charge, mass 1

What is the charge and relative mass of an electron?

-1 charge, very small mass

Why is an atom neutral overall?

As it has the same number of protons and electrons

How big is an atom?

About 1 × 10⁻¹⁰

What does the atomic number tell you?

The number of protons in an atom (also number of electrons)

What does the mass number tell you?

The number of protons and neutrons

Where is most of the mass of an atom?

In the nucleus

What are isotopes?

Atoms of the same element that has the same number of protons but a different number of neutrons

How do you calculate the relative atomic mass?

(Isotope Mass X abundance) + (Isotope Mass X abundance) / 100

How are electrons arranged in atoms?

In electron shells around the nucleus

How many shells can the first shell hold?

2 electrons

How many electrons can the second shell hold?

8 electrons

Who developed the modern periodic table?

Henry Moseley

What problems did the early tables have?

Some elements were in the wrong groups and tables were incomplete

How did Mendeleev arrange the elements?

• Mainly by atomic mass, but in some cases he did change the order because of the properties of the elements
  

• He left gaps for undiscovered elements

How is the modern periodic table arranged?

In order of increasing atomic number

What do columns (groups) represent

The number of electrons on the outer shell; this means they have similar properties

What do rows represent?

Elements with the same number of electron shells

What do metals form when they react?

Positive ions

What do non-metals form when they react?

Negative ions

What are some properties of metals?

• High boiling and melting points
  

• Good conductors of heat and electricity
  

• Malleable (bent into different shapes)
  

• Shiny
  

• Strong and hard

What are some properties of non-metals?

• Poor conductors of heat and electricity
  

• Usually dull
  

• Brittle
  

• Low melting and boiling points

What are the group 1 metals known as?

Alkalis

How many electrons are in their outer shell?

1, which makes them very reactive

What type of ions do they form

1+ ions

What are the physical properties of Group 1 metals?

• Soft
  

• Low Density
  

• Low melting/boiling points

What happens as you do go down the group?

• Reactivity increases – The outer electron is further away from the nucleus, so there is less attraction between the nucleus and the electron, so the electron is more easily lost
  

• Lower melting and boiling points - Outer electrons get further from nucleus, reducing the electrostatic attraction, requiring less energy to break the bonds

How do Group 1 metals react with water?

• They react very vigorously
  

• They produce hydrogen and also form hydroxides that dissolve in water (alkaline solutions)
  

• Lithium, sodium, and potassium float around surface, fizzing 

What are group 7 elements known as?

The Halogens

How many electrons are in their outer shell?

7

What type of ions do halogens form?

-1 ions

What happens as you go down the group?

• Reactivity decreases – it’s harder to gain an electron as the outer shell is further from the nucleus, so there is less attraction between the nucleus and the outer shell electrons, so an electron is less easily gained
  

• Boiling and melting points increase as you go down the group - molecules get bigger, so there is a stronger force of attraction between molecules, so more energy is needed to separate the molecules

What are the group 0 elements known as?

The Noble Gases

How many electrons do they have on their outer shell?

• They have 8 electrons, a full outer shell, apart from helium which has two
  

• They don’t need to gain or lose electrons, so they don’t react 
  
  

What are some properties of noble gases?

Colourless, non-flammable, and exist as monatomic gases (single atoms not bonded to each other)

What happens as you go down the group?

• As you go down the group, the boiling point increases
  

• This is because the atoms get larger, so the forces of attraction becomes stronger, so more energy is needed to overcome the forces