Topic 1: Atomic Structure and The Periodic Table

Question 1

What did John Daltons (English chemist 1766-1844) atomic theory include?

Answer 1

He said that:

• all matters are made up of tiny particles called atoms.
• atoms are tiny hard spheres that cannot be broken down into smaller parts.
• atoms cannot be created or destroyed.
• the atoms in an element are all identical (but each element has its own type of atom)

This theory explained some of the properties of matter.

Question 2

What happened in the late 19th century when a high voltage was applied to a glass tube that had most air removed?

Answer 2

When a high voltage is applied to a glass tube that has most air removed, glowing rays are seen. Some scientists thought these were ‘cathode rays’ where atoms leaving the negative electrode.

Question 3

In 1897 what did JJ Thompson discover?

Answer 3

He investigated the mass of particles in the rays and found that they were about 1800 times lighter than the lightest atom -hydrogen. Cathode rays therefore did not contain atoms but subatomic particles, which we now called electrons.

Question 4

What are the current subatomic particles that make up the structure of an atom?

Answer 4

Scientists have now worked out that atoms are made up of electrons heavier subatomic particles called protons and neutrons. All these particles have very small masses and electric charges. Most of the atom is empty space as the mass is concentrated in the centre of the nucleus.

Question 5

What are the charges and relative masses of the subatomic particles?

Answer 5

Proton: charge= +1 (positive)
mass= 1

Neutron: charge= 0 (no charge)
mass= 1

Electron: charge= -1 (negative)
mass= almost 0

Question 6

What is the structure of an atom?

Answer 6

At the centre of an atom is a tiny nucleus containing protons and neutrons. This is surrounded by fast moving electrons arranged in electron shells at different distances from the nucleus.

Question 7

Why are atoms of the same element always neutral?

Answer 7

Atoms in the same element always have equal numbers of protons and electrons and so have no overall charge because the charges cancel out.

Question 8

What did Earnest Rutherford discover in 1909?

Answer 8

He was working to investigate the structure of atoms. In one experiment tiny positive particles were fired at a thin gold foil. To their surprise most the particles passed through the gold foil with a few being deflected and a very small number bouncing back. Rutherford explained this by suggesting atoms are mostly empty space, with a small positive central nucleus which contains most of the mass.

Question 9

How is the periodic table ordered?

Answer 9

The elements were originally placed in order of masses of their atoms. However this caused some elements to be grouped with others of very different products. So a few elements were swapped around to make sure that those with similar properties were grouped together, even if it meant they were no longer in the correct order of mass.

Question 10

What experiments did Henry Moseley carry out and how is this affect the order of the periodic table?

Answer 10

Experiments carried out by him confirmed that the rearranged order of elements were actually correct. He showed they were in order of of the amount of positive charge in the nucleus

Question 11

What is the order of the modern periodic table?

Answer 11

In 1918 the proton was discovered. The modern periodic table places the elements in order of the number of protons in their atoms. This is the atomic number and it is this that defines an element - all the atoms of a particular element have the same unique atomic number (even their isotopes).

Increasing atomic number.

Question 12

What is the mass number?

Answer 12

The mass of an electron is described as negligible (it is so small it can be ignored). This explain why the nucleus of any atom contains nearly all its mass. For this reason the total number of protons and neutrons in an atoms is called the mass number.

Question 13

What represents the mass number and how is it different to the atomic number?

Answer 13

It is the top number and is the amount of neutrons and protons combined.

The atomic number is also known as the proton number and is just the number of protons. It is the bottom number underneath the mass number.

Question 14

What did James Chadwick discover in 1932 and what did this explain?

Answer 14

He discovered the neutron. His discovery explained why some atoms of the same element have different masses. These atoms are isotopes. They have the same atomic number but different mass numbers.

Question 15

What is an isotope and how do we refer to it?

Answer 15

Atoms of the same element that have different mass numbers but the same atomic number.
We refer to a specific isotope by adding its mass number to the element’s name.

Question 16

How did neutrons lead to the discovery of nuclear energy?

Answer 16

Understanding neutrons lead to the discovery of nuclear energy by firing neurones at the isotope uranium-235. It was discover nucleus can be split (nuclear fission). This produces energy new elements and transfers large amounts energy. Nuclear power stations use the energy from fission to produce electricity.

Question 17

What are relative atomic masses?

Answer 17

The mass of an atom is incredibly small so we measured it relative to an atom of Carbon-12. The isotope is used as a standard and given a mass of exactly 12.

Question 18

What is the relative mass of an isotope?

Answer 18

The relative mass of an isotope is its mass number.

Question 19

How do you calculate the relative atomic mass of an atom?

Answer 19

The symbol is Ar. A relative atomic mass is the mean mass of an atom of an element compared with carbon 12. It takes into account all the isotopes of an element and the amounts of each. RAMs are not whole numbers but most values are commonly rounded to whole numbers. The RAM of an element and its atomic number are shown in the periodic table

Question 20

Wh invented the Periodic table?

Answer 20

In 1869 Dimitri Mendeleev was a Russian scientist who was writing a chemistry text book and constructed the table to help make sense of the elements.

Question 21

How did Mendeleev first order the periodic table?

Answer 21

In order of of increasing relative atomic mass. Unlike other chemists who tried it before he left gaps to filled in later.

Question 22

Why did Mendeleev swap some positions of elements?

Answer 22

He swapped them if he thought that it better suited their chemical properties and those of their compounds.
E.g iodine has a lower RAM than tellurium so he should have placed it before tellurium due to its physical properties. Instead he placed it after tellurium so that it lines up with fluorine, chlorine and bromine as they were elements with similar chemicals properties to iodine.

Question 23

Why did Mendeleev leave gaps?

Answer 23

He assumed that elements would continue to be discovered, so he left gaps for them. This helped him to position existing so that vertical columns contained elements with increasing relative atomic mass, and horizontal rows contained elects with similar chemical properties.

Question 24

What predictions did he make about undiscovered elements?

Answer 24

He used gaps in the table to make predictions about the properties of undiscovered elements, based on the properties of nearby elements.

Question 25

Give an example of an element he made a prediction about and left a space of the periodic table for?

Answer 25

He made a prediction about an element called eka-aluminium. When gallium was discovered shortly afterwards it’s properties closely fitted those of ‘eka-aluminium’.

Question 26

What does inert mean?

Answer 26

Unreactive elements. Chemists had not predicted their existence by the end of the 19th century but they were easily fit in on the table as group 0.

Question 27

Wha did Moseley discover after his experiments?

Answer 27

He realised that an atomic number was equal to the number of positive charges in the nucleus of an atom.

Question 28

How is the modern periodic table organised?

Answer 28

• elements in a row or period are in order of increasing atomic number.
• elements with similar properties are in the same column or group.
• non metals are on the right of the periodic table and metals are on the left.

Question 29

How is the iodine tellurium pair reversal explained?

Answer 29

Iodine exists naturally as iodine-127 but tellurium has several different isotopes about 20% of its atoms are Te-126 but 2/3 of its atoms are Te-128 or Te-130, so its relative atomic mass is greater than that of iodine.

Question 30

What is an electron shell?

Answer 30

In an atom electrons occupy electron shells arranged around the nucleus. The shells can be modelled in diagrams as circles, with electrons as dots or crosses on each circle.

Question 31

What is the electronic configuration?

Answer 31

The way in which atom’s electrons are arranged is called its electronic configuration.

Question 32

How are electron shells made up?

Answer 32

Each shell can contain different numbers of electron. But…

• the first shell must contain up to 2 atoms.
• the second and third shell can contain up to 8 electrons.

Question 33

An example of the electronic configuration of sodium:

Answer 33

The first shell contains 2 electrons. The second shell contains 8 and the thirds shell contains one.
The electronic configuration is therefore…
2.8.1

Question 34

What is the electronic configuration related to?

Answer 34

The electronic configuration of an element is related to its position.

Also…

• the number of occupied shells is equal to the period number (row number)
• the number of electrons in the outer shell is equal to the group number. (Except for group 0 which have full outer shells).

Question 35

How is Mendeleev’s model of the periodic table different to that of the modern periodic table?

Answer 35

Mendeleev arranged the elements in order of relative atomic mass. He also used the properties of elements and their compounds, so he sometimes had to swap elements. For example the Ar of iodine is 126.9 so he should have put iodine before tellurium. He did this so it went into the same column as similar elements.
In the modern periodic table elements are arranged in order of increasing atomic number (the number of protons in a nucleus). This explain why Mendeleev was correct when he put placed tellurium beofre iodine.