Atoms, Elements and Compounds IGCSE

ELEMENT

• A substance made of atoms that all contain the same number of protons and cannot be split into anything simpler

• There are 118 elements found in the Periodic Table

​COMPOUND

• A pure substance made up of two or more elements chemically combined

• There is an unlimited number of compounds

• Compounds cannot be separated into their elements by physical means

• E.g. copper(II) sulfate (CuSO₄), calcium carbonate (CaCO₃), carbon dioxide (CO₂)

​MIXTURE

• A combination of two or more substances (elements and/or compounds) that are not chemically combined

• Mixtures can be separated by physical methods such as filtration or evaporation

• E.g. sand and water, oil and water, sulfur powder and iron filings

• A mixture of compounds

​MOLECULE

Two or more of the same/different elements

• All substances are made of tiny particles of matter called atoms which

• Each atom is made of subatomic particles called protons, neutrons, and electrons

• The mass of the electron is tiny, so the mass of an atom is contained within the nucleus where the protons and neutrons are located

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​Atomic Structure

Protons, Neutrons and Electrons

• The size of atoms is so tiny that we can't really compare their masses in units such as kilograms or grams, so a unit called the relative atomic mass is used

• One relative atomic mass unit is equal to 1/12^th the mass of a carbon-12 atom.

• All other elements are measured relative to the mass of a carbon-12 atom, so relative atomic mass has no units 

• Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

Finding Proton number

• The atomic number (or proton number) is the number of protons in the nucleus of an atom

• The symbol for atomic number is Z

• It is also the number of electrons present in a neutral atom and determines the position of the element on the Periodic Table

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Finding the Mass Number

• The Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom

• The symbol for nucleon number is A

• The nucleon number minus the proton number gives you the number of neutrons of an atom

• Note that protons and neutrons can collectively be called nucleons.

• The atomic number and mass number of an element can be shown using atomic notation

• The Periodic Table shows the elements together with their atomic (proton) number at the top and relative atomic mass at the bottom - there is a difference between relative atomic mass and mass number, but for your exam, you can use the relative atomic mass as the mass number (with the exception of chlorine)

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​How many subatomic particles?

• The atomic number of an atom and ion determines which element it is

• Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus.

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• Mass number = number of protons + neutrons

• Atomic number = number of protons

​Number of neutrons = Mass number - Atomic number

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​An atom is neutral and therefore has the same number of protons and electrons

Mass number = protons and neutrons

​Atomic number = Total protons and Total electrons.

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Neutrons = Mass - Atomic

Neutrons = 12 - 6 = 6​

Protons = 6

Electrons = 6​

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Example

Multiple Choice

• A

• B

• C

• D

Electronic configuration

• We can represent the structure of the atom in two ways: using diagrams called electron shell diagrams or by writing out a special notation called the electronic configuration (or electronic structure)

• Electrons orbit the nucleus in shells (or energy levels) and each shell has a different amount of energy associated with it

• The further away from the nucleus, the more energy a shell has

• Electrons fill the shell closest to the nucleus 

• When a shell becomes full of electrons, additional electrons have to be added to the next shell

• The first shell can hold 2 electrons

• The second shell can hold 8 electrons 

​Electronic configuration

• For GCSE, a simplified model is used that suggests that the third shell can hold 8 electrons

  • For the first 20 elements, once the third shell has 8 electrons, the fourth shell begins to fill

• The outermost shell of an atom is called the valence shell and an atom is much more stable if it can manage to completely fill this shell with electrons

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• The arrangement of electrons in shells can also be explained using numbers

• Instead of drawing electron shell diagrams, the number of electrons in each electron shell can be written down, separated by commas

• This notation is called the electronic configuration (or electronic structure)

  • E.g. Carbon has 6 electrons, 2 in the 1st shell and 4 in the 2nd shell

    • Its electronic configuration is 2,4

• Electronic configurations can also be written for ions

  • E.g. A sodium atom has 11 electrons, a sodium ion has lost one electron, therefore has 10 electrons; 2 in the first shell and 8 in the 2nd shell

    • Its electronic configuration is 2,8

Multiple Choice

What is the Electronic configuration of a Nitrogen atom?

• 2,5

• 3,5

• 2,6

• 2,7

Reorder

Reorder the following for the configuration of Calcium

• 2

• 8

• 8

• 2

1

2

3

4

• 1
• 2
• 3
• 4

Multiple Choice

What is the Electronic configuration of an Aluminium atom?

• 1,8,3

• 2,8,3

• 2,8

• 2,8,2

Multiple Choice

What is the Electronic configuration of a Potassium atom?

• 2,8,8

• 2,8,7,1

• 2,8,8,1

• 2,8,8,2

Electron shells and the P.Table

• There is a clear relationship between the electronic configuration and how the Periodic Table is designed

• The number of notations in the electronic configuration will show the number of occupied shells of electrons the atom has, showing the period in which that element is in

• The last notation shows the number of outer electrons the atom has, showing the group that element is in (for elements in Groups I to VII)

• Elements in the same group have the same number of outer shell electrons

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Match

Match the following

• 2,8,8,1

  1,2,3,4

• 2,8,4

  1,2,3

• 2,8

  1,2

• 2,8,8

  1,2,3

• 2,8,1

  1,2,3

Potassium

Silicon

Neon

Argon

Sodium

• Potassium

• Silicon

• Neon

• Argon

• Sodium

Isotopes

• Isotopes are different atoms of the same element that contain the same number of protons but a different number of neutrons

• The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number

• So C-14 ( or carbon-14) is the isotope of carbon which contains 6 protons, 6 electrons and 14 - 6 = 8 neutrons

  • It can also be written as ¹⁴C

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​Isotope properties

• Isotopes of the same element display the same chemical characteristics

• This is because they have the same number of electrons in their outer shells and this is what determines an atom's chemistry

• The difference between isotopes is the number of neutrons which are neutral particles within the nucleus and add mass only

• The difference in mass affects the physical properties, such as density, boiling point and melting point

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​Formation of ions

• An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons

• An atom will lose or gain electrons to become more stable

• The loss or gain of electrons takes place to gain a full outer shell of electrons which is a more stable arrangement of electrons

• The electronic configuration of an ion will be the same as that of a noble gas – such as helium, neon and argon

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Multiple Choice

• A

• B

• C

• D

Multiple Choice

• 0

• VII

• V

• II

Multiple Choice

• Lithium

• Oxygen

• Boron

• Neon

Multiple Choice

An element contains 4 electrons in its outer shell and has a neutron number of 125. Which element is it?

• Boron

• Lead

• Polonium

• Tin

Multiple Choice

Which two are isotopes of the same element?

• Q&R

• R&T

• R&S

• Q&S

Multiple Choice

• Helium

• Neon

• Argon

• Krypton

Multiple Choice

• Z, X

• W, Y

• W, X

• W, Z

Multiple Choice

• A

• B

• C

• D

Multiple Choice

• A

• B

• C

• D

Multiple Choice

What change occurs to an atom when it forms a negative ion?

• loses electrons

• gains electrons

• loses protons

• gain protons

Multiple Choice

which element does not form a stable ion and has the same electronic configuration as Argon?

• Cl

• F

• Se

• Br

Multiple Choice

• A

• B

• C

• D

Multiple Choice

• D

• C

• B

• A

Multiple Choice

• W

• X

• Y

• Z

Multiple Choice

• A

• B

• C

• D

Multiple Choice

• A

• D

• C

• B

Multiple Choice

• B

• A

• D

• C

Multiple Choice

• A

• D

• B

• C

Multiple Choice

Which sows the correct electron arrangement for lithium sulphide?

• A

• B

• C

• D

Multiple Choice

• S,T

• R,S

• Q,R

• Q,S

Multiple Choice

• P3Q

• PQ3

• P2Q2

• PQ2

Multiple Choice

• A

• B

• C

• D

Multiple Choice

• A

• B

• C

• D

Multiple Choice

• A

• B

• C

• D