TOPIC 1: KEY CONCEPTS IN CHEMISTRY
Section 1: Atomic Structure
1.1) Describe how the Dalton model of the atom has changed over time because of the discovery
of subatomic particles
Dalton presented his atomic theory based on 3 core principles:
a) Matter is made of atoms which are tiny particles – these cannot be created/destroyed/divided
b) Atoms of the same element are identical, and atoms of different elements differ
c) Different atoms combine to form new substances
1897: J. J. Thomson discovered the electron and figured out that it was negatively charged
: From this, he created the ’plum-pudding’ model
Sphere of positive charge
Negatively charged electrons
1909: Rutherford conducts his ‘gold-foil’ experiment
: Most alpha particles (α-particles) pass straight through
- Most of the atom is empty space
: Some alpha particles are deflected by a small amount
- Nucleus is positively charged so any particles that went near the nucleus were slightly
deflected
: A small number of alpha particles are fully deflected
- Most of the mass is concentrated in the nucleus so particles that collided head on with
the nucleus were fully deflected
-
1.2) Describe the structure of an atom as a nucleus containing protons and neutrons, surrounded
by electrons in shells
Electron shells
1.3) Recall the relative charge and relative mass of a) a proton, b) a neutron and c) an electron
, TOPIC 1: KEY CONCEPTS IN CHEMISTRY
Particle Relative charge Relative mass
Proton +1 1
Neutron 0 1
Electron -1 1/1800
1.4) Explain why atoms contain equal numbers of protons and electrons
Atoms contain charged subatomic particles but remain neutral
- There is an equal number of electrons and protons in the atom, so their negative and
positive charges cancel out to 0
1.5) Describe the nucleus of an atom as very small compared to the overall size of the atom
The nucleus is located in the centre of the atom and is extremely tiny in comparison to the overall
size of the atom
1.6) Recall that most of the mass of an atom is concentrated in the nucleus
Majority of an atom’s mass is concentrated in the nucleus because the neutrons and protons are
much heavier than the electrons and they are densely packed
1.7) Recall the meaning of the term mass number of an atom
1.8) Describe atoms of a given element as having the same number of protons in the nucleus and
that this number is unique to that element
Mass number, relative atomic number
23
Na
Sodium Element
11
Atomic number
Mass number: Total mass (number) of neutrons and protons
Atomic number: Number of protons in an atom
Element: A material/substance that cannot be broken down using chemical means
The proton number is unique to every element (an atom with 11 protons will always be sodium)
, TOPIC 1: KEY CONCEPTS IN CHEMISTRY
1.9) Describe isotopes as different atoms of the same element containing the same number of
protons but different numbers of neutrons in their nuclei
Isotopes: Different atoms of the same element (e.g. ⁷⁹Br, ⁸¹Br) with the same number of protons but
a different number of neutrons
1.10) Calculate the numbers of protons, neutrons and electrons in atoms given the atomic number
and mass number
Covered earlier in 1.7/1.8
1.11) Explain how the existence of isotopes results in relative atomic masses of some elements
not being whole numbers
1.12) Calculate the relative atomic mass of an element from the relative masses and abundances
of its isotopes
Relative atomic mass: Mass of the average percentage (%) abundance of an element’s isotope
relative to 1/12 of the mass of Carbon-12
³⁷Cl: 25% abundance in environment, 25/100 = 0.25
³⁵Cl: 75% abundance in environment, 75/100 = 0.75
Relative atomic mass of Cl: (0.25 x 37) + (0.75 x 35) = 35.5
As 35.5 is much closer to 35 than 37, this helps show that ³⁵Cl is much more abundant
Section 2: The Periodic Table
1.13) Describe how Mendeleev arranged the elements, known at that time, in a periodic table by
using properties of these elements and their compounds
1869: Dmitri Mendeleev created the first draft of his Periodic Table
- He organised elements vertically based on their chemical properties
- He then organised the elements horizontally in order of relative atomic mass (lightest at
the top)
- He also left gaps in his Table where no known element fit as he assumed not all
elements had been discovered yet
1.14) Describe how Mendeleev used his table to predict the existence and properties of some
elements not then discovered
Section 1: Atomic Structure
1.1) Describe how the Dalton model of the atom has changed over time because of the discovery
of subatomic particles
Dalton presented his atomic theory based on 3 core principles:
a) Matter is made of atoms which are tiny particles – these cannot be created/destroyed/divided
b) Atoms of the same element are identical, and atoms of different elements differ
c) Different atoms combine to form new substances
1897: J. J. Thomson discovered the electron and figured out that it was negatively charged
: From this, he created the ’plum-pudding’ model
Sphere of positive charge
Negatively charged electrons
1909: Rutherford conducts his ‘gold-foil’ experiment
: Most alpha particles (α-particles) pass straight through
- Most of the atom is empty space
: Some alpha particles are deflected by a small amount
- Nucleus is positively charged so any particles that went near the nucleus were slightly
deflected
: A small number of alpha particles are fully deflected
- Most of the mass is concentrated in the nucleus so particles that collided head on with
the nucleus were fully deflected
-
1.2) Describe the structure of an atom as a nucleus containing protons and neutrons, surrounded
by electrons in shells
Electron shells
1.3) Recall the relative charge and relative mass of a) a proton, b) a neutron and c) an electron
, TOPIC 1: KEY CONCEPTS IN CHEMISTRY
Particle Relative charge Relative mass
Proton +1 1
Neutron 0 1
Electron -1 1/1800
1.4) Explain why atoms contain equal numbers of protons and electrons
Atoms contain charged subatomic particles but remain neutral
- There is an equal number of electrons and protons in the atom, so their negative and
positive charges cancel out to 0
1.5) Describe the nucleus of an atom as very small compared to the overall size of the atom
The nucleus is located in the centre of the atom and is extremely tiny in comparison to the overall
size of the atom
1.6) Recall that most of the mass of an atom is concentrated in the nucleus
Majority of an atom’s mass is concentrated in the nucleus because the neutrons and protons are
much heavier than the electrons and they are densely packed
1.7) Recall the meaning of the term mass number of an atom
1.8) Describe atoms of a given element as having the same number of protons in the nucleus and
that this number is unique to that element
Mass number, relative atomic number
23
Na
Sodium Element
11
Atomic number
Mass number: Total mass (number) of neutrons and protons
Atomic number: Number of protons in an atom
Element: A material/substance that cannot be broken down using chemical means
The proton number is unique to every element (an atom with 11 protons will always be sodium)
, TOPIC 1: KEY CONCEPTS IN CHEMISTRY
1.9) Describe isotopes as different atoms of the same element containing the same number of
protons but different numbers of neutrons in their nuclei
Isotopes: Different atoms of the same element (e.g. ⁷⁹Br, ⁸¹Br) with the same number of protons but
a different number of neutrons
1.10) Calculate the numbers of protons, neutrons and electrons in atoms given the atomic number
and mass number
Covered earlier in 1.7/1.8
1.11) Explain how the existence of isotopes results in relative atomic masses of some elements
not being whole numbers
1.12) Calculate the relative atomic mass of an element from the relative masses and abundances
of its isotopes
Relative atomic mass: Mass of the average percentage (%) abundance of an element’s isotope
relative to 1/12 of the mass of Carbon-12
³⁷Cl: 25% abundance in environment, 25/100 = 0.25
³⁵Cl: 75% abundance in environment, 75/100 = 0.75
Relative atomic mass of Cl: (0.25 x 37) + (0.75 x 35) = 35.5
As 35.5 is much closer to 35 than 37, this helps show that ³⁵Cl is much more abundant
Section 2: The Periodic Table
1.13) Describe how Mendeleev arranged the elements, known at that time, in a periodic table by
using properties of these elements and their compounds
1869: Dmitri Mendeleev created the first draft of his Periodic Table
- He organised elements vertically based on their chemical properties
- He then organised the elements horizontally in order of relative atomic mass (lightest at
the top)
- He also left gaps in his Table where no known element fit as he assumed not all
elements had been discovered yet
1.14) Describe how Mendeleev used his table to predict the existence and properties of some
elements not then discovered
