Chemistry February Assessment Revision
Atoms, elements, compounds and mixtures
Atom definition : The smallest unit of matter that can exist
Element definition: A substance that is only made of only one type of atom
Compound definition: Where two or more different elements are chemically combined together.
Molecules definition: Where two or more different atoms are chemically bonded together.
Mixture definition: Where two or more different substances are not chemically joined together.
Writing Formulae and Equations
In most chemical reactants and products, are neutral charge, which means that they do not have any
charge. So for example, Cl has -1 charge, because it is in Group 7. However, K had a +1 charge,
because it is in Group 1. So when K and Cl react, they have a neutral charge (-1 + 1 = 0). But when for
example O reacts with K, as O has -2 charge and K has +1 charge there has to be two K in order to
balance out the negative charge. So it would be K 2O (+1 +1 -2 = 0)
However there are some compounds that have negative charge.
Sulphate = SO4 (-2 charge)
Nitrate = NO3 (-1 charge)
Hydroxide = OH (-1 charge)
Carbonate = CO3 (-2 charge)
Metals reacting with Oxygen.
When metals react with oxygen, they create a metal oxide. An example of a metal oxide is Iron
Oxide. It is a ionic compound, which means that ions are transferred between the metal and the
oxygen.
Metals reacting with Acids.
When metals react with acid, it creates a salt and hydrogen. A common example of this is Sodium +
Hydrochloric acid -> Sodium Chloride + Hydrogen. The name of the salt is based off of two things.
The first part of the name originates from the metal being reacting to and the second part of the
name originates from the acid name.
Atom structure and electron configuration
Atom structure is they way the atom is built, and it is stable. The atom consists of 3 subatomic
particles and 2 main parts. The 3 subatomic particles are neutron, proton and electron. Neutron has
a neutral charge and was discovered by James Chadwick in 1932. Proton has a positive charge of +1
and was discovered by Ernest Rutherford in 1919. Finally, electron has a negative charge of -1 and
was discovered by J.J Thomson in 1897.
, Neutrons and protons have the same relative mass of 1. However the relative mass of an electron is
so low that it doesn’t really matter with the mass of the atom. Electrons are also transferrable, and
this is the thing that makes chemical reactions happen.
The two parts of the atom are the neutron and the orbiting shells. The nucleus is a densely packed
centre that contains protons and neutrons. The shells that orbit it contain the electrons.
The shells in electron go in an order. The first ring can only hold 2 electrons, every ring after the first
ring has the capacity to hold 8 electrons.
One way we can demonstrate the shells of atoms is by the dot and cross method. The dot and cross
method are that we get an element, and we have to cross/dot the number of electrons the atom has
in its appropriate shells.
Relative atomic mass
Relative atomic mass is the mass of the element based off of the mass it has of its isotopes. For
example, 75% of the chlorine atoms in existence is Cl 35. Around 25% of the chlorine atoms in
existence is Cl37.
Relative Mass Formula:
Relative Atomic Mass = (Ar x %) + (Ar2 x %) / 100
e.g. Relative atomic mass = (35 x 75%) + (37 x 25%)/100
= 35.5
Therefore Chlorine has a relative atomic mass of 35.5.
Isotopes
Isotopes are atoms which have the same number of protons and electrons but different number of
neutrons. Isotopes only have different masses and apart from that they are the exact same to
another isotope of the same element.
Nearly all the elements in the Periodic Table have at least 1 isotope.
History of the atom
Democritus, a Greek philosopher, thought that stuff was made out of tiny particles with no space
around it. He called it ‘Atomos,’ Greek for indivisible. However, popular philosopher Aristotle
opposed his theory, stating instead that there are only four elements: air, water, wind and fire. His
theory became forgotten until 1808.
In 1808, a quaker with the name John Dalton sought to challenge the Aristotelian theory. Even
though Democritus’ theory was theoretical, Dalton found out that common substances always broke
down into same elements in the same proportion. He said that they each had a particular size and
mass and could not be created or destroyed. He made the Dalton model of the atom: hard spheres
that could not be broken down. Atomic theory had now been accepted into scientific community.
Atoms, elements, compounds and mixtures
Atom definition : The smallest unit of matter that can exist
Element definition: A substance that is only made of only one type of atom
Compound definition: Where two or more different elements are chemically combined together.
Molecules definition: Where two or more different atoms are chemically bonded together.
Mixture definition: Where two or more different substances are not chemically joined together.
Writing Formulae and Equations
In most chemical reactants and products, are neutral charge, which means that they do not have any
charge. So for example, Cl has -1 charge, because it is in Group 7. However, K had a +1 charge,
because it is in Group 1. So when K and Cl react, they have a neutral charge (-1 + 1 = 0). But when for
example O reacts with K, as O has -2 charge and K has +1 charge there has to be two K in order to
balance out the negative charge. So it would be K 2O (+1 +1 -2 = 0)
However there are some compounds that have negative charge.
Sulphate = SO4 (-2 charge)
Nitrate = NO3 (-1 charge)
Hydroxide = OH (-1 charge)
Carbonate = CO3 (-2 charge)
Metals reacting with Oxygen.
When metals react with oxygen, they create a metal oxide. An example of a metal oxide is Iron
Oxide. It is a ionic compound, which means that ions are transferred between the metal and the
oxygen.
Metals reacting with Acids.
When metals react with acid, it creates a salt and hydrogen. A common example of this is Sodium +
Hydrochloric acid -> Sodium Chloride + Hydrogen. The name of the salt is based off of two things.
The first part of the name originates from the metal being reacting to and the second part of the
name originates from the acid name.
Atom structure and electron configuration
Atom structure is they way the atom is built, and it is stable. The atom consists of 3 subatomic
particles and 2 main parts. The 3 subatomic particles are neutron, proton and electron. Neutron has
a neutral charge and was discovered by James Chadwick in 1932. Proton has a positive charge of +1
and was discovered by Ernest Rutherford in 1919. Finally, electron has a negative charge of -1 and
was discovered by J.J Thomson in 1897.
, Neutrons and protons have the same relative mass of 1. However the relative mass of an electron is
so low that it doesn’t really matter with the mass of the atom. Electrons are also transferrable, and
this is the thing that makes chemical reactions happen.
The two parts of the atom are the neutron and the orbiting shells. The nucleus is a densely packed
centre that contains protons and neutrons. The shells that orbit it contain the electrons.
The shells in electron go in an order. The first ring can only hold 2 electrons, every ring after the first
ring has the capacity to hold 8 electrons.
One way we can demonstrate the shells of atoms is by the dot and cross method. The dot and cross
method are that we get an element, and we have to cross/dot the number of electrons the atom has
in its appropriate shells.
Relative atomic mass
Relative atomic mass is the mass of the element based off of the mass it has of its isotopes. For
example, 75% of the chlorine atoms in existence is Cl 35. Around 25% of the chlorine atoms in
existence is Cl37.
Relative Mass Formula:
Relative Atomic Mass = (Ar x %) + (Ar2 x %) / 100
e.g. Relative atomic mass = (35 x 75%) + (37 x 25%)/100
= 35.5
Therefore Chlorine has a relative atomic mass of 35.5.
Isotopes
Isotopes are atoms which have the same number of protons and electrons but different number of
neutrons. Isotopes only have different masses and apart from that they are the exact same to
another isotope of the same element.
Nearly all the elements in the Periodic Table have at least 1 isotope.
History of the atom
Democritus, a Greek philosopher, thought that stuff was made out of tiny particles with no space
around it. He called it ‘Atomos,’ Greek for indivisible. However, popular philosopher Aristotle
opposed his theory, stating instead that there are only four elements: air, water, wind and fire. His
theory became forgotten until 1808.
In 1808, a quaker with the name John Dalton sought to challenge the Aristotelian theory. Even
though Democritus’ theory was theoretical, Dalton found out that common substances always broke
down into same elements in the same proportion. He said that they each had a particular size and
mass and could not be created or destroyed. He made the Dalton model of the atom: hard spheres
that could not be broken down. Atomic theory had now been accepted into scientific community.
