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Atoms, Elements and Compounds

Chemical Symbols

All substances are made from tiny particles calleed atoms. An atom is the smalest part of an element that can exist.

A chemical symbol:

  • consists of one or two letters
  • always starts with a capatial letter, with any other letter in lower case

Some non-metal elements exists as molecules that are made of two atoms joined together, these elements have a subscripted '2'.

  • iodine, I₂           nitrogen, N
  • bromine, Br₂     hydrogen, H
  • chlorine, Cl₂      A mnemonic can be used to remember these elements
  • flourine, F₂        I Bring Clay For Our New House
  • oxygen, O₂
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Atoms, Elements and Compounds

Chemical Formulae of Ions

An ion is a charged particle formed when an atom, or a group of atoms, looses or gains electrons. The number and sign of its electrical charges are shown in superscript text. e.g

Name of ionFormula of ionElectrical charge(s) Sodium Na+ One positive Magnesium Mg2+ Two positive Chloride Cl- One negative Oxide O2- Two negative

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Atoms, Elements and Compounds

Word Equations

A word equation represents a chemical reaction using the names of the substances invovled.

Word equations always take this form:

  • reactants → products
  • a + sign separates two or more reactants or two or more products
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Atoms, Elements and Compounds

Chemical Equations

A balenced chemical equation represents a chemical reaction using the formulae of the reactants amd products. It shows the number of units of each substance invovled.

State symbols:

State symbolMeaning (s) Solid (l) Liquid (g) Gas (aq) Aqueous solution

An aqueous solution is formed when a substance dissovles in water

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Atoms, Elements and Compounds

Balancing an Equation

The law of conservation of mass states that no atoms are lost or made during a chemical reaction

StepResult Check to see if there are an equal number of atoms of each element on both sides. There aren't. N2 + H2 → NH3 There are two nitrogen atoms on the left but only one on the right, so put a big 2 on the left of the NH3. N2 + H2 → 2NH3 Check again. There are two hydrogen atoms on the left but (2 × 3) = 6 on the right, so put a big 3 in front of the H2. N2 + 3H2 → 2NH3 Check again to see if there are equal numbers of each element on both sides. There are. (Two nitrogen atoms and six hydrogen atoms) Add the state symbols if asked to do so. N2(g) + 3H2(g) → 2NH3(g)

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Pure Substances and Mixtures

  • A pure substance consists only of one element or compund
  • A mixture consists of two or more different substances, not chemically joined together

Separating Mixtures:

Mixtures can be separated by physical processes. These processes do not invovle chemical reactions, and no new substances are made.

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Filtration and Crystallisation:


Filtration is used to separate an insoluable solid from a liquid. It is useful for separating sand from a mixture of sand and waer, or excess reactant from a reaction mixture.


Crystallisation is used to produce solid crystals from a solution. When the solution is warmed, some of solvent evaporates leaving crystals behind.

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Simple distilation is used to separate a solvent from a solution; eg producing pure water from seawater. Works because the dissolved solute has a much higher boiling point than the solvent. When the solution is heated, solvent vapour leaves the solution.

Fractional Distillation:

Fractional distillation is used to separate ddifferent liquids from a mixture of liquids. Used to separate different fractions from crude oil.

Fractional distillation works because the different liquids have different boiling points. When the mixture is heated:

  • vapours rise through a column which is hot at the bottom, and cooler at the top
  • vapours condense when they reach a part of the colomn that is below the temperature of their boiling point
  • each liquid is led away from the column
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Paper Chromatography

Paperchromatography is used to separate mixtures of soluable substances. These are often coloured substances such as food colourings, inks, dyes or plant pigments.

Chromatography relies on two different 'phases':

  • the stationary phase, which in paper chromatography is very uniform, absorbent paper
  • the mobile phase is the solvent that moves through the paper, carrying differet substances with it

Interpreting a chromatogram:

Separation by chromatography produces a chromatogram. A paper chromatogram can be used to distinguish between pure and impure substances:

  • a pure substnce produces one spot on the chromatogram
  • an impure substance, or mixture, produces two or, more spots
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Atomic Structure

Early Ideas about Atoms

John Dalton said in 1803 he thought all matter was made of tiny particles called atoms.

100 years later, JJ Thomson carried out experiements and discovered the electron, this lead to the suggestion of the plum pudding model.

In 1909 Ernest Rutherford designed an experiement to test the Plum Pudding model. In the experiement, positively charged alpha particles were fired at thin gold foil. Most particles went straight through the foil byt a few were scattered in different directions. This evidence led Rutherford to suggest a new model for the atom, called the 'nuclear model'. In the nuclear model:

  • the mass of an atom is concentrated at its centre, the nucleus
  • the nucleus is positively charged
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Atomic Structure

Developing Models of the Atom

Niels Bohr adapted Ernest Rutherford's nuclear model. Bohr calcualted that electrons orbit the nucleus in shells.

Further experiements led to the idea that the nucleus containedd small particles calledd protons, each proton has a samll positive charge. 

In 1932 James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge . These are called neutrons.

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Atomic Structure

Structure of the Atom

Nucleus and Shells:

An atom has a central nucleus. This is surrounded bby electrons arranged in shells. The nucleus is tiny compared to the atom as a whole:

  • the radius of an atom is about 01.nm
  • the radius of a nucleus(1x10^14m) is less than 1/10000 of the radius of the atom

Subatomic particles:

The nuclei of all atoms contain subatomic particles called protons. The nuclei of most atoms also contain neutrons.

Subatomic particleRelative massRelative charge Proton 1 +1 Neutron 1 0 Electron Very small -1

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Atomic Structure

Atomic Number and Mass Number

Atomic Number:

The number of protons in an atom of an element is its atomic number.

  • all atoms of a given element have the same number of protons
  • atoms of different elements have different numbers of protons

An atom contains equal numbers of protons and electrons.

Mass Number:

Atoms of different elements usually have different mass sumbers but they can be the same.

To calculate the numbers of subatomic particles in an atom.

  • number of protons = atomic number
  • number of electrons = atomic number
  • number of neutrons = masss number-atomic number
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Atomic Structure


Atoms of the same element must have the same number of protons, but they can have different numbers of neutrons. Atoms of the same elements with different numbers of neutrons are called isotopes.

Three isotopes of hydrogen:

IsotopeSymbolProtonsElectronsNeutrons Hydrogen-1 <em>{1}^{1}\textrm{H} 1 1 1 - 1 = 0 Hydrogen-2 </em>{1}^{2}\textrm{H} 1 1 2 - 1 = 1 Hydrogen-3 _{1}^{3}\textrm{H} 1 1 3 - 1 = 2

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Atomic Structure

Calculating Relative Atomic Mass

Aᵣ = total mass of atoms/total number of atoms = (75x35)+(25x37)/(75+25)


                                                                              = 35.5 (to 1d.p)

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The Periodic Table

Mendeleev's Peeriodic Table

Before the discovery of protons, neutrons and electons scientists tried to classify the elements by arranging them in order of their atomic weights (now known as the relative atomic mass). Mendeleev was a Russian chemist, he published his first periodic table of the elements in 1869, aranged in order of atomic weight. He also took into account of of the properties of their elements and their compounds; this meant that his table:

  • had gaps in it
  • showed elements with similar chemical poperties lined up in groups

Mendeleev left gaps in his table for elements not known at the time. By looking at the properties of the elementsnext to a gap, he could also predict the properties of these undiscovered elements.

Iodine has a lower atomic weight than tellurium. So iodine should be placed before tellurium in Mendeleeve's periodic table. However, iodine hass a similar chemical properties to chlorine and bromine. To make iodine line up with chlorine and bromine in his table, Mendeleev swpped the positions of iodine and tellurium.

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The Periodic Table

The Modern Periodic Table

In the modern periodic table:

  • elements are arranged in rows, called periods, in order of increasing atomic number
  • elements with similar properties are placed in vertical columns, called groups

The table is called the periodic table because elements with similar properties occur at regular intervals.

Mendeleev did not know about isotopes, but their existance explains pair reversals. The positions of iodine and tellerium were reversed in Mendeleev's table because:

  • iodine has one naturally occurring isotope, iodine-127
  • the most abundant tellurium isotopes are tellurium-128 and tellurium-130
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The Periodic Table

Electronic Structure

Electrons in Shells:

Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus. Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the lowest available energy level first.

ShellMaximum First 2 Second 8 Third 8

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The Periodic Table

Predicting an Electronic Structure:

The electronic structure of an atom can be predicted from its atomic number. e.g the atomic number for sodium is 11. Sodium atoms have 11 protons and so 11 electrons:

  • two electrons occupy the first shell
  • eight electrons occupy the second shell
  • one electron occupies the third shell

This electronic structure can be written as 2,8,1 or in a diagram like:

Structure of a sodium atom

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The Periodic Table

Electronic Structures and the Periodic Table:

The electronic structure of an element is linked to its position on the periodic table:

Electronic structure featureLink to the periodic table Number of shells Period number Number of electrons in outermost shell Group number Numbers added together Atomic number

The electronic structure of sodium (2,8,1) shows that sodium, Na:

  • is in period 3
  • is in group 1
  • has an atomic number of (2+8+1) = 11
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The Periodic Table

Properties of Metal and Non-Metals

Differrences in Chemical properties:

Most elements are metals. In their chemcal reactions, metal atoms lose electronsto form positive ions. e.g

  • when magnesium burns in air, each atom loses two electons to form a Mg^2+ ion
  • when sodium reacts with chlorine, each sodium atom loss one electron to form a Na^+ ion

How a sodium atom becomes a sodium ion when it loses an electron

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The Periodic Table

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