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Atomic structure
Atomic structure
Atoms are the building blocks of all matter. Everything you can see, feel and touch is all made of atoms. There are even things you cannot see, feel, hear or touch that are also made of atoms. Basically, everything is made up of atoms.
The atomic nucleus is the central area of the atom. It is composed of two kinds of subatomic particles: protons and neutrons.
In 1909, Ernest Rutherford led Hans Geiger and Ernest Marsden through what is known as the Gold Foil Experiments. During the experiments they would shoot particles through extremely thin sheets of gold foil. In 1911, Rutherford came to the conclusion that the atom had a dense nucleus because most of the particles shot straight through, but some of the particles were deflected due to the dense nucleus of the gold atoms. This theory would eliminate the idea that the atom was structured more like plum pudding. The plum pudding model was the leading model of atomic structure until Rutherford's findings.
Within the nucleus of an atom - protons each carry a positive charge, neutrons are neutral (no charge).
Outside the nucleus and in orbit around it - electrons each carry a negative charge
The number of protons is the same as the number of electrons therefore atoms have no electrical charge, the number of neutrons varies.
The mass of an atom is calculated by adding together the number of protons and the number of neutrons.
Atomic number and Mass number
Atomic number and Mass number
An element is identified by its atomic number which is the number of protons in one atom of the element.
Atomic number = number of protons = number of electrons
E.g. Hydrogen, carbon and silver
An atom of element hydrogen has an atomic number of 1, has 1 proton and 1 electron
An atom of carbon has an atomic number of 6, has 6 protons and 6 electrons.
An atom of nitrogen has an atomic number of 7, has 7 protons and 7 electrons
Mass number
The mass of an atom tells you its number of protons plus its number of neutrons
Mass number = number of protons + number of neutrons
By subtracting the atomic number from the mass number you can find the number of neutrons.
E.g. A boron atom has an atomic number of 5 and a mass number of 11. How many neutrons are in one atom of boron?
mass number = 11
atomic number = 5
mass number - atomic number = 11- 5 = 6
One atom of boron has 6 neutrons
Electrons and Energy levels
Electrons and Energy levels
An atom's electrons are in constant orbit at specific distances from the nucleus of the atom.
The orbits are energy level also called shells.
Electrons are most strongly attracted to the nucleus when they are in the the energy level closest to the nucleus.
The shell closest to the nucleus is the first shell. The shells get filled with electrons one after another, starting with the first shell
The closest shell to the nucleus gets filled first, and can hold a total 2 electrons
The second shell gets filled next and can hold a total of 8 electrons
The third shell gets filled next and can hold a total of 8 electrons
Example: Reactive and unreactive elements
Helium with the electron arrangement He 2, is an unreactive element, which can be used to safely inflate balloons. There are 2 electrons in the first and only shell, which is therefore full.
Hydrogen, however, with the electron arrangement H 1, is a reactive element- it explodes when burnt in air. There is only 1 electron in the first shell, which needs to gain an electron during a chemical reaction to become full.
Electron arrangements
The electronic configuration shows the number of electrons in each shell. E.g. Sodium- 2,8,1
The outermost level, called the valence shell, plays a very important part in chemical reactions
Elements are stable and unreactive when their outermost shell is full.
Elements are reactive when their outermost shell needs to gain or lose an electron or electrons to become full.
The periodic table and the first 20 elements
The periodic table and the first 20 elements
Of the first 20 elements - 12 are solids at room temperature and 8 are gases (hydrogen, helium, nitrogen, oxygen, fluorine, neon, chlorine and argon)
The vertical columns are called groups
Atoms of elements in the same group have the same number of electrons in their valence shell, so they have similar chemical properties.
The horizontal rows are called periods - Atoms of elements in the same period have the same number of electron shells.
Across a period, the number of electrons in the outer shell starts at 1 and then increases by 1 for each element until an outer shell of 8 is reached for the last element on the extreme right-hand side of the row- this element is stable since it has a full outer shell.
Each row on the periodic table corresponds to filling a shell of electrons. Hydrogen and helium are the only elements in the first period; they are filling the first shell which holds a maximum of two electrons.
Metals are on the left of the table and non-metals are on the right.
Ions and Ionic compounds
Ions and Ionic compounds
Ions
A compound is formed when there is a reaction between the atoms of two or more different elements- a compound contains at least two different kinds of elements, chemically combined.
For example, when wood burns, the element carbon C in the wood reacts with the element oxygen in the air to form a new substance, the compound carbon dioxide.
The electron in the outer shells of atoms are transferred or they may be shared during chemical reactions when atoms lose or gain electrons to become more stable.
Atoms react to gain a full outer shell of electrons and become more stable.
Atoms in the same group of the periodic table need to lose or gain the same number of electrons to become stable, so these atoms react similarly to each other.
For example: atoms of both fluorine and chlorine need to gain one electron from another element to become stable, both are poisonous, strongly reactive gases.
Atoms that have gained or lost one or more electrons in a chemical reaction are no longer electrically neutral, so they are no longer toms. They have become ions.
An ion is an atom or a group of atoms that has lost or gained one or more electrons and therefore carries a charge.
Positive ions
Atoms that lose electrons and therefore lose negative charges- become positive ions. It is easier for atoms that have 3 electrons or fewer in their outer shells to lose the 3,2 or 1 electron and become stable than it is for them to gain the 5 or more elements needed to become stable.
Example: formation of a positive ion - Sodium Na, has the electron arrangement 2,8,1. By losing 1 electron, sodium will have a full outer shell. losing 1 electron means there are no longer equal number of protons and electrons - the ion formed has an extra proton, so it carries an extra positive charge.
Negative ions
Atoms that gain electrons- and therefore gain negative charges- become negative ions. It is easier for atoms that have 5 or more electrons in their outer shells to gain 3,2 or 1 electrons and become stable than it is for them to lose the 5 or more electrons in the outer shell.
Chlorine has the electron arrangement 2,8,7.
By gaining 1 electron, chlorine will have a full outer shell. Gaining 1 electron means there are no longer equal numbers of protons and electrons- the ion formed has an extra electron so it carries an extra negative charge.
Chlorine has gone from an electron arrangement of 2,8,7 with no charge to an electron arrangement of 2,8,8 and a charge of -1. It is now a negative ion of chlorine called a chloride ion.
An atom's electronic configuration tells you how many electrons an atoms needs to gain or lose to achieve a full outer shell.
The atoms of metals lose electrons to form positive ions.
The atoms of non-metals gain electrons to form negative ions.
Ionic bonds
Positive ions and negative ions have a strong electrostatic attraction to each other. When electrons are transferred during a chemical reaction, the newly formed ions attract each other strongly. The ions form an ionic compound.
The sodium chloride compound has different properties from the poisonous and reactive elements sodium and chlorine from which it was formed. Sodium chloride is so stable that it occurs in nature and we can sprinkle it on our food.
Ionic compounds
Ionic compounds are formed from the electrostatic attraction between positive and negative ions. The overall charge on an ionic compound is zero.
Ionic compounds form strong, hard, lattice or crystal structures because the ionic bonds holding in the ions together are very strong.
Because ionic bonds are hard to break, ionic compounds are very stable. They have high melting points and boiling points. However, ionic compounds usually dissolve easily in water. When dissolved in water, ionic compounds are good conductors of electricity.
Names and formulae of ionic compounds
Most ionic compounds have two word names, the first of which is always the name of the ion carrying the positive charge (usually a metal)
Monatomic and polyatomic ions
Monatomic ions are formed from only one atom e.g. Na+ , K+, Cl-
Polyatomic ions are formed when two or more atoms join to form an ion e.g. NH4+ , OH-
The ending -ide in the name of the compound usually indicates a monatomic ion e.g. chloride Cl- , Fluoride F- and oxide O2-.
The ending -ate indicates a polyatomic ion that contains oxygen e.g. nitrate NO3- , carbonate CO32- , sulfate SO42-
Chemical formulae and equations
Chemical formulae and equations
Element- An element is a chemical formed out of one type of atom only.
Compound- A compound is a chemical in which atoms of diferent elements are joined by chemical bonds.
Compounds are new substances produced by chemical reactions
A compound is has different physical and chemical properties from those of the elements out of which it is formed.
A compound has fixed proportions of different elements. In the compound water there are always twice as many hydrogen atoms as there are oxygen atoms.
The chemical name of a compound often tells you what elements it is made of.
E.g. Carbon dioxide = carbon and oxygen
We classify substances into pure substances and mixtures:
Pure substances - There are two types of pure substances- Elements and compounds
Elements are pure substances made of one type of atom only. Elements cannot be changed into simpler chemical substances
Compounds are pure substances that contain different types of atoms bonded together. Compounds can be broken down into elements in chemical reactions.
Mixture - is an impure substance that occurs when pure substances (e.g. salt and sand) are mixed together without a chemical reaction occurring.
Chemical formulae
Chemical formulae
The elements in a compound combine in fixed amounts. This makes it possible to write a formula for a molecule.
E.g. Water is two parts hydrogen and one part oxygen. It's formula is H2O
Carbon dioxide is one part carbon and two parts oxygen. Its formula is CO2
Writing reactions
Atoms are not created or destroyed in chemical reactions. It is therefore possible to list all of the starting substances - the reactants - and show what they rearrange, or react, to form.
The symbol → stands for 'reacts to form'. The end substances of a reaction are called products. A word equation can be written to describe a reaction, e.g.
hydrogen + oxygen → water
Alternatively a formula equation can be written. These equations have to be balanced so that there are the same numbers and types of atom before the reaction as after it, e.g.
2H2 + O2 → 2H2O
Balancing equations
Small numbers after an atoms symbol tell you how many atoms are combined. For example: O2 means two oxygen atoms have combined
Large numbers in front mean more than one of whatever follows. E.g. 3O2 means three oxygen molecules, each with the formula O2
You can't alter a formula to balance an equation. You can only change the numbers in front.
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