ch1 atoms and electrons

7/30/2019 CH1 Atoms and Electrons

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Atoms are the basic building blocks of all substances

There are 92 naturally occurring elements (and somethat have been made by scientists, so there are just over 100 in all)

Elements only contain one type of atom, either on

their own, or chemically bonded together

Compounds contain two or more types of atomchemically bonded together

Elements canhave verydifferentappearancesand properties The element bromine

The element nickel

The element sulphur


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The periodic table of the elements lists all the different types of atoms from whichour world is made:

Each element has a symbol e.g. K = Potassium, Fe = IronThe first letter is always a capital, if there is a second letter it is lower case

The elements are arranged in GROUPS (vertical columns). Each group is achemical family with related properties

Rows across the periodic table are called PERIODS . H and He form the first period.


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What does an atom consist of ?

The nucleus consists of protons and neutrons .

Each differentelement has adifferent number of protons thats whatmakes each elementdifferent.

The neutrons, together with

the protons, make up themass of the atom.

The nucleus is surroundedby electrons in various

energy levels or shells. In aatom there are exactly thesame number of protons aselectrons.

Phosphorus:the element

with 15protons

Sulphur:the element

with 16protons


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none

Proton

P in the nucleusParticle Where found Charge Mass

+1

NNeutronin the nucleus

1

0E

Electron

in shells (energylevels)surrounding thenucleus

* actually about 1/2000 th of the mass of a proton or neutron


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Kpotassium

39

19

The atomic number is the smaller of the two numbers shownin the periodic table, and it corresponds to the number of protons in the nucleus of an atom of that element.

The Periodic Table is organised in atomic number order.

The mass number is the larger of the two numbers and is thetotal number of particles , both protons and neutrons , inthe nucleus. These make up the mass of the atom becauseelectrons weigh very little.

The number of neutrons can be found by subtracting the atomic number from the massnumber e.g. for potassium: neutrons = (39 19) = 20

atomicnumber

massnumber

symbol

name


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We know how many electrons an atom has the same as the number of protons(which we can find by looking up the atomic number in the periodic table).

The electrons are found in energy levels around the nucleus also referred to asshells. They are not arranged randomly in these shells.

Electrons go into the lowest unfilled energy level(that is, the shell nearest to the nucleus which has space in it).

The innermost shell is full when it contains 2 electronsThe other shells are full when they contain 8 electrons

These rules work up to Calcium (20 electrons) which is as far as you have to know !

e.g. Sodium has atomic number 11, so it has 11 electrons .

The first two go in the innermost shell which is then full

The next eight go in the second shell which is then full

The remaining one goes in the third shell.

We can show the electron arrangement for sodium as [2,8,1]

Na

e

ee

e

e

e

e

e

e

e

e


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Electron arrangements for hydrogen and helium are easy ! One and two electronsrespectively, in a single shell around the nucleus.

After that, its more complicated, but the periodic table can help:The number of shells you need to draw is the same as the PERIOD that theelement is in. Once youve drawn the right number of shells around the nucleus,

fill all shells except the outermost. (2 in the innermost shell, then 8 in each)Then draw the outer shell electrons. The number of electrons in the outermostshell = GROUP number that the element is in.

S

[2,8,6]

Sulphur is in period 3, group 6 and has 16 electrons. See how these rules works to

give 3 shells, 6 electrons in the outer shell, and the inner shells both full


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The outer shell electrons of an atom are very important.These are the electrons which get involved in formingchemical bonds .

The bonds which hold atoms together to make elementsand compounds may be formed by:

transferring electrons from one atom to another to form ions,resulting in ionic bonding e.g. in sodium chloride (NaCl)

sharing electrons between atoms, resulting in covalentbonding e.g. in water (H 2O)

S

[2,8,6]

Chemical Bonds


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When different atoms join together they form COMPOUNDS a compound isdefined as containing:

two or more different types of atom, chemically bonded together

Here are some compounds:

Here are some things that are NOT compounds:

methane, CH 4

water, H 2O

sodium chloride, NaCl

helium atoms, He sulphur molecule, S 8 graphite (carbon)


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Chemical bonds hold atoms together in elements and compounds

No matter how they are formed, they are strong and difficult to break.

In a mixture , there is more than one element orcompound intermingled . They are NOT chemicallybonded together, and can be separated by physicalmeans .

mixture

compound

H C HH

H

bond

e.g. If I mix sand and water, I CAN separate the sand from thewater molecules (by filtering the mixture) because the water isNOT chemically bonded to the sand. I CANT easily separatethe hydrogen and oxygen atoms that make up the water molecules, or the silicon and oxygen atoms which make up the

sand, because they ARE chemically bonded together.


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A chemical equation shows what happens to the substances in a chemicalreaction . It shows how the reactants are turned into the products .

reactant 1 + reactant 2 product 1 + product 2 etc.

Equations can be written in words, but it is more useful to write them using chemicalformulae (symbols) for the reactants and products, because then we know thenumber and type of atoms which are joined together in the elements andcompounds.

For example - magnesium reacts with oxygen to make magnesium oxideHeres the word equation:

magnesium + oxygen magnesium oxide

What it doesnt tell you is that oxygen comes as molecules containing two atoms, or that you need twice as much magnesium as oxygen, or that there is one oxygenatom for every magnesium atom in the product.. But the symbol equation does !

2 Mg + O 2 2 MgO


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2 Mg + O 2 2 MgO

Two atoms of the element magnesium

react with one molecule of the element oxygen(which is two oxygen atoms chemically bondedtogether)

to form magnesium oxide, which is acompound where there is one magnesiumatom for each oxygen atom.

Two magnesium oxides must be made because two magnesium atoms and twooxygen atoms are shown on the left hand side of the equation.

Mg

Mg

+O

O Mg

Mg

O

O


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When a reaction occurs, all the same atoms are there before and after , just joinedtogether in different ways. We show this in a symbol equation by making sure thereare the same number of each atom on each side of the equation then it is balanced .

We cant change numbers inside the formulae that would make themdifferent substances and it wouldnt be the same reaction. What we do ischange how many of each different molecule are involved in the reaction untilthe equation balances. That means changing numbers in front of formulae.

e.g. magnesium + hydrochloric acid magnesium chloride + hydrogen

NOT BALANCED: Mg + HCl MgCl 2 + H 2

All the formulae are correct, but there is more Cl and H on the right than on the left.

If I add another HCl to the left side, then everything will balance:

BALANCED: Mg + 2 HCl MgCl 2 + H2

Hint: Keep a tally of how many of each type of atom you have on each side of the equation whileyou are trying out different ways of getting it tobalance.


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When a reaction happens, existing chemical bonds get broken and new chemicalbonds get made .

All the atoms which are there in the elements or compounds before, are still thereafterwards , just chemically bonded together in different ways.

This means that the mass of the reactants mustbe the same as the mass of the products wecall this MASS CONSERVATION

Dont get caught out gases (including air) do weigh something !

If the mass decreases during a reaction it is because one of the products hasescaped into the atmosphere.

If mass increases during a reaction, it is because something from the atmosphere

has combined with the other reactants

reactants products

+ +

ch1 atoms and electrons

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