c2 topic 1 notes

7/25/2019 C2 Topic 1 Notes

1/4

Topic 1 Atomic structure and the periodic table

MENDELEEV

In the 1800s, a chemist called Dmitri Mendeleev used the properties of elements

known at the time to organise them into a table:

o Mendeleev arranged the elements in order of increasing atomic mass (ie in

order of increasing numbers of protons and neutrons ! see below"

o Mendeleev organised elements with similar properties into vertical

columns (called #groups$"

%nlike other chemists before him, Mendeleev:

o sometimes broke the #increasing atomic mass rule$

eg he switched tellurium and iodine around so that the& would be

in the same groups as elements with similar properties (ie b&

switching them, iodine was ne't to bromine, chlorine, fluorine"

o realised from the big )umps in atomic mass that there were still some

elements to discoverleft some gaps in his table *g he left two gaps between +inc and arsenic

ased on the known elements around them, Mendeleev predicted

the properties of the elements which should go in the gaps

In the modern periodic table, these gaps have been filled b&

gallium and germanium ! the& have ver& similar properties

to those predicted b& Mendeleev

STRUCTURE ! T"E ATM

-toms are the smallest particles of an element that can take part in chemical

reactions

The # subatomic particles protons$ neutrons$ electrons% -t the centre of each atom is a nucleus containing protons and neutrons

.ote: the nucleus is about /0,000 times smaller than the overall si+e of the atom

*lectrons are arranged in shells (or #energ& levels$" at different distances from the

nucleus

he masses and charges of subatomic particles are ver& smallthe&$re compared

to those of a proton ! these are called the #relative mass$ and the #relative charge$

Subatomic particle Relati&e mass Relati&e char'e

roton 1 2 1

.eutron 1 0

*lectron .egligible (ie 0" 3 1 Atoms and elements%

-ll atoms contain the same number of protons and electronsatoms have no

overall charge

.o two elements have the same number of protons in their atomseg h&drogen

has 1 proton, helium has / protons etc

*lements can have more than one atom:

o Different atoms of the same element alwa&s have the same number of

protons (the same number of electrons", but a different number of

neutrons ! these are called isotopes (see below"

T"E MDERN (ER)D)C TA*LE -tomic number ! number of protons in the nucleus of an atom


2/4

Mass number ! total number of protons and neutrons in the nucleus of an atom

In a chemical s&mbol, eg , the atomic number is the bottom number (the

smaller one" and the mass number is the top number (the bigger one"

4rom the s&mbol, we can calculate the number of protons, neutrons and electrons

in an atomeg for :

o -tomic number is 1515 protons

o Mass number ! atomic number 6 number of neutrons/7 3 15 6 1

neutrons

o .umber of protons 6 number of electrons15 electrons

he elements in the modern periodic table are arranged in order of increasing

atomic number rather than in order of increasing atomic mass as Mendeleev did

he hori+ontal rows are called periods

he vertical columns are called groups:

o *ach group contains elements with similar properties

o he main groups are numbered 137 from left to right

o he group on the far right is called group 0

Diagram of periodic table below:

o *lements to the left of the )agged line are metals

o *lements to the right of the line are non3metals

Relati&e atomic mass +Ar,%

-toms have ver& small massesrelative atomic mass is used instead of its actual

mass in kilograms

9elative atomic mass is the mass of an atom compared to that of carbon:

o arbon has a relative atomic mass of 1/

o Mass of a helium atom is one third that of carbonits relative atomic

mass is

In the periodic table, each element has its atomic number (the bottom number" and

its relative atomic mass (the top number" shown

)sotopes%

Isotopes are different atoms of an element with the same number of protons and

electrons, but different numbers of neutrons (ie same atomic number, different

mass number"

*g chlorine atoms alwa&s have 17 protons, but some can have 18 neutrons and

others can have /0 neutrons ! these are and

he presence of isotopes means that some relative atomic masses aren$t whole

numbers:

o 9elative abundance (proportion" of is 7;< (or 07;"o 9elative abundance of is /;< (or 0/;"


3/4

o 9elative atomic mass 6 (proportion in decimals ' mass number" 2

(proportion in decimals ' mass number"

o relative atomic mass of l 6 (07; ' 5;" 2 (0/; ' 57" 6 5;;

Relati&e atomic mass &s mass number%

he mass number is the mass of an atom ! ie the number of protons and neutrons

(because electrons have negligible relative mass"

he relative atomic mass is the average mass of all the different atoms (isotopes"

of an element (relative to carbon", taking their abundance into account

In most cases, the relative atomic mass of an element is ver& similar to the mass

number of the most common isotope (eg in the above e'ample, relative atomic

mass of l is 5;;mass number of most common isotope is 5;"

ELECTRN S"ELLS

*lectrons are arranged in shells around the nucleus of the atom

*ach shell is shown as a circle drawn around the chemical s&mbol for the atom

he wa& in which electrons are arranged in an atom is called its electronic

configuration

!indin' con-i'urations%

Different shells can contain different numbers of electrons

4or the first /0 elements:

o he first shell can contain up to / electrons

o he second and third shells both hold up to 8 electrons

he first shell fills up first, then the second shell, and so on

*lements which have the ma'imum number of electrons in their outer shells are

said to have #full outer shells$

*lectronic configurations can be worked out using atomic numberseg:o -tomic number of sodium is 11it has 11 protons11 electrons

o / electrons in first shell

o 8 electrons in second shell

o 1 electron in third (outer" shell

his can be represented b& a diagram:

Its electronic configuration can also be written in the form #/81$, whereb&:

o he numbers show how man& electrons are in each shell

o he dots separate each shell

Spottin' connections%

1 he number of occupied shells is the same as the period number:

o *g magnesium is in period 5 of the periodic tableits configuration is

#/8/$has 5 occupied shells

/ he number of outer electrons is the same as the group number (apart fromelements in group 0 which all have full outer shells":


4/4

o *g magnesium is in group / of the periodic tableits configuration is

#/8/$has / electrons in its outer shell

c2 topic 1 notes

Documents