3. Atoms, Elements & Compounds Flashcards
What are elements made out of?
Elements are made of tiny particles of matter called atoms
What are atoms made out of?
Each atom is made of subatomic particles called protons, neutrons and electrons
Why can’t we use conventional units e.g kg to compare the masses of atoms?
Their size is so tiny that we can’t really compare their masses in conventional units such as kilograms or grams
What unit is used to show the mass of an atom?
a unit called the relative atomic mass is used
What is one relative atomic mass unit equal to?
One relative atomic mass unit is equal to the mass of a carbon-12 atom.
Why does the relative atomic mass not have units?
All other elements are measured relative to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units
What is the relative mass of a proton?
1
What is the relative mass of a neutron?
1
What is the relative mass of an electron?
1/1840
What is the charge of a proton?
+1
What is the charge of a neutron?
0 (neutral)
What is the charge of an electron?
-1
What is the atomic number?
The atomic number (or proton number) is the number of protons in the nucleus of an atom.
What is the symbol for the atomic number?
Z
What does the atomic number also show? (apart from number of protons)
What does this determine?
It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table
What is the nucleon number?
Nucleon number (or mass number) is the total number of protons and neutrons in the nucleus of an atom
What is the symbol for the nucleon number?
A
How can you calculate the number of neutrons using the nucleon and proton number?
The nucleon number minus the proton number gives you the number of neutrons of an atom
What are nucleons?
protons and neutrons can collectively be called nucleons.
What do electrons move around in?
They move in orbital paths called shells
Where is the mass of an atom contained and why?
The mass of the electron is negligible, hence the mass of an atom is contained within the nucleus where the neutron and proton reside
How are elements arranged on the periodic table?
Elements are arranged on the Periodic table in order of increasing atomic number where each element has one proton more than the element preceding it
What are the vertical columns in the periodic table called?
The table is arranged in vertical columns called Groups numbered I – VIII
What are the rows in the periodic table called?
Periods
What do elements in the same group have?
Elements in the same group have the same amount of electrons in their outer shell, which gives them similar chemical properties
Isotope definition
Isotopes are atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
What is the symbol for an isotope?
The symbol for an isotope is the chemical symbol (or word) followed by a dash and then the mass number.
Into what two categories can isotopes be divided?
radioactive and non-radioactive
Why are radioisotopes unstable?
Radioactive isotopes (radioisotopes) are unstable due to the imbalance of neutrons and protons
What does the imbalance of neutrons and protons in radioisotopes lead to?
causes the nucleus to decay over time through nuclear fission and emit radiation
What are examples of radioisotopes?
tritium and carbon-14
Does decay occur at the same rate for each isotope?
NO
Decay occurs at a different rate for each isotope
What is half-life?
the time taken for the radioactivity of an isotope to decrease by 50% is constant for that particular isotope and is known as the half-life
What are the properties of non-radioactive isotopes?
Non-radioactive isotopes are stable atoms which really only differ in their mass
What are the 3 medical uses for radioisotopes?
- to treat cancer
- medical tracers
- to sterilise medical instruments
How are radioisotopes used to treat cancer?
Radiation is extremely harmful and kills cells so isotopes are used to treat cancer. The isotope cobalt-60 is frequently used for this purpose
How are radioisotopes used as medical tracers?
Medical tracers as certain parts of the body absorb isotopes and others do not. In this way an isotope can be injected into the blood and its path through the body traced with a radioactive detecting camera, revealing the flow of blood through bodily systems
How are radioisotopes used to sterilise medical equipment?
Medical instruments and materials are routinely sterilised by exposure to radiation, which kills any bacteria present
What are 3 industrial uses of radioisotopes?
- radioactive dating
- to detect leaks
- to power power plants
How are radioisotopes used in radioactive dating?
Radioactive dating uses the carbon-14 isotope to date carbon-containing materials such as organic matter, rocks and other artefacts
How are radioisotopes used in detecting leaks?
Similar to medical use, radioactive tracers are deployed to detect leaks in gas or oil pipes
How are radioisotopes used to power power plants?
The radioactive isotope uranium-235 is used as nuclear in power plants in controlled fission reactions
Do isotopes of the same element display the same chemical characteristics?
Yes
Why do isotopes of the same element display the same chemical characteristics?
- This is because they have the same number of electrons in their outer shells and this is what determines an atom’s chemistry
- The difference between isotopes is the neutrons which are neutral particles within the nucleus and add mass only
How can we represent the structure of an atom?
- using diagrams called electron shell diagrams
- by writing out a special notation called the electronic configuration
What is the correlation between the distance of the shell away from the nucleus and its energy?
The further away from the nucleus then the more energy a shell has.
What is the outermost shell called?
The outermost shell of an atom is called the valence shell
Why is it important for an atom to try and gain a full valence shell?
an atom is much more stable if it can manage to completely fill this shell with electrons
What does the last notation in the electronic configuration show?
The last notation shows the number of outer electrons the atom has, showing the Group that element is in
What do elements in the same group have in common?
Elements in the same Group have the same number of outer shell electrons
What property do all the noble gases share?
All of the noble gases are unreactive as they have full outer shells and are thus very stable
Element definition
A substance made of atoms that all contain the same number of protons (one type of atom) and cannot be split into anything simpler
Compound definition
A pure substance made up of two or more elements chemically combined
Can compounds be separated into their elements by physical means?
no
Mixture definition
A combination of two or more substances (elements and/or compounds) that are not chemically combined
Can mixtures be separated by physical methods?
YES
Mixtures can be separated by physical methods such as filtration or evaporation
What two broad types can elements be divided into?
They can be divided into two broad types: metals and nonmetals
What is a metalloid?
small number of elements display properties of both types (metal and non-metal)
What are the 6 key properties of metals?
- Conduct heat and electricity
- Are malleable and ductile (can be hammered and pulled into different shapes)
- Tend to be lustrous (shiny)
- Have high density and usually have high melting points
- Form positive ions through electron loss
- Form basic oxides
What are the 6 key properties of non-metals?
- Do not conduct heat and electricity
- Are brittle and delicate when solid and easily break up
- Tend to be dull and non-reflective
- Have low density and low melting points (many are gases at room temperature)
- Form negative ions through electron gain (except for hydrogen)
- Form acidic oxides
What are alloys?
Alloys are mixtures of metals, where the metals are mixed together but are not chemically combined
What can alloys be made out of?
They can be made from metals mixed with nonmetals such as carbon
Why are alloys preferable over pure metals?
Alloys often have properties that can be very different to the metals they contain, for example they can have more strength, hardness or resistance to corrosion or extreme temperatures
How are alloys stronger than pure metals?
- Alloys contain atoms of different sizes, which distorts the regular arrangements of atoms
- This makes it more difficult for the layers to slide over each other, so they are usually much harder than the pure metal
What is an example of an alloy?
Brass is a common example of an alloy which contains 70% copper and 30% zinc
Ion definition
An ion is an electrically charged atom or group of atoms formed by the loss or gain of electrons
Why do ions gain/loose electrons?
This loss or gain of electrons takes place to gain a full outer shell of electrons
What is the electronic structure of an ion the same as?
The electronic structure of an ion will be the same as that of a noble gas – such as helium, neon and argon
Do metals lose or gain electrons to form ions?
All metals lose electrons to other atoms to become positively charged ions
Do non-metals lose or gain electrons to form ions?
All non-metals gain electrons from other atoms to become negatively charged ions
How are ionic compounds held together?
The positive and negative charges are held together by the strong electrostatic forces of attraction between opposite charges.
What does the lattice structure refer to?
Lattice structure refers to the arrangement of the atoms of a substance in 3D space
How are atoms arranged in a lattice structure?
In lattice structures, the atoms are arranged in an ordered and repeating fashion
How is a lattice formed?
The lattices formed by ionic compounds consist of a regular arrangement of alternating positive and negative ions
How are covalent compounds formed?
Covalent compounds are formed when electrons are shared between atoms
What type of elements can participate in covalent bonding?
Only non-metal elements
What is similar between ionic and covalent bonding?
As in ionic bonding, each atom gains a full outer shell of electrons
What are molecules?
When two or more atoms are chemically bonded together, we describe them as ‘molecules’
Look at covalent compound diagrams 3.3
yes ;)
What is the difference in melting and boiling points in ionic and covalent compounds?
I - Have high melting and boiling points so ionic compounds are usually solid at room temperature
C - Have low melting and boiling points so covalent compounds are usually liquids or gases at room temperature
What is the difference in volatility in ionic and covalent compounds?
I - Not volatile so they don’t evaporate easily
C - Usually volatile which is why many covalent organic compounds have distinct aromas
What is the difference in water solubility in ionic and covalent compounds?
I - Usually water-soluble as both ionic compounds and water are polar
C - Usually not water-soluble as covalent compounds tend to be non-polar but can dissolve in organic solvents
What is the difference in ability to conduct electricity in ionic and covalent compounds?
I - Conduct electricity in molten state or in solution as they have ions that can move and carry a charge
C - Cannot conduct electricity as all electrons are involved in bonding so there are no free electrons or ions to carry the charge
Why do ionic compounds have high melting and boiling points?
This is because the oppositely charged ions in the lattice structure are attracted to each other by strong electrostatic forces which hold them firmly in place
Large amounts of energy are needed to overcome these forces so the m.p. and b.p. are high
Why do covalent compounds have lower melting and boiling points?
- Simple covalent substances, such as carbon dioxide and methane, have very strong covalent bonds between the atoms in each molecule, but much weaker intermolecular forces between individual molecules
- When one of these substances melts or boils, it is these weak intermolecular forces that break, not the strong covalent bonds
- Less energy is needed to break the molecules apart, so they have lower m.p. and b.p. than ionic compounds
What is an allotrope?
Different atomic or molecular arrangements of the same element in the same physical state
What are diamond and graphite?
allotropes of carbon which have giant covalent structures
What do giant covalent structures look like?
This class of substances contains a lot of non-metal atoms, each joined to adjacent atoms by covalent bonds forming a giant lattice structure
Do giant covalent structures have high or low melting and boiling points? Why?
Giant covalent structures have HIGH melting and boiling points as they have many strong covalent bonds that need to be broken down
What is the structure of diamond?
Each carbon atom bonds with four other carbons, forming a tetrahedron
Describe the bonds in diamond
All the covalent bonds are identical and strong with no weak intermolecular forces
3 properties of diamond
- Does not conduct electricity
- Has a very high melting point
- Is extremely hard and dense (3.51 g/cm3)
In what two ways can diamond be used?
Diamond is used in jewellery and as cutting tools
What is the structure of graphite?
Each carbon atom is bonded to three others forming layers of hexagonal-shaped forms, leaving one free electron per carbon atom
How can graphite conduct electricity?
These free electrons exist in between the layers and are free to move and carry charge, hence graphite can conduct electricity
Describe the bonds in graphite
The covalent bonds within the layers are very strong but the layers are connected to each other by weak intermolecular forces only, hence the layers can slide over each other making graphite slippery and smooth
3 properties of graphite
- Conducts electricity
- Has a very high melting point
- Is soft and slippery, less dense than diamond (2.25 g/cm3)
What are 3 uses of graphite?
- used in pencils
- used s an industrial lubricant, in engines and in locks
- used to make non-reactive electrodes for electrolysis
What type of compound is SiO2?
macromolecular compound
In what forms can SiO2 occur naturally?
sand and quartz
Describe the bonds in silicon dioxide
Each oxygen atom forms covalent bonds with 2 silicon atoms and each silicon atom in turn forms covalent bonds with 4 oxygen atoms
What shape is formed in the structure of SiO2?
A tetrahedron is formed with one silicon atom and four oxygen atoms, similar as in diamond
What giant covalent structure does SiO2 have similar properties to? Why?
SiO2 has lots of very strong covalent bonds and no intermolecular forces so it has similar properties to diamond
Properties of SiO2
It is very hard, has a very high boiling point, is insoluble in water and does not conduct electricity
What is SiO2 used for?
SiO2 is cheap since it is available naturally and is used to make sandpaper and to line the inside of furnaces
How are metal atoms held together?
Metal atoms are held together strongly by metallic bonding
How do metal ions become positively charged in a metallic lattice?
Within the metal lattice, the atoms lose their valence electrons and become positively charged
What are the electrons in a metallic lattice called and why?
The valence electrons no longer belong to any metal atom and are said to be delocalised
They move freely between the positive metal ions like a sea of electrons
Why are metallic bonds so strong?
Metallic bonds are strong and are a result of the attraction between the positive metal ions and the negatively charged delocalised electrons
What is the link between metallic bonding and the high melting and boiling points of metals?
There are many strong metallic bonds in giant metallic structures
A lot of heat energy is needed to overcome forces and break these bonds
What is the link between metallic bonding and the ability to conduct electricity of metals?
There are free electrons available to move and carry charge
Electrons entering one end of the metal cause a delocalised electron to displace itself from the other end
Hence electrons can flow so electricity is conducted
What is the link between metallic bonding and metals being malleable and ductile?
Layers of positive ions can slide over one another and take up different positions
Metallic bonding is not disrupted as the valence electrons do not belong to any particular metal atom so the delocalised electrons will move with them
Metallic bonds are thus not broken and as a result metals are strong but flexible
They can be hammered and bent into different shapes without breaking
