Atoms, Elements and Compounds
Chapter: Atoms, elements and compounds
Atoms are the smallest particles of matter that we cannot break down further by chemical means. Single atoms are far too small to see. We can assume a million sodium atoms could fit in a line across this full stop.
Generally atoms are mostly empty space. Each atom comprises of a nucleus and a cloud of particles which are called electrons that whizz around it.
Sodium is made of sodium atoms only, so it is an element. An element contains only one kind of atom. Around 90 elements have been found in earth and atmosphere. Scientists have made nearly 30 others in the lab. Many of the artificial elements are very unstable and last for just a few seconds before breaking down into other elements.
Protons, neutrons and electrons
Atoms consist of a nucleus and a cloud of electrons that move around the nucleus. The nucleus is itself a cluster of two kinds of particles protons and neutrons.
All the particles in an atom are very light. So their mass is measured in atomic mass units rather than grams. Protons and electrons also have an electric charge. Generally mass of electrons is taken as zero as they are so light.
Every atom has an equal number of protons and electrons. So atoms have no overall charge.
Nucleon number is the total number of protons and neutrons.
Isotopes are atoms of the same element, with different number of neutrons.
Some isotopes are radio active
A carbon atom behaves in a strange way and is radioactive. Its nucleus is unstable. After some time the atom breaks down naturally or decays giving out radiation in the form of rays and particles along with large amount of energy.
Radiation can harm you
If the radiation from radioisotopes gets into your body, it will kill body cells and causes radiation sickness. Victims vomit, feel tires, their hair falls out, gums bleed and they die within weeks. Also cancer is caused by the exposure of small doses of radiation.
Making use of radioisotopes
Radioisotopes are used for checking leaks in oil and gas pipes, to treat cancer by radio therapy process, to kill germs and bacteria.
Electrons are set in shells around the nucleus. The first shell which is closest to the nucleus is the lowest energy level. The shell which moves away from the nucleus, the higher will be the energy level.
The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury. The rules mentioned below are followed for writing the number of electrons in different energy levels or shells:
- The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or also called as energy level index, 1, 2, 3, So the maximum number of electrons in different shells is as follows: first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will be = 2 × 22 = 8, third orbit or M-shell will be = 2 ×32 = 18, fourth orbit or N-shell will be= 2 × 42= 32, and so on.
- The maximum number of electrons that can be accommodated in the outermost orbit is 8.
- Electrons are not accommodated in a given shell, until the inner shells are filled. So we can conclude that the shells are filled in a step-wise manner.
Metals and non-metals
There are two groups in periodic table which are separated by a zigzag line. They are metals and non-metals. The non-metals lie to the right of the line except for hydrogen. Also we can observe that there are more metals than non-metals. Around 80 % of elements known are metals.
General properties of metals
General properties of non-metals
Good conductors of electricity and heat
They do not conduct electricity or heat
High melting and boiling points
Lower melting and boiling points
Hard, strong and do not shatter if you hammer them
Solid non-metals break up easily hence called brittle in nature.
They are malleable ( hammered into sheets) and ductile ( can be drawn into wires)
They are neither malleable nor ductile
Look shiny when they re polished
Look dull in solid state
They are sonorous ( they make ringing sound)
They break up when you strike them
They have high density
They have low density
When they react they form positive ions
They often form negative ions when they react
They react with oxygen to form oxides which are bases
They react with oxygen to form oxides which are acids
The properties in last rows above are called chemical properties and the others are called physical properties.
Some exceptions to those properties
- Not all metals are hard soils. Metals like sodium and potassium can be cut with a knife and we know that mercury is a liquid at room temperature.
- Basically hydrogen is a non-metal, but it forms positive ions (H+) like metals do.
- Though carbon is non-metal, graphite a form of carbon is a good conductor.
Making use of the metals
Generally metals are hard, strong and also good conductors of electricity. So we can use them in various ways. Some are listed as below
- Iron which is the most used metal is used in buildings, bridges, cars, tin cars, needles and nails.
- Copper is used for electrical wiring in homes.
- Aluminium is used in planes and space rockets.
Non-metals are everywhere
- Air is a composition of 80% nitrogen and about 20% oxygen
- Water is termed to be a compound of hydrogen and oxygen
- Our body is mostly composed of water and hundreds of carbon compounds.
- Sand is a mixture of silicon and oxygen
Compounds, mixture and chemical changes
An element is that which contains only one kind of atom.
A compound is made of atoms of different elements, bonded together. The compound is described by a formula made from the symbols of the atoms in it.
Compounds and mixtures: the differences
A mixture contains different substances that are not bonded together.
When you heat a mixture of iron and sulfur, a chemical change takes place. The iron and sulfur atoms bond together to form a compound.
The following three signs show that a chemical change has taken place:
- One or more new chemical substance are formed.
- Energy is accepted or emerged during the reaction.
- The change is usually hard to reverse.
A chemical change is generally called a chemical reaction.
The change is a physical change if no new chemical substance is formed.
Why do atoms form bonds?
Atoms bond with each other in order to gain a stable arrangement of outer-shell electrons. We can say that they bond in order to gain 8 electrons in their outer shell.
An atom becomes an ion when it loses or gains electrons.
An ion is a charged particle. It is charged because it has unequal number of protons and electrons.
The ionic bond is the bond that forms between the ions of opposite charge.
The two ions attract each other as they have opposite charges. The force of attraction between them is so strong. It is called an ionic bond.
The compounds formed by the transfer of electrons from a metal to a non-metal are known as ionic compounds. The metal atoms lose electrons. The non-metal atoms gain them. The ion forms a lattice. No overall charge is present in this compound.
The covalent bond
Each hydrogen atom has a positive nucleus. Both nuclei attract the shared electrons and this strong force of attraction holds the two atoms together. This force of attraction is called a covalent bond.
When atoms share two electrons a single covalent bond is formed.
The two bonded hydrogen atoms above form a molecule. A molecule is a group of atoms held together by covalent bonds.
Elements made up of molecules containing two atoms are called diatomic.
A covalent bond is defined as a chemical bond that involves the sharing of electron pairs between atoms.
Metals and non-metals react together to form ionic compounds. Non-metals react together to form covalent compounds.
Comparing ionic and covalent compounds
- In ionic solids the particles (ions) are charged and the forces between them are strong.
- In molecular covalent solids the particles (molecules) are not charged and the forces between them are weak.
Properties of Ionic Compounds
- Ionic compounds are brittle in nature.
- They have high melting boiling points
- They are usually soluble in water.
- They conduct electricity when they are melted or dissolved in water.
Properties of Covalent Compounds
- They have low melting and boiling points
- They tend to be insoluble in water
- They do not conduct electricity.
Giant covalent structure
Billions of carbon atoms are bonded together in a giant covalent structure.
Diamond is a giant covalent structure. It is made of carbon atoms held in a strong lattice.
Diamond has these properties
- It is very hard because each atom is held in place by four strong covalent bonds. In fact it is the hardest substance on earth.
- It has a very high melting point of 35500C.
- It can’t conduct electricity because there are no ions or free electrons to carry the charge.
Silica is similar to diamond
Silica, SiO2 occurs naturally as quartz, the main mineral in sand. It also forms a giant covalent structure like diamond. Each silicon atom bonds covalently to four oxygen atoms. And each oxygen atom bonds covalently to two silicon atoms. The result is a very hard substance with a melting point of 1710oC.
Graphite is made only of carbon atoms. So diamond and graphite are said to be allotropes of carbon- two forms of same element.
Diamond is the hardest solid on earth. But graphite is soft.
Graphite has these properties
- Graphite is soft and slippery. Because of this nature, the sheets can slide over each other.
- Graphite is a good conductor of electricity. This is because each carbon atom has four outer electrons but it forms only three bonds making the fourth electron free to move through the graphite carrying charge.
- The bonding in metals
- These have low melting points
- Giant structures such as diamond and sodium chloride have much higher melting points. This is because the bonds between ions or atoms within giant structures are very strong.
Let us look at metals now. Metals also have high melting points-much higher than for carbon dioxide or water. So we can conclude they too might be giant structures.
The structure of metals
The atoms are packed tightly together in a regular lattice in metals. The tight packing allows outer electrons to separate from their atoms. The result is a lattice of ions in a sea of electrons that are free to move.
The metallic bond is actually the attraction between metal ions and free electrons.
Metals are crystalline as the ions are in a regular pattern.
Some key properties of metals:
1. Metals usually have high melting points
2. Metals are malleable and ductile.
The property of any metal being beaten to thin sheets is called malleability.
The capability of metals to be drawn into thin wires is called ductility
3. Metals are good conductors of electricity.
4. Metals are good conductors of heat