PARTICULATE NATURE OF MATTER

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In my last post, I made a introduction to the world of chemistry, today I will be talking about another aspect of chemistry which is the particulate nature of matter.

In particulate nature of matter, we have what we called physical and chemical change. Both these two terms have various definition and examples attached to it to help us have a proper view and understanding about what they really means.

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Physical and chemical changes

Let's look at ice, water and steam.
Ice, water and steam are just three different forms of one substance- water. If When ice melts or when water boils the change is a physical change. Physical changes are easily reversible; for example, water can change into steam and steam into water, ice can be change to water and water back to ice and so on examples like that to shows us that physical changes are reversible

When nichrome wire, sulphur, iodine, zinc oxide and lead(II) oxide are heated, no new substances are formed. The wire becomes white-hot, but is still nichrome.

Sulphur melts, but the liquid sulphur changes back to solid sulphur on cooling.
lodine does not melt but changes directly into purple iodine vapour, which forms solid iodine when it cools. Thus iodine is said to have sublimed. All these changes are physical ones. They can also be
called temporary or non-permanent changes as physical changes are not always temporary in their state as they can still be changed to to other state when subjected to some certain factors forcing the change.

A physical (or non-permanent) change is one in which no new substance is formed.

Dissolving salt in water to form a salt solution is a physical change. The change is temporary and can be reversed by evaporating salt solution. The burning of a candle in air is an example of a chemical or permanent change. The solid wax
changes into gases. Bright, shiny magnesium changes into a white powder when it burns, and ammoniumdichromate gives off gases and forms a green powder.

Blue hydrated copper(II) tetraoxosulphate(VI) or copper(II) sulphate loses its water of crystallisation and turns white, Copper(I) trioxocarbonate(IV) or copper(I) carbonate splits up and forms a black powder. It also loses a gas, but as the gas is colourless you cannot see that. New substances are formed, and therefore all these changes are chemical changes.

A chemical or permanent change is one in which a new substance is formed.

In many chemical changes, heat and light are given out. Chemical changes are not easily reversible. It is difficult or impossible to change the new substances back to the candlewax, magnesium or copper(II) carbonate.

Differences between physical and chemical changes

Now that I have talked about both the physical and chemical changes in terms of definition and examples, there is a need to mention some differences which exist between them in order to help us differentiate them

Physical change

(1). No new substances formed
(2). Usually the change is easily reversible
(3). No heat or light is given out or absorbed.
(4). No change in mass(e.g the nichrome wire does not change in mass)

Chemical change
(1). New substances formed
(2). Usually the change is not easily reversible
(3). Heat(and sometimes light)is usually given out or absorbed
(4). A change in mass(e.g the mass of magnesium increases as it burns).

Atoms and molecules

All matter is made of particles. Everything around us is made from these incredibly small particles. An atom is the smallest particle of an element that can take part in
a chemical reaction. An atom is the smallest particle of an element that can
exist on its own. An atom has all the properties of an element.

A block of copper is cut in half and in half again. If this Is repeated until the smallest piece is obtained that is still copper, this piece is an atom. Anything smaller than this can never have the properties of copper. Atoms are the building blocks of matter. All the various solids, liquids and gases are made by fitting atoms together in different ways. A molecule is formed when two or more atoms are joined. The atoms may be of the same element or different elements. The behaviour of a
particular substance is controlled by the way in which its atoms are put together.

It is hard to imagine that we are made largely of carbon, hydrogen and oxygen atoms and so is a block of wood. A compound is made by combining elements chemically. For example, water is a compound. Its molecules are made of two sorts of atoms, oxygen and hydrogen.
That is why we write the formula as H2O.

The atomic theory

Let us quickly journey back to history for better understanding of the atomic theory.
Over two thousand years ago people were thinking about the structure of matter. Democritus, an Ancient Greek thinker, suggested that all matter was made of
particles. However, he had no scientific evidence for this. More powerful people like Aristotle disagreed with Democritus and so these ideas were largely forgotten.
In 1808, John Dalton used the assumption that matter is made of atoms to explain the results of his experiments. He was the first person to use the word atom' for the smallest particles of matter. Dalton
developed his ideas further and put forward his Atomic Theory of Matter.

The main points in Dalton's atomic theory.

  1. Elements are made of small particles called atoms.

  2. Atoms can neither be created nor destroyed.

  3. Atoms of the same element are identical and have the same mass and size.

  4. Atoms combine in small whole numbers to form compounds.

Scientists found out that atoms are made up from smaller particles that they called subatomic particles. These include protons, electrons and neutrons. For
any atom they could identify the numbers of protons, neutrons and electrons by using atomic numbers and mass numbers. They discovered that some elements had different types of atom with different masses. These different types of atom they called isotopes.

They also found out that in an atom most of the mass is concentrated in a nucleus. They found that some atoms could disintegrate and form atoms of another element.

Sclentists called this radioactive decay

We can modify Daltons theory so that it still applies today.

  1. Elements are nmade of small particles called atoms.

  2. Atoms can neither be created nor destroyed during ordinary chemical reactions.

  3. All atoms of the same element have the same atomic number but may vary in mass number due to the presence of different isotopes.

  4. Atoms combine in small whole numbers to form compounds,
    The idea that matter is made of particles is not disputed. But atoms can be subdivided. Nuclear bombardments and radioactive disintegrations have challenged Dalton's ideas.

Atoms of the same element may have different masses. The existence of isotopes is a challenge to Dalton's ideas.
Some atoms of the same element can be quite different. Atoms of different elements may combine in many
different ways. These different combinations result in the formation of complex compounds.

In modern times, evidence for atoms has been obtained from photographs taken by X-rays and electron microscopes. The electron microscope can magnify objects more than one million times. Even with this high power of magnification, individual atoms and molecules cannot be seen.

Mass number and atomic number

For any atom there are two pieces of vital information you need to know if you want to work out the numbers of protons, neutrons and electrons in it. These are the
atomic number and the mass number.

The atomic number (sometimes called proton number) is the number of protons in the nucleus of an atom. As atoms are neutral, it is also the number of electrons
in an atom. It is the part of the atomic structure that definitely identifies the element. For instance, the element carbon has six protons in its nucleus. Its atomic
number, therefore, is 6. Thus, any nucleus having six protons must be a carbon nucleus.

Atomic number is very characteristic of an element. No two elements may have the same atomic number. The official symbol for atomic number is Z.

Atomic number is usually written as a subscript in front of the symbol for the atom. For example, a carbon atom where Z= 6 is written as.

The mass number is the sum of the numbers of protons and neutrons present in the nucleus of an atom. The official symbol for mass number is A. The mass number is usually written as a superscript in front of the symbol for the atom. For example, a carbon atom, where

A = 12, is written as C12up6down

A combination of mass number and atomic number of carbon gives: C12up6down

In this atom of carbon there are 6 protons (because the atomic number is 6), 6 electrons and 6 neutrons (deduct the atomic number from the mass number)

An atom of sodium (symbol Na) has a mass number of 23 and an atomic number of l1.

Isotopes

The atoms of the same element may have different numbers of neutrons. Atoms of the same element that contain different numbers of neutrons are called isotopes.

Isotopes have the same number of protons and electrons and the same atomic number. They have the same chemical properties but may have slightly
different physical properties. Isotopes have different mass numbers because they have difterent numbers of neutrons.

Most elements occur as isotopic mixtures. Some elements have a large number of isotopes; for example-
tin bas 10. Hydrogen has three isotopes. The isotopes possess 0,

Arrangement of particles in an atom

The protons and neutrons are tightly packed together in the nucleus at the centre of the atom.

Electrons in an atom are normally in pairs occupy space around the nucleus at a distance or level from it depending on the energy they possess, the levels occupied by electrons are called energy leveds or shells.

The electrons keep moving in these shells that are being attracted towards the nucdeus, by a force known as the cenitrifugal force, The electrons in the different shells are affected differently by the positive charge of the nucleus and thus they possess varyng anounts of energy

Energy levels are numbered from 1 to 7.The maximum number of electrons in a particular energy level is given by
2nsquare, where n is the number of the energy level or shell.
Therefore, if an atom has one energy level the maximum number of electrons it can have is 2. However, an energy level can be full or partially full and the electrons distribute themselves in such a way
that energy levels of lower energy (n= 1,2, 3...) are filled first, before those of higher energies. The arrangement of electrons in the various shells is sometinmes called electronic structure or electronic
configuration.

An element with atomic number 11 has three energy levels and its electron configuration is written as 2.8.1.

Helium has 2 electrons (a duplet): it is a very inactive noble gas, and therefore the first shell of 2 electrons is stable. Neon, another noble gas, has 10 electrons, 2 in
the first shell and 8 (an octet) in the second, which is further away from the nucleus. Argon has 18 electron= in shells of 2, 8 and 8.

Relative atomic mass (A)

Now many of us have heard about the atomic mass but few of us really knows what really is behind it been widely used today in science.
It was found out that It has been difficult to measure the mass of only one atom because it is such a small particle. However, it has been rather easier to determine the mass of an atom by comparison. Hydrogen was known to be the lightest element. The mass of a hydrogen-1 atom was arbitrarily fixed as 1 unit, i.e. H=1. The masses of other atoms
were then found by comparing their masses with that of a hydrogen atom. So a carbon atom, which is 12 times heavier than a hydrogen-1 atom, has a relative
atomic mass (symbol A,) of 12. Thus hydrogen was the standard for determining atomic masses of elements.

Hydrogen did not last long as a standard for measuring atomic masses. Oxygen took over but it also did not last long as standard. Hydrogen was unsuitable as
a standard because it did not combine with many elements.

References

. https://courses.lumenlearning.com/introchem/chapter/physical-and-chemical-changes-to-matter/

. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Fundamentals/Chemical_Change_vs._Physical_Change

. https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookCHEM1.html

. https://en.m.wikipedia.org/wiki/Atomic_theory

. https://courses.lumenlearning.com/introchem/chapter/john-dalton-and-atomic-theory/



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