What is the smallest unit of an element that has all the properties of that element containing a nucleus within an electron cloud?

What is a molecule?

A molecule is two or more atoms connected by chemical bonds, which form the smallest unit of a substance that retains the composition and properties of that substance. Molecules form the basis of chemistry. Molecules are noted with the element symbol and a subscript with the number of atoms.

Atoms are the fundamental unit of an element. They consist of a nucleus and surrounding electrons. When an atom has an incomplete electron shell, it is said to have valence electrons. When two or more atoms come together to share outer shell valence electrons, a chemical (covalent) bond is formed, and they enter a lower energy state. When atoms bond, energy is released in an exothermic reaction. If the covalent bond is broken and the molecule is split apart, it requires energy input and is thereby endothermic.

Diatomic molecules are when only two atoms combine. An example of a diatomic molecule is carbon monoxide (CO) made of a single atom of carbon and one of oxygen. If the two atoms are the same element, it is called a homonuclear diatomic molecule, such as oxygen (O2) and nitrogen (N2). Polyatomic molecules have more than two atoms, such as water (H2O) and carbon dioxide (CO2). Larger molecules are called polymers and may be made of thousands of atoms.

Atoms can combine in many different ways as molecules. The same atoms may combine in different proportions to form different molecules. As an example, two hydrogen atoms and one oxygen atom form water (H2O), while two hydrogen atoms and two oxygen atoms form hydrogen peroxide (H2O2). It is also possible for the same elements to combine in the same proportions but in a different physical configuration. The physical structure of the molecule can determine its properties. An example is in water: The two hydrogen atoms being positioned 120 degrees apart creates a slight directional electrical charge giving water its solvent capabilities.

A molecule's molecular weight is the sum of all its constituent atoms' atomic weights. Avogadro's number (6.02214076 × 1023) is the number of molecules that constitutes the atomic weight of a molecule in grams (g). For example, water is two hydrogen atoms with a weight of 1 g each and one oxygen atom with a weight of 16 g, meaning that one mole of water molecules weighs 18 g.

The word molecule comes from the Latin molecula meaning a unit of mass. This name was to encompass its original meaning of "the smallest unit of a substance that still retains the properties of that substance." In 1873, James Maxwell defined atom and molecule: "An atom is a body which cannot be cut in two; a molecule is the smallest possible portion of a particular substance." Since molecules were named before their true nature was discovered, it led to what is now an inexact and debated definition.

Molecules form the basis of chemistry

Nonmolecular compounds

Many compounds do not fit the strict definition of a molecule but are common in chemistry and everyday life. Some examples of structures that are not molecular in nature are crystals, minerals and metals.

Noble gases are elements that do not have valence electrons and, therefore, do not need to form covalent bonds to become stable. Some may consider them a molecule composed of only a single atom.

In salts and ionic bonds, there are not conventional covalent bonds, so they are not considered molecules. For example, table salt (sodium chloride) forms a lattice structure held together by ionic bonds. In an ionic bond, the electrons may be shared by many atoms instead of only two as in a covalent bond. These chemical bonds do not result in clear separation of individual molecules. These chemical structures are expressed as the ratios of their constituent elements.

Metals do not use covalent bonds and are not considered molecules. Instead, they form metallic bonds where the free valence electrons are shared between all the various atoms in delocalized electron clouds. The electrons are free to move throughout the entire structure instead of being localized.

See also: matter, gas, solid, plasma and liquid.

This was last updated in June 2022

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Science at a Distance

Physical Structure

Lecture Notes

a check up

Use this section to check up on the accuracy of your lecture notes. Make sure that you have written down the following definitions, explanations and important concepts in your notes.

Physical Structure - Part One

Atoms

An atom is the smallest unit of a pure substance or element that can exist and still retain the properties of the original substance or element.

common substances, such as air, water, wood and cloth are mixtures of materials and can be chemically or physically broken down into simpler substances.
a pure or elementary substance cannot be broken down into smaller or simpler constituents without loosing all its original properties; such pure substances are called elements.
each element is made up of identical particles of matter called atoms.
all the atoms in a pure substance or element are identical to one another but different to the atoms in a different element.
there are 92 natural elements and 13 more that have been created in a laboratory.

Atoms of Life

The three most abundant elements on earth are oxygen, silicon and aluminum; in living organisms the six most abundant elements are carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur.

the unique chemical properties of just a few atoms determine the form and function of all living things.
the physical and chemical properties of all elements are the result of the origination of substructure of their atoms.

Subatomic particles

There are three types of subatomic particles that are found in the atoms of all but one element.

protons are small, positively charged particles with, by definition, a mass of one unit (one a.m.u.).
neutrons are neutral particles with no electrical charge, also with a mass of one unit (one a.m.u.).
electrons are very small negatively charges particle with a very small mass that is almost negligible.

Atomic substructure

Every atom has the same substructure with a dense positively charged center (sometimes called the atomic nucleus) surrounded by diffused, rapidly moving number of negatively charged particles.

protons are always found at the atomic center.
neutrons are always found at the atomic center.
electrons are always found distributed in rapidly moving, diffuse clouds or orbitals of defined volume and shape at varying distances from the atomic center.
in a balanced, neutral atom the number of protons always equals the number of electrons.
most of an atom is empty space.
the sum of the masses of all the protons and neutrons in an atom is termed the atomic mass.
the number of protons in an atom is termed the atomic number

Physical Properties of Atoms

Two important physical properties of all atoms depend on their atomic mass and the relative number of protons and neutrons in the atomic center.

the mass or weight of an atom depends on the number of protons and neutrons it contains.
atoms of elements that contain more protons and neutrons have greater overall mass.
some elements have a range or series of atomic masses in which the number of protons is always constant but the number of neutrons varies.
some of these arrangements are unstable and the atoms spontaneously explodes and gives off radiation; these are radioactive forms of an element.

Isotopes

Isotopes are families of atoms that have the same atomic number but different atomic masses.

hydrogen is an element with three isotopes.
the most common form of hydrogen (written in chemical notation as H) has a single proton at its center and no neutrons.
another form of hydrogen, called deuterium, has a single proton and single neutron.
a third form of hydrogen, called tritium, has a single proton and two neutrons.
the atomic substructure of tritium is unstable and is radioactive.

Electron Arrangements

Electrons are negatively charged and so are attracted to the positively charged protons in the center of the atom. They are also repelled by the negative charges on other electrons. Electrons hold fixed amounts of energy and this level of energy determines how near of far from the atomic center they are located.

a balance of all the forces of energy content and charges determines the volume or shape of space an electron can occupy around the atomic center of an atom.
electrons occur in zones called energy levels at increasing distances from the atomic center.
the energy level closest to the atomic center is the smallest and lowest in energy. Low energy electrons occupy this level.
the next energy level is, for the most part, further away from the atomic center. Electrons in this energy level contain more energy than the first.
only two electrons can occupy the first energy level at any one time.
only a maximum of eight electrons can occupy the second energy level at any one time.

Orbitals

As they diffuse and move about the nucleus, electrons are restricted to certain volumes or shapes of space called orbitals.

only a maximum of two electrons can be found in or occupy any one orbital at any one time.
the lowest energy level only has one orbital, shaped like a sphere, called the 1s orbital.
two electrons are the maximum number found at any one time in this orbital.
the second energy level has four orbitals.
the 2s orbital is shaped like a sphere and holds a maximum of two electrons.
there are three 2p orbitals which are dumbbell shaped and again hold a maximum of two electrons each.

Atomic Stability

Atoms are at their most stable and least reactive when their outermost energy level is either completely filled with electrons or completely empty of electrons.

hydrogen, with only one electron in its first energy level is chemically reactive.
helium with two electrons in its first energy level is inert and has little or no chemical or reactive properties (the first energy level is not full).
sodium has one electron in its outermost energy level, it is very reactive.
chlorine has seven electrons in its outermost energy level, it too is very reactive.
the chemical properties of an element depend very heavily on the number of electrons the atoms of this element have in their outermost energy level.

Stability I: ions

One way for an atom to move to a more stable state is to gain or lose electrons from its outermost energy level until the energy level is either full, or empty of electrons.

a atom of hydrogen can give away its one electron from its first energy level.
this energy level is now empty of electron and the resulting atomic form is more stable.
the remaining atomic particle now consists of a single proton with a positive charge that is no longer neutralized by the electron.
this atomic particle is now called an ion.
atoms of the element chlorine have seven electrons in their outermost energy level.
these chlorine atoms readily accept donated electrons so as to fill this energy level.
the new atomic particle now has an extra, negatively charged electron that is not neutralized by a proton at the atomic center, but it is a more stable atomic arrangement.
this atomic particle is now a negatively charged chloride ion.
ionization is the term used for the exchange of electrons in this way and the formation of positively or negatively charged atomic particles.

Stability II: sharing

Atoms can combine together and share electrons between them. Atoms that share electrons are linked to each other in a form, called a molecule that is in a lower energy state than either of the separate atoms alone.

as two atoms of hydrogen approach one another their positively charged centers are attracted to the negatively charged electron clouds surrounding the opposite atom.
at a certain distance apart the two outermost energy levels (and orbitals) merge together and form a single, complex, molecular orbital.
this new orbital now contains two electrons, one donated from each atom in the sharing relationship.
this new arrangement is more stable than the original arrangement.
it takes energy to pull these atoms apart, so the atoms stay held together as if linked by energy glue.
these forces holding atoms together are called covalent bonds.
a single covalent bond holds two atoms together while they are sharing two electrons.

Methane: an organic molecule

Atoms of different elements can share electrons, form covalent bonds between them and create complex molecules.

carbon has only four electrons in its outermost energy level.
carbon can share these four electrons and receive four electrons from other atoms thus forming four covalent bonds.
hydrogen has only one electron in its outermost energy level.
hydrogen can share this electron thus forming one covalent bond.
one carbon atom and four hydrogen atoms can come together.
each hydrogen atom shares two electrons (one from the hydrogen atom, one from the carbon atom) with the carbon atom, forming a covalent bond.
the resulting molecule has a single carbon atom at the center and four hydrogen atoms held to it by four covalent bonds.
this is a molecule called methane.
molecules composed of carbon and hydrogen held together by covalent bonds are often called organic molecules, because they were first found in living organisms.

The Water Molecule

A molecule of water consists of one atom of oxygen held to two atoms of hydrogen by two covalent bonds. The electrons in these bonds are not shared equally by the atoms. This type of covalent bond is called polar.

oxygen is an electronegative element.
electrons are held tightly to the oxygen atom.
in a sharing arrangement between an atom of oxygen and an atom of hydrogen (a covalent bond) the electrons spend more time near or around the oxygen atom than the hydrogen atom.
the oxygen atom develops a slight negative charge (from the presence of the extra electrons).
the hydrogen atoms develop a slight positive charge (from the remaining proton which is not completely neutralized by the missing electrons).
a water molecule, therefore, has a small, very slight distribution of electrical charge; positive over the hydrogen atoms and negative over the oxygen atom.
water is a polar molecule.

Special Properties of Water I: a liquid

The polarity of the water molecule gives it some special properties, the first of which is that water is a liquid at room temperature.

water is a small molecule, smaller than molecular oxygen.
oxygen is a gas at room temperature, whereas water is a liquid.
water is polar, with slight positive and negative charges on the hydrogen and oxygen components of the molecule.
the positive charge on one water molecule attracts the negative charge on a nearby water molecule and they are briefly held together.
this tiny force of attraction is called a hydrogen bond.
billions and billions of hydrogen bonds are formed and broken between water molecules every second.
the sum of all these attractions holds the water molecules closer together than it these attractions had not formed.
the state of matter taken up by a collection of water molecules is that of a liquid rather than that of a gas.
heating water molecules causes an increase in kinetic energy, more motion and the pulling apart of the hydrogen bonds.
at high enough temperatures water becomes a gas (steam).

Special Properties of Water II: a solvent

A solvent is a liquid in which other solids and liquids will dissolve. Water is almost a universal solvent in which most other substances will dissolve to some extent.

water can dissolve more substances than any other solvent.
a solute is a dissolved substance.
the extent to which a solute will dissolve in water depends on the nature and distribution of electrical charges it has or can develop.
ions, which have strong positive or negative charges dissolve readily and easily in water. The electrical charges on the solute molecules are attracted to the polar charges on the water molecules.
such substances are called hydrophilic meaning water loving.
molecules such as methane and other similar molecules (such as those found in hydrocarbons - see later) have no charges on their surface and are not polar.
there is no attraction between water and these types of molecules so they form a boundary on interface in which one type of molecule stays on one side and the other type of molecule stays on the other side.
such substances are all hydrophobic meaning water fearing

Lecture Notes - Part 2 Lecture Notes - Part 3 Physical Structure

What is the smallest unit of an element that still has the properties of that element?

An atom is the smallest unit of a pure substance or element that can exist and still retain the properties of the original substance or element.

What is the smallest unit of elements called?

The smallest particle of an element is an atom..

"All the matter is formed of atoms, and atoms are indivisible and indestructible," according to Dalton's Atomic Theory..

Atom has a central nucleus that is surrounded by one or more electrons..

Here some examples of atoms are Neon, Argon, Iron, Calcium etc..

Which element is the smallest?

Thus, helium is the smallest element, and francium is the largest. From top to bottom in a group, orbitals corresponding to higher values of the principal quantum number (n) are being added, which are on average further away from the nucleus, thus causing the size of the atom to increase.