Medicine: Chemistry Atomic structure:
Elements, Molecules and Compounds

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# ATOMIC STRUCTURE

This is the study of the structure of an atom. An atom theoretically consists of a positively charged nucleus surrounded and neutralized by negatively charged electrons revolving in orbits at varying distances from the nucleus, the constitution of the nucleus and the arrangement of the electrons differ with various chemical elements. Following these basis, we will be looking at the various subtopics as per our plan.

 Figure 1: Atomic Structure

## ELEMENTS, MOLECULES AND COMPOUNDS.

### ELEMENTS:

An element is a substance whose atoms all have the same number of protons: another way of saying this is that all of a particular element's atoms have the same atomic number.  Elements are chemically the simplest substances and hence cannot be broken down using chemical reactions.

Some examples of elements include: hydrogen (H), sodium (Na),

Magnesium (Mg) etc…….

 Figure 2: Magnesium Element

MOLECULES:

molecule is the smallest particle in a chemical element or compound that has the chemical properties of that element or compoundMolecules are made up of atoms that are held together by chemical bonds. These bonds are formed as a result of the sharing or exchange of electrons among atoms.

 Figure 3: Water Molecule

There exists basically two types of molecules; homonuclear molecules and heteronuclear molecules.

Homonuclear molecules consists of two or more similar atoms covalently bonded together for example hydrogen ($H_2$ ), chlorine ($CL_2$ ) while heteronuclear molecules consists of two or more dissimilar atoms covalently bonded together for example hydrogen fluoride (HF), water molecule ($H_2$ O).

COMPOUNDS:

A compound is a substance formed when two or more chemical elements are chemically bonded together. Some bonds that hold the elements in a compound are covalent bonds, ionic bonds. Examples include O, NaCl. There are basically two types of compounds; covalent compounds and ionic compounds.

Covalent compounds are formed by the sharing of electron pair(s) for

the elements in a compound are covalent bonds, ionic bonds. Examples include O, NaCl. There are basically two types of compounds; covalent compounds and ionic compounds.

Covalent compounds are formed by the sharing of electron pair(s) for

 Figure 4  NaCl Compound

example O while ionic bonds are formed by the transfer of electrons from a donor to a recepient for example NaCl.

ATOMIC NUMBER, MASS NUMBER, NEUTRON NUMBER AND ATOMIC MASS

The general chemical representation of an element is $^A_ZX^C$

Where;

X= Symbol of element

A=Mass number

Z=Atomic number

C=Oxidation state

For example $_2^4He^2^+$ which represents the helium nucleus.

v  ATOMIC NUMBER: The atomic number of an element is the number of protons found in the nucleus of that element, It is denoted by the symbol Z. In a neutral atom, the atomic number is equal to the number of electrons since no electron has been lost or gained by the atom. For example sodium with atomic number 11 has 11 protons and 11 electrons.

v  MASS NUMBER (Nucleon number): The mass number of an element is the sum total of the number of protons and neutrons found in the nucleus of that element. It is also called the nucleon number and denoted by the symbol A.

A=Z+N

Where N= Number of neutrons

v  NEUTRON NUMBER: This is the difference between the mass number and the atomic number. This difference accounts for the existence of isotopes. We denote the neutron number by N.

N=A-Z

v  ATOMIC MASS: Atomic mass is the mass of a single atom of a chemical element. It includes the masses of the 3 subatomic particles that make up an atom of that element which are protons, neutrons and electrons. It is denoted by the symbol $m_A$

ISOTOPY

Isotopy is the existence of atoms of the same elements having the same atomic numbers but different mass numbers. This difference is as a result of a difference in the number of neutrons. All isotopes have similar chemical properties but have different atomic masses as a result of their difference in the number of neutrons which is a function of the atomic mass.

Some common isotopes include $^1^2_6C,^1^3_6C$  and $^1^4_6C$ , Which have same atomic number of 6 but different mass numbers.

 Figure 5 ISOTOPES OF CARBON