Atomic Structure

Atoms are the smallest particles of matter. However atoms are made up of 3 fundamental subatomic particles.

Particles	Relative mass	Charge	Location in the atom
Protons (p)	1	+1 (positive)	In the nucleus
Neutrons (n)	1	0 (zero)	In the nucleus
electrons (e)	1/1840 (or) 0.0005	–1 (negative)	Arranged in energy levels or shells around the nucleus

• The nucleus is at the center of the atom and contains the protons and neutrons.
• The protons and neutrons are collectively known as nucleons and they are larger and heavier than electrons.
• Electrons orbit around the nucleus in shells or energy levels like planets revolving around the sun.
• Protons carry up positive charge (+), neutrons have no charge therefore it is neutral and electrons have a negative charge (−).
• Atoms are neutral.
• In a neutral atom the number of protons (+) equals to the number of electrons (−) that is the number of positive charges is equal to the number of negative charges and the charges cancel each other out.
• The structure of an atom can be represented in the symbol form:

Atomic number (Z):

• Atomic number (Z) is the number of protons in the nucleus of an atom.

• It is also known as the proton number of the particular element.

ATOMIC NUMBER = number of protons

• Each element has it's own symbol and own kind of atoms with fixed number of protons.

• As the proton number for each element is fixed it can be used to identify atoms of different elements.

E.g.: - The atomic number (proton number) for:

• Hydrogen is 1, therefore it is written as H.

• Carbon is 6, therefore it is written as C.

Mass number (A):

• Mass number (A) is the sum of the number of protons and the number of neutrons in one atom of an element.

MASS NUMBER = number of protons + number of neutrons

• Mass of an atom includes mass of protons and neutrons present in that particular atom. As protons and neutrons are present in the nucleus of an atom, it is also known as Nucleon number.

• Electrons are too small to affect the mass of an atom.

Structure of atoms in symbol form

• Atomic number, mass number and the symbol f

or an element can be used ti write the structure of an atom in the symbol form. (As it appears in the periodic table)

E.g.: - Atomic number of the calcium atom is 20 and mass number is 40.

• Therefore, the structure if the atom of calcium can be represented as:

Neutron number (n): is the number of neutrons present in an atom. It can be found using the formula.

NEUTRON NUMBER = mass number − atomic number

• Therefore, for calcium atom Number of neutrons = 40 − 20 = 20

IN SUMMARY:

Atomic number = Number of protons.

Mass number = Number of protons + number of neutrons.

Neutrons number = Mass number − atomic number