Atomic structure

Atomic structure

-An atom is the smallest particle of an element
Atoms are made up of three different particles- protons, neutrons and electrons.(sub-atomic particles)
Simplified version of an atom:

The blue dot in the center of the represents the nucleus of the atom.
- The nucleus consists of...
protons-> each proton carries 1 positive charge(+1) and has a relative mass of 1.
neutrons-> neutrons carry no charge and has a relative mass of 1.

The green dot circling in the shell around the nucleus is an electron.
Each electron carries 1 negative charge(-1) and has a relative mass of 1/1840.

• The mass and substance of an atom is always concentrated in the nucleus.
• All chemical reactions happen because of changes in the electron arrangements or changes in number of electrons.

Properties 

All atoms are electrically neutral (no charge). Therefore, an atom contains an equal number of positively charged protons and negatively charged electrons so the positive and negative electric charges cancel out exactly.

-> atomic number (no. of protons)= no. of electrons

Proton/ Neutron number

The number of protons in an atom is called the proton number or the atomic number. Since atoms are neutral, the proton number can also tell the number of electrons in the atom. For example, the proton number of nitrogen is 7. This implies that the nitrogen atom has seven protons and seven electrons. Atoms of different elements have different numbers of protons. Each element has a unique proton number. This means that no other element has atoms with this proton number. e.g. Carbon has a proton number of 6, all atoms with 6 protons are therefore carbon atoms. 

• All elements are identified by their atomic number (no. of protons). Particles with the same no. of protons belong to the same element.

Nucleon/ mass number/ atomic mass

Atomic mass= no. of protons+ no. of neutrons. The mass of an atom depends on the number of protons and neutrons in the atom's nucleus. The mass of electrons in the atom is said to be negligible.

e.g. on periodic table:

Isotopes

Particles with the same number of protons (atomic no.) but different number of neutrons (atomic mass) are called isotopes of the same element.

e.g. Hydrogen isotopes:

 They are similar except that hydrogen-1 has no neutrons, hydrogen-2 has one neutron and hydrogen-3 has two neutrons. These hydrogen atoms are known as isotopes.
Isotopes are atoms of the same number of protons but different number of neutrons. Thus, isotopes of the same element have the same proton number but different nucleon numbers. Isotopes have the same chemical properties but slightly different physical properties. The chemical properties of isotopes are similar because chemical reactions involve only electrons and not the protons and neutrons. The relative masses of the isotopes differ.

Arrangement of electrons:
The electrons in an atom move around the nucleus in regions known as electron shells. Each shell can only hold a certain number of electrons.
First shell: closest to the nucleus, can hold a maximum of 2 electrons, always filled first.
2nd, 3rd and subsequent shells: can hold up to 8 electrons each, filled in order, 2nd shell to be fully-filled before the third shell and do on.

Electronic structure:

This atom has 2 electrons in the first shell and 7 electrons on the outer shell. Thus, its electronic structure or electronic configuration can also be represented as (2, 7)

The shell that is furthest from the nucleus is called the outer shell or the valence shell. The electrons in this shell are called valence electrons. The chemical properties of an element depend on the number of valence electrons. Elements of the same number of valence electrons are in the same group. This means that elements in the same group have similar chemical properties.


Ions

• Ions are formed when atoms gain or lose electrons. (chemical reaction)
• Atom loses 2 electrons-> particle now has 2 less electrons (negative charges) compared with protons (positive charges)-> Ion with 2+ charge is formed.-> CATION
• Atom gains one electron-> particle now have 1 electron more than number of protons-> ion with 1- charge is formed-> ANION.