Atoms are the basic units of matter and are composed of protons, neutrons, and electrons. Protons and neutrons reside in the nucleus at the center of the atom, while electrons exist in an orbiting cloud. Atoms become ions when they gain or lose electrons, resulting in a positive or negative charge. Molecules are made of two or more atoms bonded together, and can be formed from ions or neutral atoms. Ions are atoms or molecules with a non-zero charge due to an imbalance of protons and electrons.

1. Atoms, Molecules & Ions

2. Atoms
Atoms are the basic units of matter and the
defining structure of elements. The term
"atom" comes from the Greek word for
indivisible, because it was once thought
that atoms were the smallest things in the
universe and could not be divided. We now
know that atoms are made up of three
particles: protons, neutrons and electrons
— which are composed of even smaller
particles such as quarks.

3. Atomic particles
Protons and neutrons are heavier than electrons and reside
in the nucleus at the center of the atom. Electrons are
extremely lightweight and exist in a cloud orbiting the
nucleus. The electron cloud has a radius 10,000 times
greater than the nucleus.
Protons and neutrons have approximately the same mass.
However, one proton weighs more than 1,800 electrons.
Atoms always have an equal number of protons and
electrons, and the number of protons and neutrons is usually
the same as well. Adding a proton to an atom makes a new
element, while adding a neutron makes an isotope, or
heavier version, of that atom.

4. Nucleus
The nucleus was discovered in 1911 by Ernest
Rutherford, a physicist from New Zealand, who in
1920 proposed the name proton for the positively
charged particles of the atom. Rutherford also
theorized that there was also a neutral particle within
the nucleus, which James Chadwick, a British
physicist and student of Rutherford, was able to
confirm in 1932.

5. Protons
Protons are positively charged particles found
within atomic nuclei. Rutherford discovered them
in experiments with cathode-ray tubes conducted
between 1911 and 1919. Protons are slightly
smaller in mass than neutrons with a relative
mass of 0.9986 (as compared with the mass of
the neutron being 1) or about 1.673x10-27 kg.

6. Electrons
Electrons are tiny compared to protons and
neutrons, over 1,800 times smaller than
either a proton or a neutron. Electrons have
a relative mass of 0.0005439 (as compared
with the mass of a neutron being 1) or
about 9.109x10-31 kg.

7. Neutrons
The neutron is used as a comparison to find the
relative mass of protons and electrons (so it has a
relative mass of 1) and has a physical mass of
1.6749x10-27 kg.
The neutron's existence was theorized by
Rutherford in 1920 and discovered by Chadwick in
1932. Neutrons were found during experiments
when atoms were shot at a thin sheet of beryllium.
Subatomic particles with no charge were released

8. Molecules
A molecule is an electrically neutral group of two
or more atoms held together by chemical
bonds.[4][5][6][7][8] Molecules are distinguished
from ions by their lack of electrical charge.
However, in quantum physics, organic chemistry,
and biochemistry, the term molecule is often used
less strictly, also being applied to polyatomic ions.

9. Molecules are held together by either covalent
bonding or ionic bonding. Several types of non-
metal elements exist only as molecules in the
environment. For example, hydrogen only exists as
hydrogen molecule. A molecule of a compound is
made out of two or more elements.[14]

10. Ions
atom or molecule with non zero
charge (number of electrons does not
match number of protons) • tendency
to form ions depends on
electronegativity of element • cations
(+) or anions (-) • monoatomic: Na+ ,
Cl- , H+ , Fe2+ • molecular: NO3 - ,
SO4 2- • complex: [Fe(CN)6 ] 4-