Structure of an Atom

 

 

The modern model of an atom is based on the researches made mainly by Rutherford and Böhr.

 

According to this model,

 

Ø The atom consists of a positively charged core called the nucleus.

Ø Almost the entire mass of an atom is concentrated in its nucleus.

Ø The nucleus consists of protons and neutrons.

Ø The nucleus of hydrogen atom, however, consists of only one proton.

Ø In an atom, the nucleus is surrounded by electrons moving around it in certain fixed orbits.

Ø An atom as a whole is electrically neutral.

Ø The number of protons inside the nucleus of an atom is equal to the number of electrons surrounding the nucleus.

Ø Electrons, protons and neutrons are called the subatomic particles.

 

 

Model of an atom

 

 

Electron

Ø   The electron is a negatively charged subatomic particle.

Ø   The mass of an electron is about 1/1837 times that of a hydrogen atom. Thus, the mass of an electron is negligible.

Ø   The negative charge possessed by an electron is considered as one unit of negative charge.

 

Proton

Ø   The proton is a positively charged subatomic particle.

Ø   The mass of a proton is almost equal to that of a hydrogen atom. Thus, a proton is 1837 times heavier than an electron.

Ø   The positive charge possessed by a proton is considered as one unit of positive charge.

 

 

Neutron

Ø   The neutron is a subatomic particle which does not carry any electrical charge.

Ø   The mass of a neutron is almost equal to that of a proton.

Ø   The mass of a neutron is taken as one atomic mass unit (or 1 amu or 1 u), where 1 u = 1.66 × 10^–24 g.

Ø   The symbol for atomic mass unit is u.

  

 

 

 

 

Conventions in writing SI Units

The following convensions (rules) should be strictly followed while writing SI units.

i.    While writing a unit, only its singular form is used.

For example, the mass of any substance should be written as kg not as kgs.

Distances should be expressed in km not in kms,

e.g., ten kilogram is written as

  10 kg                          10 kgs

(correct)                      (incorrect)

ii.  The abbreviation of any unit does not have full stop (.) at the end, unless it appears at the end of a sentence. Therefore, no full stop is placed either in between or at the end of the symbol of the unit.

For example,

      kg                   kg.                                      cm                              cm.

(correct)          (incorrect)                              (correct)                      (incorrect)

iii. One space is left between the last digit of a numeral and the symbol of the unit.

For example,

10 m                      10m                                                   5 g                              5g

(correct)                (incorrect)                                    (correct)                    (incorrect)

iv.  Words and Symbols should not be mixed.

For example,

Kilogram per cubic metre    kg/m³             kilogram/m³              kg per cubic metre

(correct)                            (correct)          (incorrect)                  (incorrect)

v.  With numerals, the symbol of the unit should be written.

For example,

100 cm                       one hundred cm                   100 centimetres

(correct)                      (incorrect)                              (incorrect)

vi.  The names of the units derived from people’s names are written in small letters.

For example, the unit named after Joule is written as joule, (symbol: J).

The unit of force named after newton is written as newton, (symbol: N).

 vii.  The sign of degree (°) is not written when Kelvin scale is employed.

For example, temperature of 273 kelvin is written as 273 K and not 273°K.

  273 K                          273°K

(correct)                      (incorrect)

Prefixes in SI Units

Prefixes in SI Units

The metric system has been adopted almost around the world. 

But, over the time, a number of other units (multiples and submultiples of the SI units) have come into use. 

This has happened because the SI units of some physical quantities are either too big or too small. 

The units smaller or bigger than those based on metric system are related to the metric unit by the powers of 10. 

Different powers of 10 are indicated by prefixes.

For example, one hundredth part (10^–2) of a metre is called centimetre: centi means one-hundredth.

A list of standard prefixes is given here.