Where is the following statements??
Answer: c.They have a unique set of properties that can be used as identifiers.
Explanation:
Compound is a pure substance which is made from atoms of different elements combined together in a fixed ratio by mass.
Compounds can be decomposed into simpler constituents using chemical reactions.
Example: Water 
Compounds have different properties than the elements it is made up of.
Thus the most accurate description of compounds is that they have a unique set of properties that can be used as identifiers.
Answer:
It's because removal of electron from an atom, reduces the size of an atom.
Explanation:
When an electron is removed from an atom, it becomes an ion and in this case it will become a postive ion.
When an electron is removed from an atom, the charge balance of an atom is disturbed and positive charge increases in comparison to the negative charge. This results in increase nuclear (positive) charge which exerts greater attraction on the remaining electrons and as a result the remaining electrons are more strongly attracted by the nucleus and in this way the atomic size is decreased. Due to this increased nuclear attraction and reduced atomic size, it bcomes difficult to remove more electeon from the positively charged ion of reduced size. This is the reason that each successive ionization of electron requires a greater amount of energy.
The ionization energy has inverse relation with the size or radius of an atom. This also justifies the reason that why each successive ionization of an electron requires greater amount of energy.
This is a question similar to one wrestled with by Newton himself, who didn't understand why the planets didn't fall into each other. The reason satellites don't fly into space is because they are being pulled towards the planet by the planet's gravity.
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I forgot what quantum means to be honest, the Bohr model In atomic physics, the Bohr model or Rutherford–Bohr model, presented by Niels Bohr and Ernest Rutherford in 1913, is a system consisting of a small, dense nucleus surrounded by orbiting electrons—similar to the structure of the Solar System, but with attraction provided by electrostatic forces in place of gravity. After the cubical model (1902), the plum pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911) came the Rutherford–Bohr model or just Bohr model for short (1913). The improvement over the 1911 Rutherford model mainly concerned the new quantum physical interpretation.
