In 1871, a Russian Chemist, Dimitri Mendeleev, gave a useful scheme for classification of elements. He presented the first regular periodic table in which elements of similar chemical properties were arranged in eight vertical columns called groups. The horizontal rows of table were called periods. He arranged elements in ascending order of their atomic masses and found that elements having similar chemical properties appeared at regular intervals. This observation was called Periodic Law.
D.
This is self-regulation because when the population of the insects becomes too large, it regulates itself and starts to decrease due to a shortage of resources.
The average acceleration is -5.0 m·s⁻².
The formula for acceleration (<em>a</em>) is
= 25 m·s⁻¹; 
= 0; 
= 5.0 s
∴ 
= -5.0 m·s⁻²
The negative sign tells you that the object is <em>slowing down</em>, i.e., it is <em>decelerating</em>.
Halogens is defined as the group of 7 periodic table. As, every periodic table contains 7 valence electrons and they only need 1 more to complete an outer shell, that is why they are extremely reactive. And according to the law that recurring patterns of the properties of elements arise when they are arranged in order of increasing atomic number. As the halogen all act very similarly with each other in chemical reaction, it is true.
