Answer:
Isotopes are atoms of the same element that have different numbers of neutrons but the same number of protons and electrons. The difference in the number of neutrons between the various isotopes of an element means that the various isotopes have different masses.
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Answer:- Mass of the titanium alloy is 7.01 g, choice C is correct.
Solution:- The heat of fusion is given as 422.5 joules per gram and it also says that 2960 joules of heat is required to melt the metal completely.
The suggested equation is, 
where Q is the heat energy, m is the mass and Hf is the heat of fusion.
Since, we are asked to calculate the mass, the equation could be written as:

Let's plug in the values in it:

m = 7.01 g
So, the mass of the titanium alloy is 7.01 g, choice C is correct.
Answer: (3)
Medeleev arranged elements in order of increasing atomic weight and similar properties
He noticed that similar elements were grouped together by using this.
Answer:
D
Explanation:
Oxygen is the 8th element in the periodic table. This means that oxygen has 8 protons and 8 electrons. In order to get the number of neutrons you take the atomic weight in this case 15.9999~16 and you subtract it by the number of protons (16-8).