15. The element lithium (Li) has an atomic mass of 3 and a mass number of 7. Explain how you know that the diagram above represents a lithium atom.

What you already know:

C on this diagram is a proton and B points to the electron. A note the nucleus that contains the protons and the center of orbit to the electrons. D notes the electron cloud, radius, or the bounds to which the electron obitals exist.

If you count the number of protons this atom has it adds up to 3, which, therefore comfirms that this molecule is Li. The number of protons is specific to the atom. The question states that this element has a mass number of 7, this is because the mass number is the sum of both protons and electrons.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

every chemical equation adheres to the law of conservation of mass, which states that matter cannot be created or destroyed. therefore, there must be the same number of atoms of each element on each side of a chemical equation.