4) Answer is: ratio of protons to neutrons in a stable atom is 1 : 1.
For example, nitrogen atom (N-14) has 7 protons (p⁺) and 7 neutrons (n°). The neutron–proton ratio (N/Z ratio or nuclear ratio) is 7 : 7 ( 1 : 1 ).
Another example, neon atom (Ne-20) has 10 protons (p⁺) and 10 neutrons (n°). The neutron–proton ratio is 10 : 10 ( 1 : 1 ).
The neutron–proton ratio generally increases with increasing atomic number.
6) Answer is: beta decay.
Beta decay is radioactive decay in which a beta ray and a neutrino are emitted from an atomic nucleus.
There are two types of beta decay: beta minus and beta plus.
In beta minus decay (this example), neutron is converted to a proton and an electron and an electron antineutrino and decay atomic number Z is increased by one.
In beta plus decay, a proton is converted to a neutron, positron and an electron neutrino, so mass number does not change.
8) Answer is: millions of years.
The half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration.
Decay constant is proportionality between the size of a number of radioactive atoms (in this example uranium) and the rate at which the number of radioactive uranium atoms decreases because of radioactive decay.
9) Answer is: fussion.
Nuclei combine to form a heavier nucleus, releasing energy.
For example, fusion of hydrogen atoms at the Sun.
This fusion nuclear reaction is in the inner core of the Sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).
The inner core is the central region of the Sun and all solar energy is produced in the core by nuclear fusion.
